TON DUC THANG UNIVERSITY FACULTY OF ELECTRICAL AND ELECTRONICS ENGINEERING 402057 ELECTRONIC MATERIALS Nam T. Tran, MSc. 1 CHAPTER 4: SEMICONDUCTOR DEVICES 4.1. Semiconductor diodes n 4.2. Bipolar junction transistors n 4.3. Field effect transistors n 20/12/2015 402057 – Chapter 4: Semiconductor devices 2 OBJECTIVES Understand the structure as well as the operation principle of diode, transistor, FET… n Identify types of transistor and how to use them in electronic circuit. n 20/12/2015 402057 – Chapter 4: Semiconductor devices 3 CHAPTER 4: SEMICONDUCTOR DEVICES 4.1. Semiconductor diodes n 4.2. Bipolar junction transistors n 4.3. Field effect transistors n 20/12/2015 402057 – Chapter 4: Semiconductor devices 4 SEMICONDUCTOR DIODES Diode: the simplest and most fundamental circuit element. n An ideal diode would pass an infinite current in one direction and no current in the other direction n 20/12/2015 402057 – Chapter 4: Semiconductor devices 5 SEMICONDUCTOR DIODES n Physical structure of a diode A Al A p n The pn region is assumed to be thin (step or abrupt junction) 20/12/2015 B One-dimensional representation B diode symbol Different concentrations of electrons (and holes) of the p and n-type regions cause a concentration gradient at the boundary 402057 – Chapter 4: Semiconductor devices 6 SEMICONDUCTOR DIODES Procedure of forming pn Junction: the dynamic equilibrium of drift and diffusion movements for carriers through the junction. n 4 steps: Ø Diffusion Ø Space charge region Ø Drift Ø Equilibrium n 20/12/2015 402057 – Chapter 4: Semiconductor devices 7 SEMICONDUCTOR DIODES Space charge region (SCR) Ø Majority carriers recombining with minority carriers results in the disappearance of majority carriers Ø Electric field is established across SCR. Ø The Junction Built-In Voltage n N AND Vo = VT ln 2 ni 20/12/2015 402057 – Chapter 4: Semiconductor devices 8 SEMICONDUCTOR DIODES n Ø Ø Ø Diode characteristics The Forward-Bias Region, v > o The Reverse-Bias Region, − VZK < v < 0 The Breakdown Region, v < −VZK 20/12/2015 402057 – Chapter 4: Semiconductor devices 9 SEMICONDUCTOR DIODES n Diode characteristics 20/12/2015 402057 – Chapter 4: Semiconductor devices 10 SEMICONDUCTOR DIODES n Depletion region width ⎡ 2ε (N a + N d )(Vo − V )⎤ W = ⎢ ⎥ eN a N d ⎣ ⎦ 1/ 2 where, for forward bias, V is positive, which reduces Vo, and, for reverse bias, V is negative , so Vo is increased. n Depletion layer capacitance Cdep dQ Cdep = dV where the amount of charge on any one side of the depletion layer is |Q| = eNdWnA = eNaWpA and W = Wn + Wp 20/12/2015 402057 – Chapter 4: Semiconductor devices 11 SEMICONDUCTOR DIODES n Forward bias Ø hole diffusion electron diffusion p n hole drift electron drift + Ø Ø 20/12/2015 Applied potential lowers the potential barrier, Idiffusion > I drift Mobile carriers drift through the dep. region into neutral regions become excess minority carriers and diffuse towards terminals 402057 – Chapter 4: Semiconductor devices 12 SEMICONDUCTOR DIODES Forward bias Ø Turn-on voltage A conduction diode has approximately a constant voltage drop across it. It’s called turn-on voltage. n V D ( on ) = 0.7V For silicon V D ( on ) = 0.25V For germanium 0.3 Ø Diodes with different current rating will exhibit the turn-on voltage at different currents. 20/12/2015 402057 – Chapter 4: Semiconductor devices 13 SEMICONDUCTOR DIODES n Reverse bias Ø hole diffusion electron diffusion p n hole drift electron drift - 20/12/2015 Ø + Ø Applied potential increases the potential barrier Diffusion current is reduced Diode works in the reverse bias with a very small drift current 402057 – Chapter 4: Semiconductor devices 14 SEMICONDUCTOR DIODES Breakdown region Ø Whereas reverse breakdown is a highly undesirable effect in circuits that use conventional diodes Ø It can be extremely useful Ø When a diode is undergoing reverse breakdown and provided its maximum ratings are not exceeded the voltage appearing across it will remain substantially constant regardless of the current flowing. Ø This property makes the zener diode ideal for use as a voltage regulator n 20/12/2015 402057 – Chapter 4: Semiconductor devices 15 SEMICONDUCTOR DIODES n Breakdown mechanisms Ø Zener effect Occurs in heavily doping semiconductor ü Breakdown voltage is less than 5v. ü Carriers generated by electric field---field ionization. ü Ø Avalanche effect. Occurs in slightly doping semiconductor ü Breakdown voltage is more than 7v. ü Carriers generated by collision. ü 20/12/2015 402057 – Chapter 4: Semiconductor devices 16 SEMICONDUCTOR DIODES n Ø Zener diode Zener diodes are heavily doped silicon diodes which, unlike normal diodes, exhibit an abrupt reverse breakdown at relatively low voltages Circuit symbol 20/12/2015 402057 – Chapter 4: Semiconductor devices 17 SEMICONDUCTOR DIODES n Diode application Ø Rectifier circuits Half-wave rectifier Ø Full-wave rectifier Ø Transformer with a center-tapped secondary winding n Bridge rectifier n Ø The peak rectifier Voltage regulator Ø Limiter Ø Light emitting diode (LED) Ø 20/12/2015 402057 – Chapter 4: Semiconductor devices 18 SEMICONDUCTOR DIODES n Half-Wave Rectifier (a) Half-wave rectifier. (b) Equivalent circuit of the half-wave rectifier with the diode replaced with its battery-plus-resistance model. 20/12/2015 402057 – Chapter 4: Semiconductor devices 19 SEMICONDUCTOR DIODES n Half-Wave Rectifier (cont.) (c) Transfer characteristic of the rectifier circuit. (d) Input and output waveforms, assuming that 20/12/2015 402057 – Chapter 4: Semiconductor devices 20 SEMICONDUCTOR DIODES n Full-Wave Rectifier (a) circuit (b) transfer characteristic assuming a constant-voltage-drop model for the diodes 20/12/2015 402057 – Chapter 4: Semiconductor devices 21 SEMICONDUCTOR DIODES n Full-Wave Rectifier (cont.) (c) input and output waveforms. 20/12/2015 402057 – Chapter 4: Semiconductor devices 22 SEMICONDUCTOR DIODES n Full-Wave Rectifier (cont.) The Bridge Rectifier 20/12/2015 402057 – Chapter 4: Semiconductor devices 23 SEMICONDUCTOR DIODES n Voltage regulator Ø Voltage and current waveforms in the peak rectifier circuit with CR >> T Ø The diode is assumed ideal. 20/12/2015 402057 – Chapter 4: Semiconductor devices 24 SEMICONDUCTOR DIODES n Limiter Ø Applying a sine wave to a limiter can result in clipping off its two peaks. 20/12/2015 402057 – Chapter 4: Semiconductor devices 25 SEMICONDUCTOR DIODES n Light emitting diode (LED) LED is a pn junction diode typically made from a direct bandgap semiconductor in which the electron-hole pair (EHP) recombination results in the emission of a photon. Ø External efficiency of an LED quantifies the eficiency of conversion of electric energy into an emitted external optical energy Ø 20/12/2015 402057 – Chapter 4: Semiconductor devices 26 SEMICONDUCTOR DIODES n Seven-segment LED displays 20/12/2015 402057 – Chapter 4: Semiconductor devices 27 SEMICONDUCTOR DIODES n Variable capacitance diode Ø The capacitance of a reverse-biased diode junction will depend on the width of the depletion layer which, in turn, varies with the reverse voltage applied to the diode. Ø This allows a diode to be used as a voltage controlled Capacitor. Ø Diodes that are specially manufactured to make use of this effect (varicaps) 20/12/2015 402057 – Chapter 4: Semiconductor devices 28 CHAPTER 4: SEMICONDUCTOR DEVICES 4.1. Semiconductor diodes n 4.2. Bipolar junction transistors n 4.3. Field effect transistors n 20/12/2015 402057 – Chapter 4: Semiconductor devices 29 BIPOLAR JUNCTION TRANSISTORS Transistor is short for transfer resistor, a term which provides something of a clue as to how the device operates; Ø The current flowing in the output circuit is determined by the current flowing in the input circuit. Ø Since transistors are three-terminal devices, one electrode must remain common to both the input and the output. Ø 20/12/2015 402057 – Chapter 4: Semiconductor devices 30 BIPOLAR JUNCTION TRANSISTORS Transistors fall into two main categories: bipolar junction transistors (BJT) and field effect transistors (FET) and are also classified according to the semiconductor material employed and to their field of application. Ø Various classes of transistor are available according to the application concerned Ø 20/12/2015 402057 – Chapter 4: Semiconductor devices 31 BIPOLAR JUNCTION TRANSISTORS n BJT structure 20/12/2015 402057 – Chapter 4: Semiconductor devices 32 BIPOLAR JUNCTION TRANSISTORS Notation Ø Base is located at the middle and more thin from the level of collector and emitter. Ø The emitter and collector terminals are made of the same material while the base of the other type of material. n 20/12/2015 402057 – Chapter 4: Semiconductor devices 33 BIPOLAR JUNCTION TRANSISTORS n Transistor currents IC=the collector current IB= the base current IE= the emitter current 20/12/2015 ü The arrow is always drawn on the emitter ü The arrow always point toward the n-type ü The arrow indicates the direction of the emitter current: pnp: Eà B npn: Bà E 402057 – Chapter 4: Semiconductor devices 34 BIPOLAR JUNCTION TRANSISTORS n BJT operation (for pnp transistor) Ø One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased. Forward-biased junction of a pnp transistor 20/12/2015 Reverse-biased junction of a pnp transistor 402057 – Chapter 4: Semiconductor devices 35 BIPOLAR JUNCTION TRANSISTORS n BJT operation (for pnp transistor) ü Both biasing potentials have been applied to a pnp transistor and resulting majority and minority carrier flows indicated. ü Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material. ü A very small number of carriers (+) will through n-type material to the base terminal. Resulting IB is typically in order of microamperes. 20/12/2015 402057 – Chapter 4: Semiconductor devices 36 BIPOLAR JUNCTION TRANSISTORS n BJT operation (for pnp transistor) Ø The large number of majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal. Ø Majority carriers can cross the reverse-biased junction because the injected majority carriers will appear as minority carriers in the n-type material. Ø Applying KCL to the transistor : IE = IC + IB Ø The comprises of two components – the majority and minority carriers IC = ICmajority + ICOminority Ø ICO – IC current with emitter terminal open and is called leakage current. 20/12/2015 402057 – Chapter 4: Semiconductor devices 37 BIPOLAR JUNCTION TRANSISTORS n BJT configurations Common terminal to both input and output of the configuration. (usually the terminal closest to or at ground potential). 20/12/2015 402057 – Chapter 4: Semiconductor devices 38 BIPOLAR JUNCTION TRANSISTORS n BJT – Common base circuit 20/12/2015 402057 – Chapter 4: Semiconductor devices 39 BIPOLAR JUNCTION TRANSISTORS n BJT – Common base circuit Active region Saturation region Cut-off region 20/12/2015 402057 – Chapter 4: Semiconductor devices 40 BIPOLAR JUNCTION TRANSISTORS n BJT – Common base circuit 20/12/2015 402057 – Chapter 4: Semiconductor devices 41 BIPOLAR JUNCTION TRANSISTORS n BJT – Common emitter circuit 20/12/2015 402057 – Chapter 4: Semiconductor devices 42 BIPOLAR JUNCTION TRANSISTORS n BJT – Common emitter circuit n n n n n Input characteristics for a common-emitter NPN transistor 20/12/2015 IB is microamperes compared to miliamperes of IC. IB will flow when VBE > 0.7V for silicon and 0.3V for germanium Before this value IB is very small and no IB. Base-emitter junction is forward bias Increasing VCE will reduce IB for different values. 402057 – Chapter 4: Semiconductor devices 43 BIPOLAR JUNCTION TRANSISTORS n BJT – Common emitter circuit n n n n n Output characteristics 20/12/2015 For small VCE (VCE <VCESAT) IC increase linearly with increasing of VCE VCE > VCESAT IC not totally depends on VCE à constant IC IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC IB=0 A à ICEO occur. Noticing the value when IC=0A. There is still some value of current flows. 402057 – Chapter 4: Semiconductor devices 44 BIPOLAR JUNCTION TRANSISTORS n BJT – Common emitter circuit Amplification factor 𝛽= 𝐼↓𝐶 /𝐼↓𝐵 20/12/2015 402057 – Chapter 4: Semiconductor devices 45 BIPOLAR JUNCTION TRANSISTORS n BJT – Common emitter circuit Example: From output characteristics of common emitter configuration, find βac and βdc with an Operating point at IB=25 µA and VCE =7.5V. 20/12/2015 402057 – Chapter 4: Semiconductor devices 46 BIPOLAR JUNCTION TRANSISTORS n BJT – Common emitter circuit Solution 20/12/2015 402057 – Chapter 4: Semiconductor devices 47 BIPOLAR JUNCTION TRANSISTORS n BJT – Common collector circuit 20/12/2015 402057 – Chapter 4: Semiconductor devices 48 BIPOLAR JUNCTION TRANSISTORS n BJT – Common collector circuit 20/12/2015 402057 – Chapter 4: Semiconductor devices 49 BIPOLAR JUNCTION TRANSISTORS n BJT – Small signal equivalent model 20/12/2015 402057 – Chapter 4: Semiconductor devices 50 BIPOLAR JUNCTION TRANSISTORS n BJT – Small signal equivalent model 20/12/2015 402057 – Chapter 4: Semiconductor devices 51 BIPOLAR JUNCTION TRANSISTORS n European system of transistor coding Ø First letter – semiconductor material: A Germanium B Silicon Ø Second letter – application: C Low-power, low-frequency D High-power, low-frequency F Low-power, high-frequency L High-power, high-frequency Ø Third letter – in the case of transistors for specialized applications, the third letter does not generally have any particular significance 20/12/2015 402057 – Chapter 4: Semiconductor devices 52 CHAPTER 4: SEMICONDUCTOR DEVICES 4.1. Semiconductor diodes n 4.2. Bipolar junction transistors n 4.3. Field effect transistors n 20/12/2015 402057 – Chapter 4: Semiconductor devices 53 FIELD EFFECT TRANSISTORS Ø Field effect transistors (FET) comprise a channel of Ptype or N-type material surrounded by material of the opposite polarity. Ø The ends of the channel (in which conduction takes place) form electrodes known as the source and drain. Ø The effective width of the channel (in which on conduction takes place) is controlled by a charge placed on the third (gate) electrode. Ø The effective resistance between the source and drain is thus determined by the voltage present at the gate. 20/12/2015 402057 – Chapter 4: Semiconductor devices 54 FIELD EFFECT TRANSISTORS n Classification FET n JFET n-Channel JFET p-Channel JFET MOSFET (IGFET) Depletion MOSFET Enhancement MOSFET n-Channel EMOSFET 20/12/2015 p-Channel EMOSFET n-Channel DMOSFET 402057 – Chapter 4: Semiconductor devices p-Channel DMOSFET 55 JFET n JFET Construction Two types of JFET: nchannel (most widely used) and p-channel Ø 3 terminal: Drain (D), Source (S) connect to n-channel Gate (G) Ø 20/12/2015 402057 – Chapter 4: Semiconductor devices 56 JFET n JFET Operation 20/12/2015 402057 – Chapter 4: Semiconductor devices 57 JFET n JFET characteristics Ø A typical mutual characteristic (ID plotted against VGS) for a small signal general-purpose N-channel JFET operating in common-source mode. Ø This characteristic shows that the drain current is progressively reduced as the gate-source voltage is made more negative. Ø At a certain value of VGS the drain current falls to zero and the device is said to be cut-off. 20/12/2015 402057 – Chapter 4: Semiconductor devices 58 JFET n JFET characteristics 20/12/2015 402057 – Chapter 4: Semiconductor devices 59 JFET 20/12/2015 402057 – Chapter 4: Semiconductor devices 60 JFET 20/12/2015 402057 – Chapter 4: Semiconductor devices 61 JFET n JFET parameters The gain offered by a field effect transistor is normally expressed in terms of its forward transfer conductance (gfs or Yfs) in common source mode. Ø In this mode, the input voltage is applied to the gate and the output current appears in the drain . Ø The common-source forward transfer conductance is given by: Ø 20/12/2015 402057 – Chapter 4: Semiconductor devices 62 JFET n JFET – Common source circuit 20/12/2015 402057 – Chapter 4: Semiconductor devices 63 JFET n JFET – Common gate circuit 20/12/2015 402057 – Chapter 4: Semiconductor devices 64 MOSFET Ø The metal-oxide-semiconductor field-effect transistor (MOSFET) became a practical reality in the 1970s. Ø The MOSFET, compared to BJTs, can be made very small . Ø Since digital circuits can be designed using only MOSFETs, with essentially no resistors or diodes required, highdensity VLSI circuits, including microprocessors and memories, can be fabricated 20/12/2015 402057 – Chapter 4: Semiconductor devices 65 MOSFET Ø In the MOSFET, the current is controlled by an electric field applied perpendicular to both the semiconductor surface and to the direction of current. Ø The phenomenon used to modulate the conductance of a semiconductor, or control the current in a semiconductor, by applying an electric field perpendicular to the surface is called the field effect. Ø The basic transistor principle is that the voltage between two terminals controls the current through the third terminal. 20/12/2015 402057 – Chapter 4: Semiconductor devices 66 MOSFET n MOSFET – Voltage threshold 20/12/2015 402057 – Chapter 4: Semiconductor devices 67 MOSFET n CMOS Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. Ø CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors ( CMOS sensor), data converters, and highly integrated transceivers for many types of communication. Ø In 1963, while working for Fairchild Semiconductor, Frank Wanlass patented CMOS Ø 20/12/2015 402057 – Chapter 4: Semiconductor devices 68 MOSFET n CMOS 20/12/2015 402057 – Chapter 4: Semiconductor devices 69 SUMMARY In this chapter, we have learnt the structure, operational principle as well as the characteristics and applications of important semiconductor devices: Ø Diodes Ø Bipolar junction transistors Ø Field effect transistors n 20/12/2015 402057 – Chapter 2: Electromagnetic properties of solids and applications 70 ASSIGNMENTS n Exercise 1 Given circuit. Determine Vo, know that VB E = 0.7 V, β = 120,V1=2.8v 20/12/2015 402057 – Chapter 4: Semiconductor devices 71 ASSIGNMENTS n Exercise 2 Draw output signal form, know that vi=10 sinΩt, Vγ=0.7 R3 A B R D9 A R3 B R D10 DIODE SOURCE VOLTAGE DIODE SOURCE VOLTAGE D11 +5V DIODE BATTERY 0 20/12/2015 402057 – Chapter 4: Semiconductor devices 0 72 ASSIGNMENTS n Exercise 3 Design a voltage regulator using the circuit . The voltage regulator is to power a car radio at VL = 9 V from an automobile battery whose voltage may vary between 11 and 13.6 V. The current in the radio will vary between 0 (off) to 100 mA (full volume). 20/12/2015 402057 – Chapter 4: Semiconductor devices 73
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