LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY UNIT 1 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY INTRODUCTION: A p–n junction is formed by joining P-type and N-type semiconductors together in very close contact. The term junction refers to the boundary interface where the two regions of the semiconductor meet. If they were constructed of two separate pieces this would introduce a grain boundary, so p–n junctions are created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion ofdopants, or by epitaxy (growing a layer of crystal doped with one type of dopant on top of a layer of crystal doped with another type of dopant). P-N junctions are elementary "building blocks" of almost all semiconductor electronic devices such as diodes, transistors, solar cells,LEDs, and integrated circuits; they are the active sites where the electronic action of the device takes place. For example, a common type of transistor, the bipolar junction transistor, consists of two p–n junctions in series, in the form n–p–n or p–n–p. Normally, p–n junctions are manufactured from a single crystal with different dopant concentrations diffused across it. Creating a semiconductor from two separate pieces of material would introduce a grain boundary between the semiconductors which severely inhibits its utility by scattering the electrons and holes.[citation needed]. However, in the case of solar cells,polycrystalline silicon is often used to reduce expense, despite the lower efficiency. Properties of a p-n junction: The p–n junction possesses some interesting properties which have useful applications in modern electronics. A p-doped semiconductor is relatively conductive. The same is true of an n-doped semiconductor, but the junction between them can become depleted of charge carriers, and hence nonconductive, depending on the relative voltages of the two semiconductor regions. By manipulating this non-conductive layer, p–n junctions are commonly used as diodes: circuit elements that allow a flow of electricity in one direction but not in the other (opposite) direction. This property is explained in terms of forward bias and reverse bias, where the term bias refers to an application of electric voltage to the p–n junction. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Equilibrium (zero bias): In a p–n junction, without an external applied voltage, an equilibrium condition is reached in which a potential difference is formed across the junction. This potential difference is called built-in potential Vbi. After joining p-type and n-type semiconductors, electrons near the p–n interface tend to diffuse into the p region. As electrons diffuse, they leave positively charged ions (donors) in the n region. Similarly, holes near the p–n interface begin to diffuse into the n-type region leaving fixed ions (acceptors) with negative charge. The regions nearby the p–n interfaces lose their neutrality and become charged, forming the space charge region or depletion layer (see figure A). Figure A. A p–n junction in thermal equilibrium with zero bias voltage applied. Electrons and holes concentration are reported respectively with blue and red lines. Gray regions are charge neutral. Light red zone is positively charged. Light blue zone is negatively charged. The electric field is shown on the bottom, the electrostatic force on electrons and holes and the direction in which the diffusion tends to move electrons and holes. The electric field created by the space charge region opposes the diffusion process for both electrons and holes. There are two concurrent phenomena: the diffusion process that tends to generate more space charge, and the electric field generated by the space charge that tends to counteract the diffusion. The carrier concentration profile at equilibrium is shown in figure Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY A with blue and red lines. Also shown are the two counterbalancing phenomena that establish equilibrium. Figure B. A p–n junction in thermal equilibrium with zero bias voltage applied. Under the junction, plots for the charge density, the electric field and the voltage are reported. The space charge region is a zone with a net charge provided by the fixed ions (donors or acceptors) that have been left uncovered by majority carrier diffusion. When equilibrium is reached, the charge density is approximated by the displayed step function. In fact, the region is completely depleted of majority carriers (leaving a charge density equal to the net doping level), and the edge between the space charge region and the neutral region is quite sharp Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY (see figure B, Q(x) graph). The space charge region has the same charge on both sides of the p–n interfaces, thus it extends farther on the less doped side (the n side in figures A and B). Forward bias: In forward bias, the p-type is connected with the positive terminal and the n-type is connected with the negative terminal. PN junction operation in forward bias mode showing reducing depletion width. Both p and n junctions are doped at a 1e15/cm3 doping level, leading to built-in potential of ~0.59V. Reducing depletion width can be inferred from the shrinking charge profile, as fewer dopants are exposed with increasing forward bias. With a battery connected this way, the holes in the P-type region and the electrons in the N-type region are pushed towards the junction. This reduces the width of the depletion zone. The positive charge applied to the P-type material repels the holes, while the negative charge applied to the N-type material repels the electrons. As electrons and holes are pushed towards the junction, the distance between them decreases. This lowers the barrier in potential. With increasing forward-bias voltage, the depletion zone eventually becomes thin enough that the zone's electric field can't counteract charge carrier motion across the p–n junction, consequently reducing electrical resistance. The electrons which cross the p–n junction into the P-type material (or holes which cross into the N-type material) will diffuse in the near-neutral region. Therefore, the amount of minority diffusion in the near-neutral zones determines the amount of current that may flow through the diode. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Only majority carriers (electrons in N-type material or holes in P-type) can flow through a semiconductor for a macroscopic length. With this in mind, consider the flow of electrons across the junction. The forward bias causes a force on the electrons pushing them from the N side toward the P side. With forward bias, the depletion region is narrow enough that electrons can cross the junction and inject into the P-type material. However, they do not continue to flow through the P-type material indefinitely, because it is energetically favorable for them to recombine with holes. The average length an electron travels through the P-type material before recombining is called the diffusion length, and it is typically on the order of microns. Although the electrons penetrate only a short distance into the P-type material, the electric current continues uninterrupted, because holes (the majority carriers) begin to flow in the opposite direction. The total current (the sum of the electron and hole currents) is constant in space, because any variation would cause charge buildup over time (this is Kirchhoff's current law). The flow of holes from the P-type region into the N-type region is exactly analogous to the flow of electrons from N to P (electrons and holes swap roles and the signs of all currents and voltages are reversed). Therefore, the macroscopic picture of the current flow through the diode involves electrons flowing through the N-type region toward the junction, holes flowing through the P-type region in the opposite direction toward the junction, and the two species of carriers constantly recombining in the vicinity of the junction. The electrons and holes travel in opposite directions, but they also have opposite charges, so the overall current is in the same direction on both sides of the diode, as required. Reverse bias: A silicon p–n junction in reverse bias. Reverse biased usually refers to how a diode is used in a circuit. If a diode is reverse biased, the voltage at the cathode is higher than that at the anode. Therefore, no current will flow until the Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY diode breaks down. Connecting the P-type region to the negative terminal of the battery and the N-type region to the positive terminal, corresponds to reverse bias. The connections are illustrated in the following diagram: (a) Blocks of P and N semiconductor in contact have no exploitable properties. (b) Single crystal doped with P and N type impurities develops a potential barrier. This separation of charges at the PN junction constitutes a potential barrier. This potential barrier must be overcome by an external voltage source to make the junction conduct. The formation of the junction and potential barrier happens during the manufacturing process. The magnitude of the potential barrier is a function of the materials used in manufacturing. Silicon PN junctions have a higher potential barrier than germanium junctions. In Figure below(a) the battery is arranged so that the negative terminal supplies electrons to the N-type material. These electrons diffuse toward the junction. The positive terminal removes electrons from the P-type semiconductor, creating holes that diffuse toward the junction. If the battery voltage is great enough to overcome the junction potential (0.6V in Si), the N-type electrons and P-holes combine annihilating each other. This frees up space within the lattice for more carriers to flow toward the junction. Thus, currents of N-type and P-type majority carriers flow toward the junction. The recombination at the junction allows a battery current to flow through the PN junction diode. Such a junction is said to be forward biased. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY (a) Forward battery bias repells carriers toward junction, where recombination results in battery current. (b) Reverse battery bias attracts carriers toward battery terminals, away from junction. Depletion region thickness increases. No sustained battery current flows. If the battery polarity is reversed as in Figure above(b) majority carriers are attracted away from the junction toward the battery terminals. The positive battery terminal attracts N-type majority carriers, electrons, away from the junction. The negative terminal attracts P-type majority carriers, holes, away from the junction. This increases the thickness of the nonconducting depletion region. There is no recombination of majority carriers; thus, no conduction. This arrangement of battery polarity is called reverse bias. The diode schematic symbol is illustrated in Figure below(b) corresponding to the doped semiconductor bar at (a). The diode is a unidirectional device. Electron current only flows in one direction, against the arrow, corresponding to forward bias. The cathode, bar, of the diode symbol corresponds to N-type semiconductor. The anode, arrow, corresponds to the P-type semiconductor. To remember this relationship, Not-pointing (bar) on the symbol corresponds to N-type semiconductor. Pointing (arrow) corresponds to P-type. (a) Forward biased PN junction, (b) Corresponding diode schematic symbol (c) Silicon Diode I vs V characteristic curve. If a diode is forward biased as in Figure above(a), current will increase slightly as voltage is increased from 0 V. In the case of a silicon diode a measurable current flows when the voltage approaches 0.6 V at (c). As the voltage is increases past 0.6 V, current increases considerably after the knee. Increasing the voltage well beyond 0.7 V may result in high enough current to destroy the diode. The forward voltage, VF, is a characteristic of the semiconductor: 0.6 to 0.7 V for silicon, 0.2 V for germanium, a few volts for Light Emitting Diodes (LED). The forward current ranges from a few mA for point contact diodes to 100 mA for small signal diodes to tens or thousands of amperes for power diodes. If the diode is reverse biased, only the leakage current of the intrinsic semiconductor flows. This is plotted to the left of the origin in Figure above(c). This current will only be as high as 1 µA for Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY the most extreme conditions for silicon small signal diodes. This current does not increase appreciably with increasing reverse bias until the diode breaks down. At breakdown, the current increases so greatly that the diode will be destroyed unless a high series resistance limits current. We normally select a diode with a higher reverse voltage rating than any applied voltage to prevent this. Silicon diodes are typically available with reverse break down ratings of 50, 100, 200, 400, 800 V and higher. It is possible to fabricate diodes with a lower rating of a few volts for use as voltage standards. We previously mentioned that the reverse leakage current of under a µA for silicon diodes was due to conduction of the intrinsic semiconductor. This is the leakage that can be explained by theory. Thermal energy produces few electron hole pairs, which conduct leakage current until recombination. In actual practice this predictable current is only part of the leakage current. Much of the leakage current is due to surface conduction, related to the lack of cleanliness of the semiconductor surface. Both leakage currents increase with increasing temperature, approaching a µA for small silicon diodes. Because the p-type material is now connected to the negative terminal of the power supply, the 'holes' in the P-type material are pulled away from the junction, causing the width of the depletion zone to increase. Similarly, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction. Therefore the depletion region widens, and does so increasingly with increasing reverse-bias voltage. This increases the voltage barrier causing a high resistance to the flow of charge carriers thus allowing minimal electric current to cross the p–n junction. The increase in resistance of the p-n junction results in the junction to behave as an insulator. This is important for radiation detection because if current was able to flow, the charged particles would just dissipate into the material. The reverse bias ensures that charged particles are able to make it to the detector system. The strength of the depletion zone electric field increases as the reverse-bias voltage increases. Once the electric field intensity increases beyond a critical level, the p–n junction depletion zone breaks-down and current begins to flow, usually by either the Zener or avalanche breakdown processes. Both of these breakdown processes are non-destructive and are reversible, so long as the amount of current flowing does not reach levels that cause the semiconductor material to overheat and cause thermal damage. This effect is used to one's advantage in zener diode regulator circuits. Zener diodes have a certain - low - breakdown voltage. A standard value for breakdown voltage is for instance 5.6V. This means that the voltage at the cathode can never be more than 5.6V higher than the voltage at the anode, because the diode will break down - and therefore conduct - if the voltage gets any higher. This effectively regulates the voltage over the diode. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY DIFFUSION CAPACITANCE: Diffusion capacitance is the capacitance due to transport of charge carriers between two terminals of a device, for example, the diffusion of carriers from anode to cathode in forward bias mode of a diode or from emitter to base (forward-biased junction in active region) for a transistor. In a semiconductor device with a current flowing through it (for example, an ongoing transport of charge by diffusion) at a particular moment there is necessarily some charge in the process of transit through the device. If the applied voltage changes to a different value and the current changes to a different value, a different amount of charge will be in transit in the new circumstances. The change in the amount of transiting charge divided by the change in the voltage causing it is the diffusion capacitance. The adjective "diffusion" is used because the original use of this term was for junction diodes, where the charge transport was via the diffusion mechanism. To implement this notion quantitatively, at a particular moment in time let the voltage across the device be V. Now assume that the voltage changes with time slowly enough that at each moment the current is the same as the DC current that would flow at that voltage, say I = I(V) (the quasistatic approximation). Suppose further that the time to cross the device is theforward transit time τF. In this case the amount of charge in transit through the device at this particular moment, denoted Q, is given by Q = I(V)τF. Consequently, the corresponding diffusion capacitance:Cdiff. is . In the event the quasi-static approximation does not hold, that is, for very fast voltage changes occurring in times shorter than the transit time τF, the equations governing time-dependent transport in the device must be solved to find the charge in transit, for example the Boltzmann equation. Zener diode: A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this electrical property. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY A conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by circuitry, the diode will be permanently damaged. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction builtin voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations. A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. By contrast with the conventional device, a reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V even if reverse bias voltage applied across it is more than its Zener voltage. The Zener diode is therefore ideal for applications such as the generation of a reference voltage (e.g. for an amplifier stage), or as a voltage stabilizer for low-current applications. The Zener diode's operation depends on the heavy doping of its p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides.[1] The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts. Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient[1]. In a 5.6 V diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6 V diode is the component of choice in temperature-critical applications. Modern manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of "Zener diode". Uses: Zener diode shown with typical packages. Reversecurrent − iZ is shown. Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point on, the relatively low impedance of the diode keeps the voltage across the diode at that value. In this circuit, a typical voltage reference or regulator, an input voltage, U IN, is regulated down to a stable output voltage UOUT. The intrinsic voltage drop of diode D is stable over a wide current range and holds UOUT relatively constant even though the input voltage may fluctuate over a fairly wide range. Because of the low impedance of the diode when operated like this, Resistor R is used to limit current through the circuit. In the case of this simple reference, the current flowing in the diode is determined using Ohms law and the known voltage drop across the resistor R. IDiode = (UIN - UOUT) / RΩ The value of R must satisfy two conditions: Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY 1. R must be small enough that the current through D keeps D in reverse breakdown. The value of this current is given in the data sheet for D. For example, the common BZX79C5V6[2] device, a 5.6 V 0.5 W Zener diode, has a recommended reverse current of 5 mA. If insufficient current exists through D, then UOUT will be unregulated, and less than the nominal breakdown voltage (this differs to voltage regulator tubes where the output voltage will be higher than nominal and could rise as high as U IN). When calculating R, allowance must be made for any current through the external load, not shown in this diagram, connected across UOUT. 2. R must be large enough that the current through D does not destroy the device. If the current through D is ID, its breakdown voltage VB and its maximum power dissipation PMAX, then IDVB < PMAX. A load may be placed across the diode in this reference circuit, and as long as the zener stays in reverse breakdown, the diode will provide a stable voltage source to the load. Shunt regulators are simple, but the requirements that the ballast resistor be small enough to avoid excessive voltage drop during worst-case operation (low input voltage concurrent with high load current) tends to leave a lot of current flowing in the diode much of the time, making for a fairly wasteful regulator with high quiescent power dissipation, only suitable for smaller loads. Zener diodes in this configuration are often used as stable references for more advanced voltage regulator circuits. These devices are also encountered, typically in series with a base-emitter junction, in transistor stages where selective choice of a device centered around the avalanche/Zener point can be used to introduce compensating temperature co-efficient balancing of the transistor PN junction. An example of this kind of use would be a DC error amplifier used in a regulated power supply circuit feedback loop system. Zener diodes are also used in surge protectors to limit transient voltage spikes. Another notable application of the zener diode is the use of noise caused by its avalanche breakdown in a random number generator that never repeats. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY UNIT 2 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY INTRODUCTION: A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known asrectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes,mercury arc valves, and other components. A device which performs the opposite function (converting DC to AC) is known as an inverter. When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium rectifier stacks were used. Half-wave rectification: In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one-phase supply, or with three diodes in a three-phase supply. The output DC voltage of a half wave rectifier can be calculated with the following two ideal equations: Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Full-wave rectification: A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. (See semiconductors, diode). Four diodes arranged this way are called a diode bridge or bridge rectifier: Graetz bridge rectifier: a full-wave rectifier using 4 diodes. For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) can form a full-wave rectifier. Twice as many windings are required on the transformer secondary to obtain the same output voltage compared to the bridge rectifier above. Full-wave rectifier using a transformer and 2 diodes. The average and root-mean-square output voltages of an ideal single phase full wave rectifier can be calculated as: Where: Vdc,Vav - the average or DC output voltage, Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Vp - the peak value of half wave, Vrms - the root-mean-square value of output voltage. π = ~ 3.14159 A Full Wave Rectifier is a circuit, which converts an ac voltage into a pulsating dc voltage using both half cycles of the applied ac voltage. It uses two diodes of which one conducts during one half cycle while the other conducts during the other half cycle of the applied ac voltage. During the positive half cycle of the input voltage, diode D1 becomes forward biased and D2 becomes reverse biased. Hence D1 conducts and D2 remains OFF. The load current flows through D1 and the voltage drop across RL will be equal to the input voltage. During the negative half cycle of the input voltage, diode D1 becomes reverse biased and D2 becomes forward biased. Hence D1 remains OFF and D2 conducts. The load current flows through D2 and the voltage drop across RL will be equal to the input voltage. Ripple Factor The ripple factor for a Full Wave Rectifier is given by Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY the average voltage or the dc voltage available across the load resistance is RMS value of the voltage at the load resistance is Efficiency Efficiency, is the ratio of the dc output power to ac input power The maximum efficiency of a Full Wave Rectifier is 81.2%. Transformer Utilization Factor Transformer Utilization Factor, TUF can be used to determine the rating of a transformer secondary. It is determined by considering the primary and the secondary winding separately and it gives a value of 0.693. Form Factor Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Form factor is defined as the ratio of the rms value of the output voltage to the average value of the output voltage. Peak Factor Peak factor is defined as the ratio of the peak value of the output voltage to the rms value of the output voltage. Peak inverse voltage for Full Wave Rectifier is 2Vm because the entire secondary voltage appears across the non-conducting diode. This concludes the explanation of the various factors associated with Full Wave Rectifier. Rectifier with Filter The output of the Full Wave Rectifier contains both ac and dc components. A majority of the applications, which cannot tolerate a high value ripple, necessitates further processing of the rectified output. The undesirable ac components i.e. the ripple, can be minimized using filters. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The output of the rectifier is fed as input to the filter. The output of the filter is not a perfect dc, but it also contains small ac components. Some important filters are 1. 2. 3. 4. Inductor Filter Capacitor Filter LC Filter CLC or Filter Inductor Filter The figure shows an inductor filter. When the output of the rectifier passes through an inductor, it blocks the ac component and allows only the dc component to reach the load. Ripple factor of the inductor filter is given by Faculty Name: VINAY CHOWDARY . Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The above equation shows that ripple will decrease when L is increased and RL is decreased. Thus the inductor filter is more effective only when the load current is high (small RL). The larger value of the inductor can reduce the ripple and at the same time the output dc voltage will be lowered as the inductor has a higher dc resistance. The operation of the inductor filter depends on its property to oppose any change of current passing through it. To analyze this filter for full wave, the Fourier series can be written as The dc component is . Assuming the third and higher terms contribute little output, the output voltage is The diode, choke and transformer resistances can be neglected since they are very small compared with RL. Therefore the dc component of current The impedance of series combination of L and RL at 2 is Therefore for the ac component, Therefore, the resulting current i is given by, The ripple factor which can be defined as the ratio of the rms value of the ripple to the dc value of the wave, is Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY If , then a simplified expression for is In case, the load resistance is infinity i.e., the output is an open circuit, then the ripple factor is . This is slightly less than the value of 0.482. The difference being attributable to the omission of higher harmonics as mentioned. It is clear that the inductor filter should only be used where RL is consistently small. Capacitor Filter A capacitor filter connected directly across the load is shown above. The property of a capacitor is that it allows ac component and blocks dc component. The operation of the capacitor filter is to short the ripple to ground but leave the dc to appear at output when it is connected across the pulsating dc voltage. During the positive half cycle, the capacitor charges upto the peak vale of the transformer secondary voltage, Vm and will try to maintain this value as the full wave input drops to zero. Capacitor will discharge through RL slowly until the transformer secondary voltage again increase to a value greater than the capacitor voltage. The diode conducts for a period, which depends on the capacitor voltage. The diode will conduct when the transformer secondary voltage becomes more than the diode voltage. This is called the cut in voltage. The diode stops conducting when the transformer voltage becomes less than the diode voltage. This is called cut out voltage. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Referring to the figure below, with slight approximation the ripple voltage can be assumed as triangular. From the cut-in point to the cut-out point, whatever charge the capacitor acquires is equal to the charge the capacitor has lost during the period of non-conduction, i.e., from cut-out point to the next cut-in point. The charge it has acquired The charge it has lost If the value of the capacitor is fairly large, or the value of the load resistance is very large, then it can be assumed that the time T2 is equal to half the periodic time of the waveform. From the above assumptions, the ripple waveform will be triangular and its rms value is given by Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The ripple may be decreased by increasing C or RL (both) with a resulting increase in the dc. output voltage. LC Filter: - The ripple factor is directly proportional to the load resistance RL in the inductor filter and inversely proportional to RL in the capacitor filter. Therefore if these two filters are combined as LC filter or L section filter as shown in figure the ripple factor will be independent of RL. If the value of inductance is increased it will increase the time of conduction. At some critical value of inductance, one diode, either D1 or D2 will always conducting. From Fourier series, the output voltage can be expressed as The dc output voltage, The ripple factor Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY CLC or Filter The above figure shows CLC or type filter, which basically consists of a capacitor filter, followed by LC section. This filter offers a fairly smooth output and is characterized by highly peaked diode currents and poor regulation. As in L section filter the analysis is obtained as follows. Procedure: EDWin 2000 -> Schematic Editor: The circuit diagram is drawn by loading components from the library. Wiring and proper net assignment has been made. The values are assigned for relevant components. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY EDWin 2000 -> Mixed Mode Simulator: The circuit is preprocessed. The test points and waveform markers are placed in input and output of the circuit. GND net is set as reference net. The Transient Analysis parameters have been set. The Transient Analysis is executed and output waveform is observed in Waveform Viewer. Result: The output waveform for Full Wave Rectifier with filter and without filter may be observed in the waveform viewer. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Peak loss: An aspect of most rectification is a loss from the peak input voltage to the peak output voltage, caused by the built-in voltage drop across the diodes (around 0.7 V for ordinary silicon p-njunction diodes and 0.3 V for Schottky diodes). Half-wave rectification and full-wave rectification using two separate secondaries will have a peak voltage loss of one diode drop. Bridge rectification will have a loss of two diode drops. This may represent significant power loss in very low voltage supplies. In addition, the diodes will not conduct below this voltage, so the circuit is only passing current through for a portion of each half-cycle, causing short segments of zero voltage to appear between each "hump". Rectifier output with fiters: While half-wave and full-wave rectification suffice to deliver a form of DC output, neither produces constant-voltage DC. In order to produce steady DC from a rectified AC supply, a smoothing circuit or filter is required.[1] In its simplest form this can be just a reservoir capacitor or smoothing capacitor, placed at the DC output of the rectifier. There will still remain an amount of AC ripple voltage where the voltage is not completely smoothed. RC-Filter Rectifier Sizing of the capacitor represents a tradeoff. For a given load, a larger capacitor will reduce ripple but will cost more and will create higher peak currents in the transformer secondary and in the supply feeding it. In extreme cases where many rectifiers are loaded onto a power distribution circuit, it may prove difficult for the power distribution authority to maintain a correctly shaped sinusoidal voltage curve. For a given tolerable ripple the required capacitor size is proportional to the load current and inversely proportional to the supply frequency and the number of output peaks of the rectifier per input cycle. The load current and the supply frequency are generally outside the control of the Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY designer of the rectifier system but the number of peaks per input cycle can be affected by the choice of rectifier design. A half-wave rectifier will only give one peak per cycle and for this and other reasons is only used in very small power supplies. A full wave rectifier achieves two peaks per cycle and this is the best that can be done with single-phase input. For three-phase inputs a three-phase bridge will give six peaks per cycle and even higher numbers of peaks can be achieved by using transformer networks placed before the rectifier to convert to a higher phase order. To further reduce this ripple, a capacitor-input filter can be used. This complements the reservoir capacitor with a choke (inductor) and a second filter capacitor, so that a steadier DC output can be obtained across the terminals of the filter capacitor. The choke presents a high impedance to the ripple current. A more usual alternative to a filter, and essential if the DC load is very demanding of a smooth supply voltage, is to follow the reservoir capacitor with a voltage regulator. The reservoir capacitor needs to be large enough to prevent the troughs of the ripple getting below the voltage the DC is being regulated to. The regulator serves both to remove the last of the ripple and to deal with variations in supply and load characteristics. It would be possible to use a smaller reservoir capacitor (these can be large on high-current power supplies) and then apply some filtering as well as the regulator, but this is not a common strategy. The extreme of this approach is to dispense with the reservoir capacitor altogether and put the rectified waveform straight into a choke-input filter. The advantage of this circuit is that the current waveform is smoother and consequently the rectifier no longer has to deal with the current as a large current pulse, but instead the current delivery is spread over the entire cycle. The downside is that the voltage output is much lower – approximately the average of an AC half-cycle rather than the peak. The capacitor-input filter, also called pi filter due to its shape that looks like the Greek letter pi, is a type of electronic filter. Filter circuits are used to remove unwanted or undesired frequencies from a signal. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY A typical capacitor input filter consists of a filter capacitor C1, connected across the rectifier output, an inductor L, in series and another filter capacitor, C2, connected across the load, RL. A filter of this sort is designed for use at a particular frequency, generally fixed by the AC line frequency and rectifier configuration. When used in this service, filter performance is often characterized by its regulation and ripple. 1. The capacitor C1 offers low reactance to the AC component of the rectifier output while it offers infinite reactance to the DC component. As a result the capacitor shunts an appreciable amount of the AC component while the DC component continues its journey to the inductor L 2. The inductor L offers high reactance to the AC component but it offers almost zero reactance to the DC component. As a result the DC component flows through the inductor while the AC component is blocked. 3. The capacitor C2 bypasses the AC component which the inductor had failed to block. As a result only the DC component appears across the load RL. Passive filters: Passive implementations of linear filters are based on combinations of resistors (R), inductors (L) and capacitors (C). These types are collectively known as passive filters, because they do not depend upon an external power supply and/or they do not contain active components such as transistors. Inductors block high-frequency signals and conduct low-frequency signals, while capacitors do the reverse. A filter in which the signal passes through an inductor, or in which a capacitor provides a path to ground, presents less attenuation to low-frequency signals than high-frequency signals and is a low-pass filter. If the signal passes through a capacitor, or has a path to ground through an inductor, then the filter presents less attenuation to high-frequency signals than lowfrequency signals and is a high-pass filter. Resistors on their own have no frequency-selective properties, but are added to inductors and capacitors to determine the time-constants of the circuit, and therefore the frequencies to which it responds. The inductors and capacitors are the reactive elements of the filter. The number of elements determines the order of the filter. In this context, an LC tuned circuit being used in a band-pass or band-stop filter is considered a single element even though it consists of two components. At high frequencies (above about 100 megahertz), sometimes the inductors consist of single loops or strips of sheet metal, and the capacitors consist of adjacent strips of metal. These inductive or capacitive pieces of metal are called stubs. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Single element types A low-pass electronic filter realised by an RC circuit The simplest passive filters, RC and RL filters, include only one reactive element, except hybrid LC filter which is characterized by inductance and capacitance integrated in one element.[1]. L filter: An L filter consists of two reactive elements, one in series and one in parallel. T and π filters: Main article: Capacitor-input filter Low-pass π filter High-pass T filter: Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Three-element filters can have a 'T' or 'π' topology and in either geometries, a low-pass,highpass, band-pass, or band-stop characteristic is possible. The components can be chosen symmetric or not, depending on the required frequency characteristics. The high-pass T filter in the illustration, has a very low impedance at high frequencies, and a very high impedance at low frequencies. That means that it can be inserted in a transmission line, resulting in the high frequencies being passed and low frequencies being reflected. Likewise, for the illustrated lowpass π filter, the circuit can be connected to a transmission line, transmitting low frequencies and reflecting high frequencies. Using m-derived filter sections with correct termination impedances, the input impedance can be reasonably constant in the pass band Voltage regulator: A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator is an example of a negative feedback control loop. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages. Electronic voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other elements. In automobilealternators and central power station generator plants, voltage regulators control the output of the plant. In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line. Electronic voltage regulators: Electronic voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element in such a way as to reduce the voltage error. This forms a negative feedback control loop; increasing the open-loop gain tends to increase regulation accuracy but reduce stability (avoidance of oscillation, or ringing during step changes). There will also be a trade-off between stability and the speed of the response to changes. If the output voltage is too low (perhaps due to input voltage reducing or load current increasing), the regulation element is commanded, up to a point, to produce a higher output voltage - by dropping less of the input voltage (for linear series Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY regulators and buck switching regulators), or to draw input current for longer periods (boosttype switching regulators); if the output voltage is too high, the regulation element will normally be commanded to produce a lower voltage. However, many regulators have over-current protection, so that they will entirely stop sourcing current (or limit the current in some way) if the output current is too high, and some regulators may also shut down if the input voltage is outside a given range (see also: crowbar circuits). Measures of regulator quality: The output voltage can only be held roughly constant; the regulation is specified by two measurements: load regulation is the change in output voltage for a given change in load current (for example: "typically 15mV, maximum 100mV for load currents between 5mA and 1.4A, at some specified temperature and input voltage"). line regulation or input regulation is the degree to which output voltage changes with input (supply) voltage changes - as a ratio of output to input change (for example "typically 13mV/V"), or the output voltage change over the entire specified input voltage range (for example "plus or minus 2% for input voltages between 90V and 260V, 50-60Hz"). Other important parameters are: Temperature coefficient of the output voltage is the change in output voltage with temperature (perhaps averaged over a given temperature range), while... Initial accuracy of a voltage regulator (or simply "the voltage accuracy") reflects the error in output voltage for a fixed regulator without taking into account temperature or aging effects on output accuracy. Dropout voltage - the minimum difference between input voltage and output voltage for which the regulator can still supply the specified current. A Low Drop-Out (LDO) regulator is designed to work well even with an input supply only a Volt or so above the output voltage. Absolute maximum ratings are defined for regulator components, specifying the continuous and peak output currents that may be used (sometimes internally limited), the maximum input voltage, maximum power dissipation at a given temperature, etc. Output noise (thermal white noise) and output dynamic impedance may be specified as graphs versus frequency, while output ripple noise (mains "hum" or switch-mode "hash" noise) may be given as peak-to-peak or RMS voltages, or in terms of their spectra. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Quiescent current in a regulator circuit is the current drawn internally, not available to the load, normally measured as the input current while no load is connected (and hence a source of inefficiency; some linear regulators are, surprisingly, more efficient at very low current loads than switch-mode designs because of this). Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY UNIT 3 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY INTRODUCTION: A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits. The transistor is the fundamental building block of modern electronic devices, and is ubiquitous in modern electronic systems. Following its release in the early 1950s the transistor revolutionised the field of electronics, and paved the cheaper radios, calculators, andcomputers, amongst other things. way for smaller and History: A replica of the first working transistor. Physicist Julius Edgar Lilienfeld filed the first patent for a transistor in Canada in 1925, describing a device similar to a Field Effect Transistor or "FET" However, Lilienfeld did not publish any research articles about his devices,[citation needed] nor did his patent cite any examples of devices actually constructed. In 1934, German inventor Oskar Heil patented a similar device From 1942 Herbert Mataré experimented with so-called Duodiodes while working on a detector for a Doppler RADAR system. The duodiodes built by him had two separate but very close metal contacts on the semiconductor substrate. He discovered effects that could not be explained by two independently operating diodes and thus formed the basic idea for the later point contact transistor. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY In 1947, John Bardeen and Walter Brattain at AT&T's Bell Labs in the United States observed that when electrical contacts were applied to a crystal of germanium, the output power was larger than the input. Solid State Physics Group leader William Shockley saw the potential in this, and over the next few months worked to greatly expand the knowledge of semiconductors. The term transistor was coined by John R. Pierce According to physicist/historian Robert Arns, legal papers from the Bell Labs patent show that William Shockley and Gerald Pearson had built operational versions from Lilienfeld's patents, yet they never referenced this work in any of their later research papers or historical articles The name transistor is a portmanteau of the term "transfer resistor" The first silicon transistor was produced by Texas Instruments in 1954.This was the work of Gordon Teal, an expert in growing crystals of high purity, who had previously worked at Bell Labs.The first MOS transistor actually built was by Kahng and Atalla at Bell Labs in 1960. Importance The transistor is the key active component in practically all modern electronics, and is considered by many to be one of the greatest inventions of the twentieth century.Its importance in today's society rests on its ability to be mass produced using a highly automated process (semiconductor device fabrication) that achieves astonishingly low per-transistor costs. Although several companies each produce over a billion individually packaged (known as discrete) transistors every year, the vast majority of transistors now produced are in integrated circuits (often shortened to IC, microchips or simply chips), along with diodes, resistors, capacitors and other electronic components, to produce complete electronic circuits. A logic gate consists of up to about twenty transistors whereas an advanced microprocessor, as of 2009, can use as many as 2.3 billion transistors (MOSFETs).[11] "About 60 million transistors were built this year [2002] ... for [each] man, woman, and child on Earth."[12] The transistor's low cost, flexibility, and reliability have made it a ubiquitous device. Transistorized mechatronic circuits have replaced electromechanical devices in controlling appliances and machinery. It is often easier and cheaper to use a standard microcontroller and write a computer program to carry out a control function than to design an equivalent mechanical control function. Usage: The bipolar junction transistor, or BJT, was the most commonly used transistor in the 1960s and 70s. Even after MOSFETs became widely available, the BJT remained the transistor of choice for many analog circuits such as simple amplifiers because of their greater linearity and ease of Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY manufacture. Desirable properties of MOSFETs, such as their utility in low-power devices, usually in the CMOS configuration, allowed them to capture nearly all market share for digital circuits; more recently MOSFETs have captured most analog and power applications as well, including modern clocked analog circuits, voltage regulators, amplifiers, power transmitters, motor drivers, etc. Simplified operation Simple circuit to show the labels of a bipolar transistor. The essential usefulness of a transistor comes from its ability to use a small signal applied between one pair of its terminals to control a much larger signal at another pair of terminals. This property is called gain. A transistor can control its output in proportion to the input signal; that is, it can act as an amplifier. Alternatively, the transistor can be used to turn current on or off in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements. The two types of transistors have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing from the base to the emitter) can control or switch a much larger current between the collector and emitter terminals. For a field-effect transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate can control a current between source and drain. The image to the right represents a typical bipolar transistor in a circuit. Charge will flow between emitter and collector terminals depending on the current in the base. Since internally the base and emitter connections behave like a semiconductor diode, a voltage drop develops between base and emitter while the base current exists. The amount of this voltage depends on the material the transistor is made from, and is referred to asVBE. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Transistor as a switch: BJT used as an electronic switch, in grounded-emitter configuration. Transistors are commonly used as electronic switches, for both high power applications including switched-mode power supplies and low power applications such as logic gates. In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises the base and collector current rise exponentially, and the collector voltage drops because of the collector load resistor. The relevant equations: VRC = ICE × RC, the voltage across the load (the lamp with resistance RC) VRC + VCE = VCC, the supply voltage shown as 6V If VCE could fall to 0 (perfect closed switch) then Ic could go no higher than VCC / RC, even with higher base voltage and current. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off,[13] or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant. Transistor as an amplifier: Amplifier circuit, standard common-emitter configuration. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The common-emitter amplifier is designed so that a small change in voltage in (Vin) changes the small current through the base of the transistor and the transistor's current amplification combined with the properties of the circuit mean that small swings in Vin produce large changes in Vout. Various configurations of single transistor amplifier are possible, with some providing current gain, some voltage gain, and some both. From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal processing. The first discrete transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved. Modern transistor audio amplifiers of up to a few hundred watts are common and relatively inexpensive. Comparison with vacuum tubes Prior to the development of transistors, vacuum (electron) tubes (or in the UK "thermionic valves" or just "valves") were the main active components in electronic equipment. Advantages: The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are Small size and minimal weight, allowing the development of miniaturized electronic devices. Highly automated manufacturing processes, resulting in low per-unit cost. Lower possible operating voltages, making transistors suitable for small, batterypowered applications. No warm-up period for cathode heaters required after power application. Lower power dissipation and generally greater energy efficiency. Higher reliability and greater physical ruggedness. Extremely long life. Some transistorized devices have been in service for more than 50 years. Complementary devices available, facilitating the design of complementarysymmetry circuits, something not possible with vacuum tubes. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications. Limitations: Silicon transistors do not operate at voltages higher than about 1,000 volts (SiC devices can be operated as high as 3,000 volts). In contrast, electron tubes have been developed that can be operated at tens of thousands of volts. High power, high frequency operation, such as that used in over-the-air television broadcasting, is better achieved in electron tubes due to improved electron mobility in a vacuum. Silicon transistors are much more vulnerable than electron tubes an electromagnetic pulse generated by a high-altitude nuclear explosion. PNP P-channel NPN N-channel BJT to JFET BJT and JFET symbols P-channel Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY N-channel JFET MOSFET enh MOSFET dep JFET and IGFET symbols Transistors are categorized by Semiconductor material: germanium, silicon, gallium arsenide, silicon carbide, etc. Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types" Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs) Maximum power rating: low, medium, high Maximum operating frequency: low, medium, high, radio frequency (RF), microwave (The maximum effective frequency of a transistor is denoted by the term fT, an abbreviation for "frequency of transition". The frequency of transition is the frequency at which the transistor yields unity gain). Application: switch, general purpose, audio, high voltage, super-beta, matched pair Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array, power modules Amplification factor hfe (transistor beta)[14] Thus, a particular transistor may be described as silicon, surface mount, BJT, NPN, low power, high frequency switch. Bipolar junction transistor: transistors are so named because they conduct by using both majority and minority carriers. The bipolar junction transistor (BJT), the first type of transistor to be massproduced, is a combination of two junction diodes, and is formed of either a thin layer of p-type semiconductor sandwiched between two n-type semiconductors (an n-p-n transistor), or a thin layer of n-type semiconductor sandwiched between two p-type semiconductors (a p-n-p transistor). This construction produces two p-n junctions: a base–emitter junction and a base–collector junction, separated by a thin region of semiconductor known as the base region (two junction diodes wired together without sharing an intervening semiconducting region will not make a transistor). Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The BJT has three terminals, corresponding to the three layers of semiconductor an emitter, abase, and a collector. It is useful in amplifiers because the currents at the emitter and collector are controllable by a relatively small base current."[15] In an NPN transistor operating in the active region, the emitter-base junction is forward biased (electrons and holes recombine at the junction), and electrons are injected into the base region. Because the base is narrow, most of these electrons will diffuse into the reverse-biased (electrons and holes are formed at, and move away from the junction) base-collector junction and be swept into the collector; perhaps one-hundredth of the electrons will recombine in the base, which is the dominant mechanism in the base current. By controlling the number of electrons that can leave the base, the number of electrons entering the collector can be controlled.[15] Collector current is approximately β (common-emitter current gain) times the base current. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications. A bipolar (junction) transistor (BJT) is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolartransistors are so named because their operation involves both electrons and holes. Charge flow in a BJT is due to bidirectional diffusion of charge carriers across a junction between two regions of different charge concentrations. This mode of operation is contrasted with unipolar transistors, such as field-effect transistors, in which only one carrier type is involved in charge flow due to drift. By design, most of the BJT collector current is due to the flow of charges injected from a high-concentration emitter into the base where they are minority carriersthat diffuse toward the collector, and so BJTs are classified as minority-carrier devices. PNP NPN Schematic PNPBJTs. Faculty Name: VINAY CHOWDARY symbols and for NPN-type Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY NPN BJT with forward-biased E–B junction and reverse-biased B–C junction An NPN transistor can be considered as two diodes with a shared anode. In typical operation, the base-emitter junction is forward biased and the base–collector junction is reverse biased. In an NPN transistor, for example, when a positive voltage is applied to the base–emitter junction, the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the base region. These electrons wander (or "diffuse") through the base from the region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base. To minimize the percentage of carriers that recombine before reaching the collector–base junction, the transistor's base region must be thin enough that carriers can diffuse across it in much less time than the semiconductor's minority carrier lifetime. In particular, the thickness of the base must be much less than the diffusion length of the electrons. The collector–base junction is reverse-biased, and so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector–base junction. The thin shared base and asymmetric collector–emitter doping is what differentiates a bipolar transistor from twoseparate and oppositely biased diodes connected in series. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Voltage, current, and charge control: The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control). These views are related by the current–voltage relation of the base–emitter junction, which is just the usual exponential current– voltage curve of a p-n junction (diode).[1] The physical explanation for collector current is the amount of minority-carrier charge in the base region.[1][2][3] Detailed models of transistor action, such as the Gummel–Poon model, account for the distribution of this charge explicitly to explain transistor behavior more exactly.[4] The charge-control view easily handles phototransistors, where minority carriers in the base region are created by the absorption of photons, and handles the dynamics of turn-off, or recovery time, which depends on charge in the base region recombining. However, because base charge is not a signal that is visible at the terminals, the current- and voltage-control views are generally used in circuit design and analysis. In analog circuit design, the current-control view is sometimes used because it is approximately linear. That is, the collector current is approximately βF times the base current. Some basic circuits can be designed by assuming that the emitter–base voltage is approximately constant, and that collector current is beta times the base current. However, to accurately and reliably design production BJT circuits, the voltage-control (for example, Ebers–Moll) model is required[1]. The voltage-control model requires an exponential function to be taken into account, but when it is linearized such that the transistor can be modelled as a transconductance, as in the Ebers–Moll model, design for circuits such as differential amplifiers again becomes a mostly linear problem, so the voltage-control view is often preferred. For translinear circuits, in which the exponential I–V curve is key to the operation, the transistors are usually modelled as voltage controlled with transconductance proportional to collector current. In general, transistor level circuit design is performed using SPICE or a comparable analogue circuit simulator, so model complexity is usually not of much concern to the designer. Turn-on, turn-off, and storage delay: The Bipolar transistor exhibits a few delay characteristics when turning on and off. Most transistors, and especially power transistors, exhibit long base storage time that limits maximum frequency of operation in switching applications. One method for reducing this storage time is by using a Baker clamp. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Transistor 'alpha' and 'beta': The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. The heavy doping of the emitter region and light doping of the base region cause many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. The common-emitter current gain is represented by βFor hfe; it is approximately the ratio of the DC collector current to the DC base current in forward-active region. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications. Another important parameter is the common-base current gain, αF. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.98 and 0.998. Alpha and beta are more precisely related by the following identities (NPN transistor): Structure: Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Simplified cross section of a planar NPN bipolar junction transistor A BJT consists of three differently doped semiconductor regions, the emitter region, thebase region and the collector region. These regions are, respectively, p type, n type andp type in a PNP, and n type, p type and n type in a NPN transistor. Each semiconductor region is connected to a terminal, appropriately labeled: emitter (E), base (B) andcollector (C). The base is physically located between the emitter and the collector and is made from lightly doped, high resistivity material. The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β. A cross section view of a BJT indicates that the collector–base junction has a much larger area than the emitter– base junction. The bipolar junction transistor, unlike other transistors, is usually not a symmetrical device. This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode. Because the transistor's internal structure is usually optimized for forward-mode operation, interchanging the collector and the emitter makes the values of α and β in reverse operation much smaller than those in forward operation; often the α of the reverse mode is lower than 0.5. The lack of symmetry is primarily due to the doping ratios of the emitter and the collector. The emitter is heavily doped, while the collector is lightly Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY doped, allowing a large reverse bias voltage to be applied before the collector–base junction breaks down. The collector–base junction is reverse biased in normal operation. The reason the emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers injected by the emitter to those injected by the base. For high current gain, most of the carriers injected into the emitter–base junction must come from the emitter. The low-performance "lateral" bipolar transistors sometimes used in CMOS processes are sometimes designed symmetrically, that is, with no difference between forward and backward operation. Small changes in the voltage applied across the base–emitter terminals causes the current that flows between the emitter and the collector to change significantly. This effect can be used to amplify the input voltage or current. BJTs can be thought of as voltage-controlled current sources, but are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base. Early transistors were made from germanium but most modern BJTs are made from silicon. A significant minority are also now made from gallium arsenide, especially for very high speed applications (see HBT, below). NPN The symbol of an NPN Bipolar Junction Transistor. NPN is one of the two types of bipolar transistors, in which the letters "N" (negative) and "P" (positive) refer to the majority charge carriers inside the different regions of the transistor. Most bipolar transistors used today are NPN, because electron mobility is higher than hole mobility in semiconductors, allowing greater currents and faster operation. NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. In other terms, an NPN transistor is "on" when its base is pulled high relative to the emitter. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode. One mnemonic device for identifying the symbol for the NPN transistor is "not pointing in, or 'not pointing, no' " PNP The other type of BJT is the PNP with the letters "P" and "N" referring to the majority charge carriers inside the different regions of the transistor. The symbol of a PNP Bipolar Junction Transistor. PNP transistors consist of a layer of N-doped semiconductor between two layers of P-doped material. A small current leaving the base in common-emitter mode is amplified in the collector output. In other terms, a PNP transistor is "on" when its base is pulled low relative to the emitter. The arrow in the PNP transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode. One mnemonic device for identifying the symbol for the PNP transistor is "pointing in proudly, or 'pointing in - pah'." Regions of operation: Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Applied voltages Mode E<B<C Forward active E<B>C Saturation E>B<C Cut-off E>B>C Reverse-action Bipolar transistors have five distinct regions of operation, defined by BJT junction biases. The modes of operation can be described in terms of the applied voltages (this description applies to NPN transistors; polarities are reversed for PNP transistors): Forward active: base higher than emitter, collector higher than base (in this mode the collector current is proportional to base current by βF). Saturation: base higher than emitter, but collector is not higher than base. Cut-Off: base lower than emitter, but collector is higher than base. It means the transistor is not letting conventional current to go through collector to emitter. Reverse-action: base lower than emitter, collector lower than base: reverse conventional current goes through transistor. In terms of junction biasing: ('reverse biased base–collector junction' means Vbc < 0 for NPN, opposite for PNP) Forward-active (or simply, active): The base–emitter junction is forward biased and the base–collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βF, in forward-active mode. If this is the case, the collector–emitter current is approximately proportional to the base current, but many times larger, for small base current variations. Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Because most BJTs are designed to maximize current gain in forward-active mode, the βF in inverted mode is several (2–3 for the ordinary germanium transistor) times smaller. This transistor mode is seldom used, usually being Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY considered only for failsafe conditions and some types of bipolar logic. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region. Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch. Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current, which corresponds to a logical "off", or an open switch. Avalanche breakdown region Although these regions are well defined for sufficiently large applied voltage, they overlap somewhat for small (less than a few hundred millivolts) biases. For example, in the typical grounded-emitter configuration of an NPN BJT used as a pulldown switch in digital logic, the "off" state never involves a reverse-biased junction because the base voltage never goes below ground; nevertheless the forward bias is close enough to zero that essentially no current flows, so this end of the forward active region can be regarded as the cutoff region. Active-mode NPN transistors in circuits Structure and use of NPN transistor. Arrow according to schematic. The diagram opposite is a schematic representation of an NPN transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from C to E, VBE must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs at room temperature but can be different Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY depending on the type of transistor and its biasing. This applied voltage causes the lower P-N junction to 'turn-on' allowing a flow of electrons from the emitter into the base. In active mode, the electric field existing between base and collector (caused by VCE) will cause the majority of these electrons to cross the upper P-N junction into the collector to form the collector current IC. The remainder of the electrons recombine with holes, the majority carriers in the base, making a current through the base connection to form the base current, IB. As shown in the diagram, the emitter current, IE, is the total transistor current, which is the sum of the other terminal currents (i.e., ). In the diagram, the arrows representing current point in the direction of conventional current – the flow of electrons is in the opposite direction of the arrows because electrons carry negative electric charge. In active mode, the ratio of the collector current to the base current is called the DC current gain. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value (for example see op-amp). The value of this gain for DC signals is referred to as hFE, and the value of this gain for AC signals is referred to ashfe. However, when there is no particular frequency range of interest, the symbol β is used. It should also be noted that the emitter current is related to VBE exponentially. At room temperature, an increase in VBE by approximately 60 mV increases the emitter current by a factor of 10. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way. Active-mode PNP transistors in circuits Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Structure and use of PNP transistor: The diagram opposite is a schematic representation of a PNP transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from E to C, VEB must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs at room temperature but can be different depending on the type of transistor and its biasing. This applied voltage causes the upper P-N junction to 'turn-on' allowing a flow of holes from the emitter into the base. In active mode, the electric field existing between the emitter and the collector (caused by VCE) causes the majority of these holes to cross the lower P-N junction into the collector to form the collector current IC. The remainder of the holes recombine with electrons, the majority carriers in the base, making a current through the base connection to form the base current, IB. As shown in the diagram, the emitter current, IE, is the total transistor current, which is the sum of the other terminal currents (i.e., ). In the diagram, the arrows representing current point in the direction of conventional current – the flow of holes is in the same direction of the arrows because holes carry positive electric charge. In active mode, the ratio of the collector current to the base current is called theDC current gain. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value. The value of this gain for DC signals is referred to as hFE, and the value of this gain for AC signals is referred to as hfe. However, when there is no particular frequency range of interest, the symbol β is used. It should also be noted that the emitter current is related to VEB exponentially. At room temperature, an increase in VEB by approximately 60 mV increases the emitter current by a factor of 10. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way. TRANSISTOR CONFIGURATIONS: Transistor circuits may be classified into three configurations based on which terminal is common to both the input and the output of the circuit. These configurations are: 1) the common-emitter configuration; 2) the common-base configuration; and 3) the commoncollector configuration. The common-emitter (CE) transistor configuration is shown in Figure 1. In this configuration, the transistor terminal common to both the input and the output of the circuit is the emitter. The common-emitter configuration, which is also known as the 'grounded-emitter' configuration, is the most widely used among the three configurations. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY configurations. Figure 1. Common-Emitter Transistor Configuration The input current and output voltage of the common-emitter configuration, which are the base current Ib and the collector-emitter voltage Vce, respectively, are often considered as the independent variables in this circuit. Its dependent variables, on the other hand, are the base-emitter voltage Vbe (which is the input voltage) and the collector current Ic (which is the output current). A plot of the output current Ic against the collector-emitter voltage Vce for different values of Ib may be drawn for easier analysis of a transistor's input/output characteristics, as shown in this Diagram of Vce-Ic Curves. The common-base (CB) transistor configuration, which is also known as the 'grounded base' configuration, is shown in Figure 2. In this configuration, the terminal common to both the input and the output of the circuit is the base. Figure 2. Common-Base Transistor Configuration The input current and output voltage of the common-base configuration, which are the emitter current Ie and the collector-base voltage Vcb, respectively, are often considered as the independent variables in this circuit. Its dependent variables, on the other hand, are the emitter-base voltage Veb (which is the input voltage) and the collector current Ic (which is the output current). A plot of the output current Ic against the collector-base voltage Vcb for different values of Ie may be drawn for easier analysis of a transistor's input/output characteristics, as shown in this Diagram of Vcb-Ic Curves. The common-collector (CC) transistor configuration is shown in Figure 3. In this configuration, the collector is common to both the input and the output of the circuit. This is basically the same as the common-emitter configuration, except that the load is in the Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY emitter instead of the collector. Just like in the common-emitter circuit, the current flowing through the load when the transistor is reverse-biased is zero, with the collector current being very small and equal to the base current. As the base current is increased, the transistor slowly gets out of cut-off, goes into the active region, and eventually becomes saturated. Once saturated, the voltage across the load becomes maximum, while the voltage Vce across the collector and emitter of the transistor goes down to a very low value, i.e., as low as a few tens of millivolts for germanium and 0.2 V for silicon transistors. Figure 3. Common-Collector Transistor Configuration Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY UNIT 4 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY BIASING: Bipolar transistor amplifiers must be properly biased to operate correctly. In circuits made with individual devices (discrete circuits), biasing networks consisting of resistors are commonly employed. Much more elaborate biasing arrangements are used in integrated circuits, for example, bandgap voltage references and current mirrors. The operating point of a device, also known as bias point, quiescent point, or Q-point, is the point on the output characteristics that shows the DC collector–emitter voltage (Vce) and the collector current (Ic) with no input signal applied. The term is normally used in connection with devices such as transistors. Bias circuit requirements Signal requirements for Class A amplifiers For analog circuit operation, the Q-point is placed so the transistor stays in active mode (does not shift to operation in the saturation region or cut-off region) when input is applied. For digital operation, the Q-point is placed so the transistor does the contrary - switches from "on" to "off" state. Often, Q-point is established near the center of active region of transistor characteristic to allow similar signal swings in positive and negative directions. Q-point should be stable. In particular, it should be insensitive to variations in transistor parameters (for example, should not shift if transistor is replaced by another of the same type), variations in temperature, variations in power supply voltage and so forth. The circuit must be practical: easily implemented and costeffective. Thermal considerations At constant current, the voltage across the emitter–base junction VBE of a bipolar transistor decreases 2 mV (silicon) and 1.8mV (germanium) for each 1°C rise in temperature (reference being 25°C). By the Ebers–Moll model, if the base–emitter voltage VBE is held constant and the temperature rises, the current through the base–emitter diode IB will increase, and thus the collector current IC will also increase. Depending on the bias point, the power dissipated in the transistor may also increase, which will further increase its temperature and exacerbate the problem. This deleterious positive feedback results in thermal runaway.[1] There are several approaches to mitigate bipolar transistor thermal runaway. For example, Negative feedback can be built into the biasing circuit so that increased collector current leads to decreased base current. Hence, the increasing collector current throttles its source. Heat sinks can be used that carry away extra heat and prevent the base–emitter temperature from rising. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The transistor can be biased so that its collector is normally less than half of the power supply voltage, which implies that collector–emitter power dissipation is at its maximum value. Runaway is then impossible because increasing collector current leads to a decrease in dissipated power; this notion is known as the half-voltage principle. The circuits below primarily demonstrate the use of negative feedback to prevent thermal runaway. Types of bias circuit for Class A amplifiers The following discussion treats five common biasing circuits used with Class A bipolar transistor amplifiers: 1. Fixed bias 2. Collector-to-base bias 3. Fixed bias with emitter resistor 4. Voltage divider bias 5. Emitter bias Fixed bias (base bias) Fixed bias (Base bias): This form of biasing is also called base bias. In the example image on the right, the single power source (for example, a battery) is used for both collector and base of transistor, although separate batteries can also be used. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY In the given circuit, Vcc = IBRB + Vbe Therefore, IB = (Vcc - Vbe)/RB For a given transistor, Vbe does not vary significantly during use. As Vce is of fixed value, on selection of RB, the base current IB is fixed. Therefore this type is called fixed bias type of circuit. Also for given circuit, Vcc = ICRC + Vce Therefore, Vce = Vcc - ICRC The common-emitter current gain of a transistor is an important parameter in circuit design, and is specified on the data sheet for a particular transistor. It is denoted as β on this page. Because IC = βIB we can obtain IC as well. In this manner, operating point given as (Vce,IC) can be set for given transistor. Merits: It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (RB). A very small number of components are required. Demerits: The collector current does not remain constant with variation in temperature or power supply voltage. Therefore the operating point is unstable. Changes in Vbe will change IB and thus cause RE to change. This in turn will alter the gain of the stage. When the transistor is replaced with another one, considerable change in the value of β can be expected. Due to this change the operating point will shift. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY For small-signal transistors (e.g., not power transistors) with relatively high values of β (i.e., between 100 and 200), this configuration will be prone to thermal runaway. In particular, thestability factor, which is a measure of the change in collector current with changes in reverse saturation current, is approximately β+1. To ensure absolute stability of the amplifier, a stability factor of less than 25 is preferred, and so small-signal transistors have large stability factors.[citation needed] Usage: Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits (i.e., those circuits which use the transistor as a current source). Instead, it is often used in circuits where transistor is used as a switch. However, one application of fixed bias is to achieve crude automatic gain control in the transistor by feeding the base resistor from a DC signal derived from the AC output of a later stage. Collector-to-base bias: Collector-to-base bias This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point. In this form of biasing, the base resistor RB is connected to the collector instead of connecting it to the DC source Vcc. So any thermal runaway will induce a voltage drop across the RC resistor that will throttle the transistor's base current. From Kirchhoff's voltage law, the voltage Faculty Name: VINAY CHOWDARY across the base resistor Rb is Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY By the Ebers–Moll model, Ic = βIb, and so From Ohm's law, the base current , and so Hence, the base current Ib is If Vbe is held constant and temperature increases, then the collector current Ic increases. However, a larger Ic causes the voltage drop across resistor Rc to increase, which in turn reduces the voltage across the base resistor Rb. A lower base-resistor voltage drop reduces the base current Ib, which results in less collector current Ic. Because an increase in collector current with temperature is opposed, the operating point is kept stable. Merits: Circuit stabilizes the operating point against variations in temperature and β (ie. replacement of transistor) Demerits: In this circuit, to keep Ic independent of β, the following condition must be met: which is the case when As β-value is fixed (and generally unknown) for a given transistor, this relation can be satisfied either by keeping Rc fairly large or making Rb very low. If Rc is large, a high Vcc is necessary, which increases cost as well as precautions necessary while handling. If Rb is low, the reverse bias of the collector–base region is small, which limits the range of collector voltage swing that leaves the transistor in active mode. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The resistor Rb causes an AC feedback, reducing the voltage gain of the amplifier. This undesirable effect is a trade-off for greater Q-point stability. Usage: The feedback also decreases the input impedance of the amplifier as seen from the base, which can be advantageous. Due to the gain reduction from feedback, this biasing form is used only when the trade-off for stability is warranted. Fixed bias with emitter resistor: Fixed bias with emitter resistor The fixed bias circuit is modified by attaching an external resistor to the emitter. This resistor introduces negative feedback that stabilizes the Q-point. From Kirchhoff's voltage law, the voltage across the base resistor is VRb = VCC - IeRe - Vbe. From Ohm's law, the base current is Ib = VRb / Rb. The way feedback controls the bias point is as follows. If Vbe is held constant and temperature increases, emitter current increases. However, a larger Ie increases the emitter voltage Ve = IeRe, which in turn reduces the voltage VRb across the base resistor. A lower base-resistor voltage drop reduces the base current, which results in less collector current because Ic = ß IB. Collector current and emitter current are related by Ic = α Ie with α ≈ 1, so increase in emitter current with temperature is opposed, and operating point is kept stable. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Similarly, if the transistor is replaced by another, there may be a change in IC (corresponding to change in β-value, for example). By similar process as above, the change is negated and operating point kept stable. For the given circuit, IB = (VCC - Vbe)/(RB + (β+1)RE). Merits: The circuit has the tendency to stabilize operating point against changes in temperature and βvalue. Demerits: In this circuit, to keep IC independent of β the following condition must be met: which is approximately the case if ( β + 1 )RE >> RB. As β-value is fixed for a given transistor, this relation can be satisfied either by keeping R E very large, or making RB very low. If RE is of large value, high VCC is necessary. This increases cost as well as precautions necessary while handling. If RB is low, a separate low voltage supply should be used in the base circuit. Using two supplies of different voltages is impractical. In addition to the above, RE causes ac feedback which reduces the voltage gain of the amplifier. Usage: The feedback also increases the input impedance of the amplifier when seen from the base, which can be advantageous. Due to the above disadvantages, this type of biasing circuit is used only with careful consideration of the trade-offs involved. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Voltage divider bias: Voltage divider bias The voltage divider is formed using external resistors R1 and R2. The voltage across R2 forward biases the emitter junction. By proper selection of resistors R1 and R2, the operating point of the transistor can be made independent of β. In this circuit, the voltage divider holds the base voltage fixed independent of base current provided the divider current is large compared to the base current. However, even with a fixed base voltage, collector current varies with temperature (for example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with emitter resistor. In this circuit the base voltage is given by: voltage across provided . Also For the given circuit, Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Merits: Unlike above circuits, only one dc supply is necessary. Operating point is almost independent of β variation. Operating point stabilized against shift in temperature. Demerits: In this circuit, to keep IC independent of β the following condition must be met: which is approximately the case if where R1 || R2 denotes the equivalent resistance of R1 and R2 connected in parallel. As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE fairly large, or making R1||R2 very low. If RE is of large value, high VCC is necessary. This increases cost as well as precautions necessary while handling. If R1 || R2 is low, either R1 is low, or R2 is low, or both are low. A low R1 raises VB closer to VC, reducing the available swing in collector voltage, and limiting how large RC can be made without driving the transistor out of active mode. A low R2 lowers Vbe, reducing the allowed collector current. Lowering both resistor values draws more current from the power supply and lowers the input resistance of the amplifier as seen from the base. AC as well as DC feedback is caused by RE, which reduces the AC voltage gain of the amplifier. A method to avoid AC feedback while retaining DC feedback is discussed below. Usage: The circuit's stability and merits as above make it widely used for linear circuits. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Voltage divider with AC bypass capacitor: Voltage divider with capacitor The standard voltage divider circuit discussed above faces a drawback - AC feedback caused by resistor RE reduces the gain. This can be avoided by placing a capacitor (C E) in parallel with RE, as shown in circuit diagram. This capacitor is usually chosen to have a low enough reactance at the signal frequencies of interest such that RE is essentially shorted at AC, thus grounding the emitter. Feedback is therefore only present at DC to stabilize the operating point, in which case any AC advantages of feedback are lost. Of course, this idea can be used to shunt only a portion of RE, thereby retaining some AC feedback. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Emitter bias: Emitter bias When a split supply (dual power supply) is available, this biasing circuit is the most effective, and provides zero bias voltage at the emitter or collector for load. The negative supply VEE is used to forward-bias the emitter junction through RE. The positive supply VCC is used to reversebias the collector junction. Only two resistors are necessary for the common collector stage and four resistors for the common emitter or common base stage. We know that, VB - VE = Vbe If RB is small enough, base voltage will be approximately zero. Therefore emitter current is, IE = (VEE - Vbe)/RE The operating point is independent of β if RE >> RB/β Merit: Good stability of operating point similar to voltage divider bias. Demerit: This type can only be used when a split (dual) power supply is available. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY PROBLEMS 1.Find the stability factor for the circuit given below: Solution: Given circuit is of self and its stability factor is given by: Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY 4. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY UNIT 5 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY INTRODUCTION: H – Parameter model :- → The equivalent circuit of a transistor can be dram using simple approximation by retaining its essential features. → These equivalent circuits will aid in analyzing transistor circuits easily and rapidly. Two port devices & Network Parameters:- → A transistor can be treated as a two part network. The terminal behaviour of any two part network can be specified by the terminal voltages V1 & V2 at parts 1 & 2 respectively and current i1 and i2, entering parts 1 & 2, respectively, as shown in figure. Two port network → Of these four variables V1, V2, i1 and i2, two can be selected as independent variables and the remaining two can be expressed in terms of these independent variables. This leads to various two part parameters out of which the following three are more important. 1. Z – Parameters (or) Impedance parameters 2. Y – Parameters (or) Admittance parameters 3. H – Parameters (or) Hybrid parameters. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Hybrid parameters (or) h – parameters:- → If the input current i1 and output Voltage V2 are takes as independent variables, the input voltage V1 and output current i2 can be written as V1 = h11 i1 + h12 V2 i2 = h21 i1 + h22 V2 The four hybrid parameters h11, h12, h21 and h22 are defined as follows. h11 = [V1 / i1] with V2 = 0 = Input Impedance with output part short circuited. h22 = [i2 / V2] with i1 = 0 = Output admittance with input part open circuited. h12 = [V1 / V2] with i1 = 0 = reverse voltage transfer ratio with input part open circuited. h21 = [i2 / i1] with V2 = 0 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY = Forward current gain with output part short circuited. The dimensions of h – parameters are as follows: h11 - Ω h22 – mhos h12, h21 – dimension less. → as the dimensions are not alike, (ie) they are hybrid in nature, and these parameters are called as hybrid parameters. I = 11 = input ; 0 = 22 = output ; F = 21 = forward transfer ; r = 12 = Reverse transfer. Notations used in transistor circuits:- hie = h11e = Short circuit input impedance h0e = h22e = Open circuit output admittance hre = h12e = Open circuit reverse voltage transfer ratio hfe = h21e = Short circuit forward current Gain. The Hybrid Model for Two-port Network:- V1 = h11 i1 + h12 V2 I2 = h1 i1 + h22 V2 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY ↓ V1 = h1 i1 + hr V2 I2 = hf i1 + h0 V2 The Hybrid Model for Two-port Network Transistor Hybrid model:- Use of h – parameters to describe a transistor have the following advantages. 1. h – parameters are real numbers up to radio frequencies . 2. They are easy to measure 3. They can be determined from the transistor static characteristics curves. 4. They are convenient to use in circuit analysis and design. 5. Easily convert able from one configuration to other. 6.Readily supplied by manufactories. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY CE Transistor Circuit To Derive the Hybrid model for transistor consider the CE circuit shown in figure.The variables are iB, ic, vB(=vBE) and vc(=vCE). iB and vc are considered as independent variables. Then , vB= f1(iB, vc ) ----------------------(1) iC= f2(iB, vc ) ----------------------(2) Making a Taylor’s series expansion around the quiescent point IB, VC and neglecting higher order terms, the following two equations are obtained. ΔvB = (∂f1/∂iB)Vc . Δ iB + (∂f1/∂vc)IB . ΔvC ---------------(3) Δ iC = (∂f2/∂iB)Vc . Δ iB + (∂f2/∂vc)IB . ΔvC ----------------(4) The partial derivatives are taken keeping the collector voltage or base current constant as indicated by the subscript attached to the derivative. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY ΔvB , ΔvC , Δ iC , Δ iB represent the small signal(increment) base and collector voltages and currents,they are represented by symbols vb , vc , ib and ic respectively. Eqs (3) and (4) may be written as Vb = hie ib + hre Vc ic = hfe ib + hoe Vc Where hie =(∂f1/∂iB)Vc = (∂vB/∂iB)Vc = (ΔvB /ΔiB)Vc = (vb / ib)Vc hre =(∂f1/∂vc)IB = (∂vB/∂vc) IB = (ΔvB /Δvc) IB = (vb /vc) IB hfe =(∂f2/∂iB)Vc = (∂ic /∂iB)Vc = (Δ ic /ΔiB)Vc = (ic / ib)Vc hoe= (∂f2/∂vc)IB = (∂ic /∂vc) IB = (Δ ic /Δvc) IB = (ic /vc) IB The above equations define the h-parameters of the transistor in CE configuration.The same theory can be extended to transistors in other configurations. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Hybrid Model and Equations for the transistor in three different configurations are are given below. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Unit 6 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY INTRODUCTION: The field-effect transistor (FET) relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with the dual-carrier-type operation of bipolar (junction) transistors (BJT). The concept of the FET predates the BJT, though it was not physically implemented until after BJTs due to the limitations of semiconductor materials and the relative ease of manufacturing BJTs compared to FETs at the time. History: The principle of field-effect transistors was first patented by Julius Edgar Lilienfeld in 1925 and by Oskar Heil in 1934, but practical semi-conducting devices (the JFET, junction gate fieldeffect transistor) were only developed much later after the transistor effect was observed and explained by the team of William Shockley at Bell Labs in 1947. The MOSFET (metal–oxide– semiconductor field-effect transistor), which largely superseded the JFET and had a more profound effect on electronic development, was first proposed by Dawon Kahng in 1960.[1] [edit]Terminals Cross section of an n-type MOSFET All FETs have a gate, drain, and source terminal that correspond roughly to the base, collector, and emitter of BJTs. Aside from the JFET, all FETs also have a fourth terminal called the body, base, bulk, or substrate. This fourth terminal serves to bias the transistor into operation; it is rare to make non-trivial use of the body terminal in circuit designs, but its presence is important when setting up the physical layout of anintegrated circuit. The size of the gate, length L in the diagram, is the distance between source and drain. The width is the extension of the transistor, in the diagram perpendicular to the cross section. Typically the width is much Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY larger than the length of the gate. A gate length of 1µm limits the upper frequency to about 5 GHz, 0.2µm to about 30 GHz. The names of the terminals refer to their functions. The gate terminal may be thought of as controlling the opening and closing of a physical gate. This gate permits electrons to flow through or blocks their passage by creating or eliminating a channel between the source and drain. Electrons flow from the source terminal towards the drain terminal if influenced by an applied voltage. The body simply refers to the bulk of the semiconductor in which the gate, source and drain lie. Usually the body terminal is connected to the highest or lowest voltage within the circuit, depending on type. The body terminal and the source terminal are sometimes connected together since the source is also sometimes connected to the highest or lowest voltage within the circuit, however there are several uses of FETs which do not have such a configuration, such as transmission gates and cascode circuits. FET operation: I–V characteristics and output plot of a JFET n-channel transistor. The FET controls the flow of electrons (or electron holes) from the source to drain by affecting the size and shape of a "conductive channel" created and influenced by voltage (or lack of voltage) applied across the gate and source terminals (For ease of discussion, this assumes body and source are connected). This conductive channel is the "stream" through which electrons flow from source to drain. In an n-channel depletion-mode device, a negative gate-to-source voltage causes a depletion region to expand in width and encroach on the channel from the sides, narrowing the channel. If the depletion region expands to completely close the channel, the resistance of the channel from source to drain becomes large, and the FET is effectively turned off like a switch. Likewise a positive gate-to-source voltage increases the channel size and allows electrons to flow easily. Conversely, in an n-channel enhancement-mode device, a positive gate-to-source voltage is necessary to create a conductive channel, since one does not exist naturally within the transistor. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel. But first, enough electrons must be attracted near the gate to counter the dopant ions added to the body of the FET; this forms a region free of mobile carriers called a depletion region, and the phenomenon is referred to as the threshold voltage of the FET. Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to create a conductive channel from source to drain; this process is called inversion. For either enhancement- or depletion-mode devices, at drain-to-source voltages much less than gate-to-source voltages, changing the gate voltage will alter the channel resistance, and drain current will be proportional to drain voltage (referenced to source voltage). In this mode the FET operates like a variable resistor and the FET is said to be operating in a linear modeor ohmic mode.[2][3] If drain-to-source voltage is increased, this creates a significant asymmetrical change in the shape of the channel due to a gradient of voltage potential from source to drain. The shape of the inversion region becomes "pinched-off" near the drain end of the channel. If drain-to-source voltage is increased further, the pinch-off point of the channel begins to move away from the drain towards the source. The FET is said to be in saturation mode;[4] some authors refer to it as active mode, for a better analogy with bipolar transistor operating regions.[5][6] The saturation mode, or the region between ohmic and saturation, is used when amplification is needed. The inbetween region is sometimes considered to be part of the ohmic or linear region, even where drain current is not approximately linear with drain voltage. Even though the conductive channel formed by gate-to-source voltage no longer connects source to drain during saturation mode, carriers are not blocked from flowing. Considering again an nchannel device, a depletion region exists in the p-type body, surrounding the conductive channel and drain and source regions. The electrons which comprise the channel are free to move out of the channel through the depletion region if attracted to the drain by drain-to-source voltage. The depletion region is free of carriers and has a resistance similar to silicon. Any increase of the drain-to-source voltage will increase the distance from drain to the pinch-off point, increasing resistance due to the depletion region proportionally to the applied drain-to-source voltage. This proportional change causes the drain-to-source current to remain relatively fixed independent of changes to the drain-to-source voltage and quite unlike the linear mode operation. Thus in saturation mode, the FET behaves as a constant-current source rather than as a resistor and can be used most effectively as a voltage amplifier. In this case, the gate-to-source voltage determines the level of constant current through the channel. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Composition: The FET can be constructed from a number of semiconductors, silicon being by far the most common. Most FETs are made with conventional bulk semiconductor processing techniques, using the single crystal semiconductor wafer as the active region, or channel. Among the more unusual body materials are amorphous silicon, polycrystalline silicon or other amorphous semiconductors in thin-film transistors or organic field effect transistors that are based on organic semiconductors and often apply organic gate insulators and electrodes. Types of field-effect transistors: The channel of a FET is doped to produce either an N-type semiconductor or a P-type semiconductor. The drain and source may be doped of opposite type to the channel, in the case of depletion mode FETs, or doped of similar type to the channel as in enhancement mode FETs. Field-effect transistors are also distinguished by the method of insulation between channel and gate. Types of FETs are: CNFET The DEPFET is a FET formed in a fully-depleted substrate and acts as a sensor, amplifier and memory node at the same time. It can be used as an image (photon) sensor. The DGMOSFET is a MOSFET with dual gates. The DNAFET is a specialized FET that acts as a biosensor, by using a gate made of singlestrand DNA molecules to detect matching DNA strands. The FREDFET (Fast Reverse or Fast Recovery Epitaxial Diode FET) is a specialized FET designed to provide a very fast recovery (turn-off) of the body diode. The HEMT (High Electron Mobility Transistor), also called an HFET (heterostructure FET), can be made using bandgapengineering in a ternary semiconductor such as AlGaAs. The fully depleted wide-band-gap material forms the isolation between gate and body. The IGBT (Insulated-Gate Bipolar Transistor) is a device for power control. It has a structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These are commonly used for the 200-3000 V drain-to-source voltage range of operation. Power MOSFETs are still the device of choice for drain-to-source voltages of 1 to 200 V. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The ISFET is an Ion-Sensitive Field Effect Transistor used to measure ion concentrations in a solution; when the ion concentration (such as H+, see pH electrode) changes, the current through the transistor will change accordingly. The JFET (Junction Field-Effect Transistor) uses a reverse biased p-n junction to separate the gate from the body. The MESFET (Metal–Semiconductor Field-Effect Transistor) substitutes the p-n junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials. The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure formed by graded doping of the active region. The MOSFET (Metal–Oxide–Semiconductor Field-Effect an insulator (typically SiO2) between the gate and the body. The NOMFET is a Nanoparticle Organic Memory Field-Effect Transistor.[1] The OFET is an Organic Field-Effect Transistor using an organic semiconductor in its channel. Transistor) utilizes Uses: IGBTs see application in switching internal combustion engine ignition coils, where fast switching and voltage blocking capabilities are important. The most commonly used FET is the MOSFET. The CMOS (complementary-symmetry metal oxide semiconductor) process technology is the basis for modern digital integrated circuits. This process technology uses an arrangement where the (usually "enhancement-mode") pchannel MOSFET and n-channel MOSFET are connected in series such that when one is on, the other is off. The fragile insulating layer of the MOSFET between the gate and channel makes it vulnerable to electrostatic damage during handling. This is not usually a problem after the device has been installed in a properly designed circuit. In FETs electrons can flow in either direction through the channel when operated in the linear mode, and the naming convention of drain terminal and source terminal is somewhat arbitrary, as the devices are typically (but not always) built symmetrically from source to drain. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Structure: Circuit symbolfor an n-Channel JFET Circuit symbol for a p-Channel JFET The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers (p-type), or of negative carriers (n-type). Contacts at each end form the source(S) and drain(D). The gate(G) (control) terminal has doping opposite to that of the channel, which surrounds it, so that there is a P-N junction at the interface. Terminals to connect with the outside are usually made ohmic. Function: JFET operation is like that of a garden hose. The flow of water through a hose can be controlled by squeezing it to reduce the cross section; the flow of electric charge through a JFET is controlled by constricting the current-carrying channel. The current depends also on the electric field between source and drain (analogous to the difference in pressure on either end of the hose). Schematic symbols: The JFET gate is sometimes drawn in the middle of the channel (instead of at the drain or source electrode as in these examples). This symmetry suggests that "drain" and "source" are interchangeable, so the symbol should be used only for those JFETs where they are indeed interchangeable (which is not true of all JFETs). Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Officially, the style of the symbol should show the component inside a circle (representing the envelope of a discrete device). This is true in both the US and Europe. The symbol is usually drawn without the circle when drawing schematics of integrated circuits. More recently, the symbol is often drawn without its circle even for discrete devices. In every case the arrow head shows the polarity of the P-N junction formed between the channel and gate. As with an ordinary diode, the arrow points from P to N, the direction of conventional current when forward-biased. An English mnemonic is that the arrow of an N-channel device "points in". To pinch off the channel, it needs a certain reverse bias (VGS) of the junction. This "pinch-off voltage" varies considerably, even among devices of the same type. For example, V GS(off) for the Temic J201 device varies from -0.8V to -4V.[1] Typical values vary from -0.3V to -10V. To switch off an n-channel device requires a negative gate-source voltage (VGS). Conversely, to switch off a p-channel device requires VGS positive. In normal operation, the electric field developed by the gate must block conduction between the source and the drain. Comparison with other transistors: JFET gate current (the reverse leakage of the gate-to-channel junction) is comparable to that of a MOSFET (which has insulating oxide between gate and channel), but much less than the base current of a bipolar junction transistor. The JFET has highertransconductance than the MOSFET and is therefore used in some low-noise, high input-impedance op-amps. Pinch off voltage: The current in N-JFET due to a small voltage VDS is given by: where Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY 2a = channel thickness W = width L = length q = electronic charge = 1.6 x 10-19 C μn = electron mobility Nd = n type doping concentration In the saturation region: In the linear region or (in terms of IDSS): METAL OXIDE FIELD EFFECT TRANSISTOR: The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a device used for amplifying or switching electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925. In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain. The channel can be of n-type or p-type (see article on semiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common. The 'metal' in the name is now often a misnomer because the previously metal gate material is now often a layer of polysilicon (polycrystalline silicon). Aluminium had been the gate material until the mid 1970s, when polysilicon became dominant, due to its capability to form self-aligned gates. Metallic gates are regaining popularity, since it is difficult to increase the speed of operation of transistors without metal gates. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY IGFET is a related term meaning insulated-gate field-effect transistor, and is almost synonymous with MOSFET, though it can refer to FETs with a gate insulator that is not oxide. Another synonym is MISFET for metal–insulator–semiconductor FET. Composition: Photomicrograph of two metal-gate MOSFETs in a test pattern. Probe pads for two gates and three source/drain nodes are labeled. Usually the semiconductor of choice is silicon, but some chip manufacturers, most notably IBMand intel, recently started using achemical compound of silicon and germanium (SiGe) in MOSFET channels. Unfortunately, many semiconductors with better electrical properties than silicon, such as gallium arsenide, do not form good semiconductor-to-insulator interfaces, thus are not suitable for MOSFETs. Research continues on creating insulators with acceptable electrical characteristics on other semiconductor material. In order to overcome power consumption increase due to gate current leakage, high-κ dielectric replaces silicon dioxide for the gate insulator, while metal gates return by replacing polysilicon (see Intel announcement[1]). The gate is separated from the channel by a thin insulating layer, traditionally of silicon dioxide and later of silicon oxynitride. Some companies have started to introduce a high-κ dielectric + metal gate combination in the 45 nanometer node. When a voltage is applied between the gate and body terminals, the electric field generated penetrates through the oxide and creates an "inversion layer" or "channel" at the semiconductorinsulator interface. The inversion channel is of the same type, P-type or N-type, as the source and drain, thus it provides a channel through which current can pass. Varying the voltage between Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY the gate and body modulates the conductivity of this layer and allows to control the current flow between drain and source. Circuit symbols: A variety of symbols are used for the MOSFET. The basic design is generally a line for the channel with the source and drain leaving it at right angles and then bending back at right angles into the same direction as the channel. Sometimes three line segments are used for enhancement mode and a solid line for depletion mode. Another line is drawn parallel to the channel for the gate. The bulk connection, if shown, is shown connected to the back of the channel with an arrow indicating PMOS or NMOS. Arrows always point from P to N, so an NMOS (N-channel in Pwell or P-substrate) has the arrow pointing in (from the bulk to the channel). If the bulk is connected to the source (as is generally the case with discrete devices) it is sometimes angled to meet up with the source leaving the transistor. If the bulk is not shown (as is often the case in IC design as they are generally common bulk) an inversion symbol is sometimes used to indicate PMOS, alternatively an arrow on the source may be used in the same way as for bipolar transistors (out for nMOS, in for pMOS). Comparison of enhancement-mode and depletion-mode MOSFET symbols, along with JFET symbols (drawn with source and drain ordered such that higher voltages appear higher on the page than lower voltages): P-channel N-channel JFET MOSFET enh MOSFET enh (no bulk) MOSFET dep For the symbols in which the bulk, or body, terminal is shown, it is here shown internally connected to the source. This is a typical configuration, but by no means the only important configuration. In general, the MOSFET is a four-terminal device, and in integrated circuits many Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY of the MOSFETs share a body connection, not necessarily connected to the source terminals of all the transistors. MOSFET operation: Example application of an N-Channel MOSFET. When the switch is pushed the LED lights up. Metal–oxide–semiconductor structure on P-type silicon Metal–oxide–semiconductor structure: A traditional metal–oxide–semiconductor (MOS) structure is obtained by growing a layer of silicon dioxide (SiO2) on top of a silicon substrate and depositing a layer of metal or polycrystalline silicon (the latter is commonly used). As the silicon dioxide is a dielectric material, its structure is equivalent to a planar capacitor, with one of the electrodes replaced by a semiconductor. When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor. If we consider a P-type semiconductor (with NA the density of acceptors, p the density of holes; p = NA in neutral bulk), a positive voltage, VGB, from gate to body (see figure) creates a depletion layer by forcing the positively charged holes away from the gateinsulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see doping (semiconductor)). If VGB is high enough, a high concentration Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY of negative charge carriers forms in an inversion layer located in a thin layer next to the interface between the semiconductor and the insulator. Unlike the MOSFET, where the inversion layer electrons are supplied rapidly from the source/drain electrodes, in the MOS capacitor they are produced much more slowly by thermal generation through carrier generation and recombination centers in the depletion region. Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the threshold voltage. This structure with p-type body is the basis of the N-type MOSFET, which requires the addition of an N-type source and drain regions MOSFET structure and channel formation Cross section of an NMOS without channel formed: OFF state Cross section of an NMOS with channel formed: ON state Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY A metal–oxide–semiconductor field-effect transistor (MOSFET) is based on the modulation of charge concentration by a MOS capacitance between a body electrode and a gate electrode located above the body and insulated from all other device regions by a gate dielectric layer which in the case of a MOSFET is an oxide, such as silicon dioxide. If dielectrics other than an oxide such as silicon dioxide (often referred to as oxide) are employed the device may be referred to as a metal–insulator–semiconductor FET (MISFET). Compared to the MOS capacitor, the MOSFET includes two additional terminals (source and drain), each connected to individual highly doped regions that are separated by the body region. These regions can be either p or n type, but they must both be of the same type, and of opposite type to the body region. The source and drain (unlike the body) are highly doped as signified by a '+' sign after the type of doping. If the MOSFET is an n-channel or nMOS FET, then the source and drain are 'n+' regions and the body is a 'p' region. As described above, with sufficient gate voltage, holes from the body are driven away from the gate, forming an inversion layer or n-channel at the interface between the p region and the oxide. This conducting channel extends between the source and the drain, and current is conducted through it when a voltage is applied between source and drain. For gate voltages below the threshold value, the channel is lightly populated, and only a very small subthreshold leakage current can flow between the source and the drain. If the MOSFET is a p-channel or pMOS FET, then the source and drain are 'p+' regions and the body is a 'n' region. When a negative gate-source voltage (positive source-gate) is applied, it creates a p-channel at the surface of the n region, analogous to the n-channel case, but with opposite polarities of charges and voltages. When a voltage less negative than the threshold value (a negative voltage for p-channel) is applied between gate and source, the channel disappears and only a very small subthreshold current can flow between the source and the drain. The source is so named because it is the source of the charge carriers (electrons for n-channel, holes for p-channel) that flow through the channel; similarly, the drain is where the charge carriers leave the channel. The device may comprise a Silicon On Insulator (SOI) device in which a Buried OXide (BOX) is formed below a thin semiconductor layer. If the channel region between the gate dielectric and a Buried Oxide (BOX) region is very thin, the very thin channel region is referred to as an Ultra Thin Channel (UTC) region with the source and drain regions formed on either side thereof in and/or above the thin semiconductor layer. Alternatively, the device may comprise a SEMiconductor On Insulator (SEMOI) device in which semiconductors other than silicon are employed. Many alternative semiconductor materials may be employed. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY When the source and drain regions are formed above the channel in whole or in part, they are referred to as Raised Source/Drain (RSD) regions. Modes of operation: The operation of a MOSFET can be separated into three different modes, depending on the voltages at the terminals. In the following discussion, a simplified algebraic model is used that is accurate only for old technology. Modern MOSFET characteristics require computer models that have rather more complex behavior. For an enhancement-mode, n-channel MOSFET, the three operational modes are: Cutoff, subthreshold, or weak-inversion mode When VGS < Vth: where Vth is the threshold voltage of the device. According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and source. In reality, the Boltzmann distribution of electron energies allows some of the more energetic electrons at the source to enter the channel and flow to the drain, resulting in a subthreshold current that is an exponential function of gate–source voltage. While the current between drain and source should ideally be zero when the transistor is being used as a turned-off switch, there is a weak-inversion current, sometimes called subthreshold leakage. In weak inversion the current varies exponentially with gate-to-source bias VGS as given approximately by:[3][4] , where ID0 = current at VGS = Vth and the slope factor n is given by n = 1 + CD / COX, with CD = capacitance of the depletion layer and COX = capacitance of the oxide layer. In a long-channel device, there is no drain voltage dependence of the current onceVDS > > VT, but as channel length is reduced drain-induced barrier lowering introduces drain voltage dependence that depends in a complex way upon the device geometry (for example, the channel doping, the junction doping and so on). Frequently, threshold voltage Vth for this mode is defined as the gate voltage at which a selected value of current ID0occurs, for example, ID0 = 1 μA, which may not be the same Vth-value used in the equations for the following modes. Some micropower analog circuits are designed to take advantage of subthreshold conduction.[5][6][7] By working in the weak-inversion region, the MOSFETs in these Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY circuits deliver the highest possible transconductance-to-current ratio, namely: gm / ID = 1 / (nVT), almost that of a bipolar transistor.[8] The subthreshold I–V curve depends exponentially upon threshold voltage, introducing a strong dependence on any manufacturing variation that affects threshold voltage; for example: variations in oxide thickness, junction depth, or body doping that change the degree of drain-induced barrier lowering. The resulting sensitivity to fabricational variations complicates optimization for leakage and performance.[9][10] MOSFET drain current vs. drain-to-source voltage for several values of VGS − Vth; the boundary between linear(Ohmic) and saturation (active) modes is indicated by the upward curving parabola. Cross section of a MOSFET operating in the linear (Ohmic) region; strong inversion region present even near drain Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Cross section of a MOSFET operating in the saturation (active) region; channel exhibits pinchoff near drain Triode mode or linear region (also known as the ohmic mode When VGS > Vth and VDS < ( VGS – Vth ) The transistor is turned on, and a channel has been created which allows current to flow between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as: where μn is the charge-carrier effective mobility, W is the gate width, L is the gate length and Cox is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest. Saturation or active mode[13][14] When VGS > Vth and VDS > ( VGS – Vth ) The switch is turned on, and a channel has been created, which allows current to flow between the drain and source. Since the drain voltage is higher than the gate voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate. The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate–source voltage, and modeled very approximately as: The additional factor involving λ, the channel-length modulation parameter, models current dependence on drain voltage due to the Early effect, or channel length modulation. According to this equation, a key design parameter, the MOSFET transconductance is: , where the combination Vov = VGS – Vth is called the overdrive voltage. Another key design parameter is the MOSFET output resistance rout given by: Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY . rout is the inverse of gDS where region. . VDS is the expression in saturation If λ is taken as zero, an infinite output resistance of the device results that leads to unrealistic circuit predictions, particularly in analog circuits. As the channel length becomes very short, these equations become quite inaccurate. New physical effects arise. For example, carrier transport in the active mode may become limited by velocity saturation. When velocity saturation dominates, the saturation drain current is more nearly linear than quadratic in VGS. At even shorter lengths, carriers transport with near zero scattering, known as quasi-ballistic transport. In addition, the output current is affected by drain-induced barrier lowering of the threshold voltage. [edit]Body effect Ohmic contact to body to ensure no body bias; top left:subthreshold, top right:Ohmic mode, bottom left:Active mode at onset of pinch-off, bottom right: Active mode well into pinch-off – channel length modulation evident The body effect describes the changes in the threshold voltage by the change in the source-bulk voltage, approximated by the following equation: , where VTN is the threshold voltage with substrate bias present, and VTO is the zero-VSB value of threshold voltage, γ is the body effect parameter, and 2φ is the surface potential parameter. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The body can be operated as a second gate, and is sometimes referred to as the "back gate"; the body effect is sometimes called the "back-gate effect". The primacy of MOSFETs: In 1959, Dawon Kahng and Martin M. (John) Atalla at Bell Labs invented the metal–oxide– semiconductor field-effect transistor (MOSFET).[17] Operationally and structurally different from the bipolar junction transistor,[18] the MOSFET was made by putting an insulating layer on the surface of the semiconductor and then placing a metallic gate electrode on that. It used crystalline silicon for the semiconductor and a thermally oxidized layer of silicon dioxide for the insulator. The silicon MOSFET did not generate localized electron traps at the interface between the silicon and its native oxide layer, and thus was inherently free from the trapping and scattering of carriers that had impeded the performance of earlier field-effect transistors. Following the (expensive) development of clean rooms to reduce contamination to levels never before thought necessary, and of photolithography[19] and the planar process to allow circuits to be made in very few steps, the Si–SiO2 system possessed such technical attractions as low cost of production (on a per circuit basis) and ease of integration. Largely because of these two factors, the MOSFET has become the most widely used type of transistor in integrated circuits. Advantages of BJT over MOSFET: BJTs have some advantages over MOSFETs for at least two digital applications. Firstly, in high speed switching, they do not have the "larger" capacitance from the gate, which when multiplied by the resistance of the channel gives the intrinsic time constant of the process. The intrinsic time constant places a limit on the speed a MOSFET can operate at because higher frequency signals are filtered out. Widening the channel reduces the resistance of the channel, but increases the capacitance by exactly the same amount. Reducing the width of the channel increases the resistance, but reduces the capacitance by the same amount. R*C=Tc1, 0.5R*2C=Tc1, 2R*0.5C=Tc1. There is no way to minimize the intrinsic time constant for a certain process. Different processes using different channel lengths, channel heights, gate thicknesses and materials will have different intrinsic time constants. This problem is mostly avoided with a BJT because it does not have a gate. The second application where BJTs have an advantage over MOSFETs stems from the first. When driving many other gates, called fanout, the resistance of the MOSFET is in series with the gate capacitances of the other FETs, creating a secondary time constant. Delay circuits use this fact to create a fixed signal delay by using a small CMOS device to send a signal to many other, many times larger CMOS devices. The secondary time constant can be minimized by increasing the driving FET's channel width to decrease its resistance and decreasing the channel widths of Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY the FETs being driven, decreasing their capacitance. The drawback is that it increases the capacitance of the driving FET and increases the resistance of the FETs being driven, but usually these drawbacks are a minimal problem when compared to the timing problem. BJTs are better able to drive the other gates because they can output more current than MOSFETs, allowing for the FETs being driven to charge faster. Many chips use MOSFET inputs and BiCMOS outputs (see above) Depletion type MOSFET: The channel is of silicon. It can be a p-type or n-type channel; it is still mostly silicon. Next, we take note that silicon dioxide is simply glass, which is a good insulator. So we can form a thin layer of silicon dioxide along one surface of the channel, and then lay our metal gate region down over the glass. The result is shown to the left. This device is sometimes known as an insulated-gate field effect transistor, or IGFET. More commonly, noting the construction of the gate, it is called a metal-oxidesemiconductor FET, or MOSFET. With no voltage applied to the gate (G) electrode, the channel really is just a semiconductor resistance, and will conduct current according to the voltage applied between source (S) and drain (D). There is no pn junction, so there is no depletion region. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY With an appropriate voltage applied between source and drain, current will flow through the channel, as a semiconductor resistance. However, if we now apply a negative voltage to the gate, as shown to the right, it will amount to a small negative static charge on the gate. This negative voltage will repel electrons, with their negative charge, away from the gate. But free electrons are the majority current carriers in the n-type silicon channel. By repelling them away from the gate region, the applied gate voltage creates a depletion region around the gate area, thus restricting the usable width of the channel just as the pn junction did. Because this type of FET operates by creating a depletion region within an existing channel, it is called a depletion-mode MOSFET. The mechanical structure of this device is shown to the right. In an IC, we would place two n-type regions side by side within a p-type area and then place the gate between the n-type regions. However, the important region still consists of the two n-type regions and the ptype area between them. This is the portion we have depicted to the right. With no applied bias, we have what amounts to an npn transistor with no base connection. The two n-type regions are isolated from each other, and are electrically separate. Even with a voltage applied between the two n-type regions, there is no channel present and no current flow. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY While we still apply the usual positive voltage to the drain with respect to the source, this time we will also apply a positive voltage to the gate region. This has the effect of attracting free electrons towards the gate. The larger the positive gate voltage, the wider its electric field and the more free electrons it will attract. You might not think this would have any effect on the p-type region, where the majority current carriers are holes. However, there are some free electrons here as well. In addition, the source junction is forward biased, so the positive gate voltage can attract electrons across this junction towards the gate. The net result is that the electrons attracted towards the gate actually enhance a channel within the p-type region, as shown to the left. This is a channel formed of free electrons, and actually bridges the gap between source and drain. Now we have a channel, which can conduct current from source to drain through the device. Enhancement type MOSFET: In these devices operate by having a channel enhanced in the semiconductor material where no channel was constructed, they are known as enhancement-mode MOSFETs. It is just as easy to construct p-channel versions of these devices as n-channel versions. Indeed CMOS logic ICs consist of nothing but these devices, constructed and used in pairs such that one will be turned off while the other is turned on. This is the source of the designation CMOS: Complementary MOS. Enhancement-mode MOSFETs have the same advantages and disadvantages as their depletion-mode cousins. However, when they are constructed as part of an IC rather than as individual devices, they are not readily subject to random static charges. Such ICs are constructed with input protection circuitry for any MOSFET input that must be made accessible to external circuitry. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Unit 7 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY INTRODUCTION: Generally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The relationship of the input to the output of an amplifier—usually expressed as a function of the input frequency—is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain. In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to make the human voice louder or play recorded music. Amplifiers may be classified according to the input (source) they are designed to amplify (such as a guitar amplifier, to perform with an electric guitar), the device they are intended to drive (such as a headphone amplifier), the frequency range of the signals (Audio, IF, RF, and VHF amplifiers, for example), whether they invert the signal (inverting amplifiers and non-inverting amplifiers), or the type of device used in the amplification (valve or tube amplifiers, FET amplifiers, etc.). A related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or asensor. COMMON SOURCE AMPLIFIER: In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as avoltage or transconductance amplifier. The easiest way to tell if a FET is common source, common drain, or common gate is to examine where the signal enters and leaves. The remaining terminal is what is known as "common". In this example, the signal enters the gate, and exits the drain. The only terminal remaining is the source. This is a common-source FET circuit. The analogous bipolar junction transistor circuit is the common-emitter amplifier. The common-source (CS) amplifier may be viewed as a transconductance amplifier or as a voltage amplifier. (See classification of amplifiers). As a transconductance amplifier, the input voltage is seen as modulating the current going to the load. As a voltage amplifier, input voltage modulates the amount of current flowing through the FET, changing the voltage across the output resistance according to Ohm's law. However, the FET device's output resistance typically is not high enough for a reasonable transconductance amplifier (ideally infinite), nor low enough for a decent voltage amplifier (ideally zero). Another major drawback is the amplifier's limited high-frequency response. Therefore, in practice the output often is routed through either a voltage follower (common-drain or CD stage), or a current follower (common-gate or CG stage), Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY to obtain more favorable output and frequency characteristics. The CS–CG combination is called a cascode amplifier. Characteristics: At low frequencies and using signal characteristics can be derived. a Definition simplified hybrid-pi model, the following small- Expression Current gain Voltage gain Input impedance Output impedance Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY COMMON DRAIN AMPLIFIER: In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field effect transistor (FET) amplifier topologies, typically used as avoltage buffer. In this circuit the gate terminal of the transistor serves as the input, the source is the output, and the drain is common to both (input and output), hence its name. The analogous bipolar junction transistor circuit is the common-collector amplifier. In addition, this circuit is used to transform impedances. For example, the Thévenin resistance of a combination of a voltage follower driven by a voltage source with high Thévenin resistance is reduced to only the output resistance of the voltage follower, a small resistance. That resistance reduction makes the combination a more ideal voltage source. Conversely, a voltage follower inserted between a driving stage and a high load (ie a low resistance) presents an infinite resistance (low load) to the driving stage, an advantage in coupling a voltage signal to a large load. Characteristics: Basic N-channel JFET source follower circuit (neglecting biasingdetails). At low frequencies, the source follower pictured at right has the following small signal characteristics.[1] Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Voltage gain: Current gain: Input impedance: Output impedance: (the parallel notation indicates the impedance of components A and B that are connected in parallel) The variable gm that is not listed in Figure 1 is the transconductance of the device (usually given in units of siemens). [ Figure 3: Basic N-channel MOSFET common-source amplifier with active load ID. Figure 4: Small-signal circuit for N-channel MOSFET common-source amplifier. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY FET parameters: Figure 2: Simplified, low-frequency hybrid-piMOSFET model. A basic, low-frequency hybrid-pi model for the MOSFET is shown in figure 2. The various parameters are as follows. is the transconductance in siemens, evaluated in the Shichman-Hodges model in terms of the Qpoint drain current ID by (see Jaeger and Blalock[3]): , where: ID is the quiescent drain current (also called the drain bias or DC drain current) Vth = threshold voltage and VGS = gate-to-source voltage. The combination: often is called the overdrive voltage. is the output resistance due to channel length modulation, calculated using the Shichman-Hodges model as , using the approximation for the channel length modulation parameter λ[4] . Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Here VE is a technology-related parameter (about 4 V/μm for the 65 nm technology node[4]) and L is the length of the source-to-drain separation. The reciprocal of the output resistance is named the drain conductance . Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY UNIT 8 Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY SILICON CONTROLLED RECTIFIER: silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state device that controls current. The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957. Construction of SCR: An SCR consists of four layers of alternating P and N type semiconductor materials. Silicon is used as the intrinsic semiconductor, to which the proper dopants are added. The junctions are either diffused or alloyed. The planar construction is used for low power SCRs (and all the junctions are diffused). The mesa type construction is used for high power SCRs. In this case, junction J2 is obtained by the diffusion method and then the outer two layers are alloyed to it, since the PNPN pellet is required to handle large currents. It is properly braced with tungsten or molybdenum plates to provide greater mechanical strength. One of these plates is hard soldered to a copper stud, which is threaded for attachment of heat sink. The doping of PNPN will depend on the application of SCR, since its characteristics are similar to those of the thyraton. Today, the term thyristor applies to the larger family of multilayer devices that exhibit bistable state-change behaviour, that is, switching either ON or OFF. Modes of operation: In the normal "off" state, the device restricts current to the leakage current. When the gate-tocathode voltage exceeds a certain threshold, the device turns "on" and conducts current. The device will remain in the "on" state even after gate current is removed so long as current through the device remains above the holding current. Once current falls below the holding current for an appropriate period of time, the device will switch "off". If the gate is pulsed and the current through the device is below the holding current, the device will remain in the "off" state. If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge into the gate to trigger the device into the "on" state; this is referred to as "dv/dt triggering." This is usually prevented by limiting the rate of voltage rise across the device, perhaps by using a snubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly and the partially-triggered SCR may dissipate more power than is usual, possibly harming the device. SCRs can also be triggered by increasing the forward voltage beyond their rated breakdown voltage (also called as break over voltage), but again, this does not rapidly switch the entire device into conduction and so may be harmful so this mode of operation is also usually avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown voltage, so the exact trigger point will vary from device to device. This device is generally used in switching applications. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Reverse Bias: SCR are available with or without reverse blocking capability. Reverse blocking capability adds to the forward voltage drop because of the need to have a long, low doped P1 region. Usually, the reverse blocking voltage rating and forward blocking voltage rating are the same. The typical application for reverse blocking SCR is in current source inverters. SCR incapable of blocking reverse voltage are known as asymmetrical SCR, abbreviated ASCR. They typically have a reverse breakdown rating in the 10's of volts. ASCR are used where either a reverse conducting diode is applied in parallel (for example, in voltage source inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC traction choppers). Asymmetrical SCR can be fabricated with a reverse conducting diode in the same package. These are known as RCT, for reverse conducting thyristor. Application of SCRs: SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation makes them suitable for use in medium to high-voltage AC power control applications, such as lamp dimming, regulators and motor control. UNI JUNCTION TRANSISTOR: A unijunction transistor (UJT) is an electronic semiconductor device that has only one junction. The UJT has three terminals: an emitter (E) and two bases (B1 and B2). The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type and it is heavily doped. The resistance between B1 and B2, when the emitter is open-circuit is called interbase resistance. There are two types of unijunction transistor: The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length, defining the device parameter η. The 2N2646 is the most commonly used version of the UJT. The programmable unijunction transistor, or PUT, is a close cousin to the thyristor. Like the thyristor it consists of four P-N layers and has an anode and a cathode connected to the first and the last layer, and a gate connected to one of the inner layers. They are not directly interchangeable with conventional UJTs but perform a similar function. In a proper circuit configuration with two "programming" resistors for setting the parameter η, they behave like a conventional UJT. The 2N6027 is an example of such a device. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY The UJT is biased with a positive voltage between the two bases. This causes a potential drop along the length of the device. When the emitter voltage is driven approximately one diode voltage above the voltage at the point where the P diffusion (emitter) is, current will begin to flow from the emitter into the base region. Because the base region is very lightly doped, the additional current (actually charges in the base region) causes conductivity modulation which reduces the resistance of the portion of the base between the emitter junction and the B2 terminal. This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected. Overall, the effect is a negative resistance at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuits. Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1970s and early 1980s because they allowed simple oscillators to be built using just one active device. Later, as integrated circuits became more popular, oscillators such as the 555 timer IC became more commonly used. In addition to its use as the active device in relaxation oscillators, one of the most important applications of UJTs or PUTs is to trigger thyristors (SCR, TRIAC, etc.). In fact, a DC voltage can be used to control a UJT or PUT circuit such that the "on-period" increases with an increase in the DC control voltage. This application is important for large AC current control. Varactor diode: Varicap schematic symbol In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Applications: Varactors are used as voltage-controlled capacitors, rather than as rectifiers. They are commonly used in parametric amplifiers, parametric oscillators and voltage-controlled oscillators as part of phase-locked loops and frequency synthesizers. Operation Internal structure of a varicap Operation of a varicap Varactors are operated reverse-biased so no current flows, but since the thickness of the depletion zone varies with the applied bias voltage, the capacitance of the diode can be made to vary. Generally, the depletion region thickness is proportional to the square root of the applied Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY voltage; and capacitance is inversely proportional to the depletion region thickness. Thus, the capacitance is inversely proportional to the square root of applied voltage. All diodes exhibit this phenomenon to some degree, but specially made varactor diodes exploit the effect to boost the capacitance and variability range achieved - most diode fabrication attempts to achieve the opposite. In the figure we can see an example of a crossection of a varactor with the depletion layer formed of a p-n-junction. But the depletion layer can also be made of a MOS-diode or a Schottky diode. This is very important in CMOS and MMIC technology. Tunnel diode: Tunnel diode schematic symbol 1N3716 tunnel diode Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwavefrequency region, by using quantum mechanical effects. It was invented in August 1957 by Leo Esaki when he was with Tokyo Tsushin Kogyo, now known as Sony. In 1973 he received the Nobel Prize in Physics, jointly with Brian Josephson, for discovering the electron tunneling effect used in these diodes. Robert Noyceindependently came up with the idea of a tunnel diode while working for William Shockley, but was discouraged from pursuing it. These diodes have a heavily doped p–n junction only some 10 nm (100 Å) wide. The heavy doping results in a broken bandgap, whereconduction band electron states on the n-side are more or less aligned with valence band hole states on the p-side. Tunnel diodes were manufactured by Sony for the first time in 1957 followed by General Electric and other companies from about 1960, and are still made in low volume today, Tunnel diodes are usually made from germanium, but can also be made in gallium arsenide andsilicon materials. They can be used as oscillators, amplifiers, frequency converters and detectors Forward bias operation: Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow p–n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the p-n junction. As voltage increases further these states become more misaligned and the current drops – this is called negative resistance because current decreases with increasing voltage. As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the p–n junction, and no longer by tunneling through the p–n junction barrier. Thus the most important operating region for a tunnel diode is the negative resistance region. Reverse bias operation: When used in the reverse direction they are called back diodes and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction). Under reverse bias filled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the pn junction barrier in reverse direction – this is the Zener effect that also occurs in zener diodes. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Technical comparisons: A rough approximation of the VI curve for a tunnel diode, showing the negative differential resistance region In a conventional semiconductor diode, conduction takes place while the p–n junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the “reverse breakdown voltage” when conduction begins (often accompanied by destruction of the device). In the tunnel diode, the dopant concentration in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction. However, when forward-biased, an odd effect occurs called “quantum mechanical tunnelling” which gives rise to a region where an increase in forward voltage is accompanied by a decrease in forward current. This negative resistance region can be exploited in a solid state version of the dynatron oscillator which normally uses a tetrode thermionic valve (or tube). The tunnel diode showed great promise as an oscillator and high-frequency threshold (trigger) device since it would operate at frequencies far greater than the tetrode would, well into the microwave bands. Applications for tunnel diodes included local oscillators for UHF television tuners, trigger circuits in oscilloscopes, high speed counter circuits, and very fast-rise time pulse generator circuits. The tunnel diode can also be used as low-noise microwave amplifier.[5] However, since its discovery, more conventional semiconductor devices have Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY surpassed its performance using conventional oscillator techniques. For many purposes, a threeterminal device, such as a field-effect transistor, is more flexible than a device with only two terminals. Practical tunnel diodes operate at a few millamperes and a few tenths of a volt, making them low-power devices. The Gunn diode has similar high frequency capability and can handle more power. Tunnel diodes are also relatively resistant to nuclear radiation, as compared to other diodes. This makes them well suited to higher radiation environments, such as those found in space applications. Longevity: Esaki diodes are notable for their longevity; devices made in the 1960s still function. Writing in Nature, Esaki and coauthors state that semiconductor devices in general are extremely stable, and suggest that their shelf life should be "infinite" if kept at room temperature. They go on to report that a small-scale test of 50-year-old devices revealed a "gratifying confirmation of the diode's longevity". Schottky barrier diode: Schottky barrier, named after Walter H. Schottky, is a potential barrier formed at a metal– semiconductor junction which has rectifying characteristics, suitable for use as a diode. The largest differences between a Schottky barrier and a p–n junction are its typically lower junction voltage, and decreased (almost nonexistent) depletion width in the metal. Not all metal–semiconductor junctions form Schottky barriers. A metal–semiconductor junction that does not rectify current is called an ohmic contact. Rectifying properties depend on the metal's work function, the band gap of the intrinsic semiconductor, the type and concentration of dopants in the semiconductor, and other factors. Design of semiconductor devices requires familiarity with the Schottky effect to ensure Schottky barriers are not created accidentally where an ohmic connection is desired. Advantages: Schottky barriers, with their lower junction voltage, find application where a device better approximating an ideal diode is desired. They are also used in conjunction with normal diodes and transistors, where their lower junction voltage is used for circuit protection (among other things). Because one of the materials in a Schottky diode is a metal, lower resistance devices are often possible. In addition, the fact that only one type of dopant is needed may greatly Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY simplifyfabrication. And because of their majority carrier conduction mechanism, Schottky diodes can achieve greater switching speeds than p-n junction diodes, making them appropriate to rectify high frequency signals. Devices: A metal–semiconductor junction that forms a Schottky barrier as a device by itself is known as a Schottky diode. A bipolar junction transistor with a Schottky barrier between the base and the collector is known as a Schottky transistor. Because the junction voltage of the Schottky barrier is small, the transistor is prevented from saturating too deeply, which improves the speed when used as a switch. This is the basis for the Schottky and Advanced Schottky TTL families, as well as their low power variants. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY A MESFET, or Metal–Semiconductor FET, is a device similar in operation to the JFET, which utilizes a reverse biased Schottky barrier to provide the depletion region. A particularly interesting variant of this device is the HEMT, or High Electron Mobility Transistor, which also utilizes a heterojunction to provide a device with extremely high conductance. Schottky barriers are commonly used also in semiconductor electrical characterization techniques. In fact, in the semiconductor, a depletion region is created by the metal electrons, which "push" away semiconductor electrons (simplification, see depletion region article). In the depletion region, dopants remain ionized and give rise to a "space charge" which, in turn, give rise to a capacitance of the junction. The metal-semiconductor interface and the opposite boundary of the depleted area act like two capacitor plates, with the depletion region acting as adielectric. By applying a voltage to the junction it is possible to vary the depletion width: if we reverse bias the junction, the dopants electrons will be emitted and pushed away; if we forward bias the junction, the electrons will be captured. By analyzing the emission and capture of electrons by dopants (or, more frequently, by crystallographic defects or dislocations, or other electron traps) is possible to characterize the semiconductor material. The most popular electrical characterization techniques that use this type of junction are DLTS and CV profiling. A Schottky barrier carbon nanotube FET uses the nonideal contact between a metal and a carbon nanotube (CNT) to form a Schottky barrier that can be used to make Schottky diodes or transistors, or so on. The scaling of semiconductor devices to ever-smaller sizes is rapidly approaching fundamental limits. Carbon nanotubes may become a practical alternative to customary devices due to their small size and unique mechanical and electronic properties. Photodiode: Symbol for photodiode. A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction Unijunction transistor: Unijunction transistors Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY Circuit symbol: A unijunction transistor (UJT) is an electronic semiconductor device that has only one junction. The UJT has three terminals: an emitter (E) and two bases (B1 and B2). The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type and it is heavily doped. The resistance between B1 and B2, when the emitter is open-circuit is called interbase resistance. There are two types of unijunction transistor: The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length, defining the device parameter η. The 2N2646 is the most commonly used version of the UJT. The programmable unijunction transistor, or PUT, is a close cousin to the thyristor. Like the thyristor it consists of four P-N layers and has an anode and a cathode connected to the first and the last layer, and a gate connected to one of the inner layers. They are not directly interchangeable with conventional UJTs but perform a similar function. In a proper circuit configuration with two "programming" resistors for setting the parameter η, they behave like a conventional UJT. The 2N6027 is an example of such a device. The UJT is biased with a positive voltage between the two bases. This causes a potential drop along the length of the device. When the emitter voltage is driven approximately one diode voltage above the voltage at the point where the P diffusion (emitter) is, current will begin to flow from the emitter into the base region. Because the base region is very lightly doped, the additional current (actually charges in the base region) causes conductivity modulation which reduces the resistance of the portion of the base between the emitter junction and the B2 terminal. This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected. Overall, the effect is anegative resistance at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuits. Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1970s and early 1980s because they allowed simple oscillators to be built using just one active device. Later, as integrated circuits became more popular, oscillators such as the 555 timer IC became more commonly used. In addition to its use as the active device in relaxation oscillators, one of the most important applications of UJTs or PUTs is to trigger thyristors (SCR,TRIAC, etc.). In fact, a DC voltage Faculty Name: VINAY CHOWDARY Page No: LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY can be used to control a UJT or PUT circuit such that the "on-period" increases with an increase in the DC control voltage. This application is important for large AC current control. Faculty Name: VINAY CHOWDARY Page No: