LORDS INSTITUTE OF ENGINEERING&TECHNOLOGY
UNIT 1
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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.
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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
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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
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(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.
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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
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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.
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(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
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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.
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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.
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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.
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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:
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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.
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UNIT 2
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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:
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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,
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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
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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
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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.
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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
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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
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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.
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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
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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
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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.
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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.
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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
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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.
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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.
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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:
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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
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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.
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
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).
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UNIT 3
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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.
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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
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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.
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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.
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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.
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
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
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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).
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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.
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symbols
and
for
NPN-type
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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.
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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.
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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:
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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
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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.
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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:
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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
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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
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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
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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.
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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
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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
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UNIT 4
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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.
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
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.
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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.
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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
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across the base resistor Rb is
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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.
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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.
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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.
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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,
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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.
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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.
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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.
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PROBLEMS
1.Find the stability factor for the circuit given below:
Solution:
Given circuit is of self and its stability factor is given by:
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4.
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UNIT 5
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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.
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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
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= 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
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↓
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.
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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.
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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.
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Hybrid Model and Equations for the transistor in three different configurations are are
given below.
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Unit 6
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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
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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.
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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.
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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.
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
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
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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).
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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
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
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.
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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
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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
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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
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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
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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.
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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
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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
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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:
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.
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.
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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
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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.
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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.
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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.
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Unit 7
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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),
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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
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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]
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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.
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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]
.
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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
.
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UNIT 8
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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.
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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.
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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.
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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
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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
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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.
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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
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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
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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.
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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.
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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
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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
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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.
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