Transistorswitchamplifier a 3 terminal device Source Gate Incoherent

Transistor/switch/amplifier – a 3 terminal device Source Gate Incoherent Light Coherent Light Vein Artery Valve Gain medium Drain Laser Dam Emitter Collector Heart Ion Channel Base BJT MOSFET Axonal conduction ECE 663

All of these share a feature with… • Output current can toggle between large and small (Switching Digital logic; create 0 s and 1 s) • Small change in ‘valve’ (3 rd terminal) creates Large change in output between 1 st and 2 nd terminal (Amplification Analog applications; Turn 0. 5 50)

Example: BJT common emitter characteristics Gain = 300

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Aim of this chapter • How can we get ‘Gain’? • What is the structure of the device to get gain? • What is the equation for gain? • How can we use this equation to maximize gain? • How can we model this device as a circuit element? • What are its AC characteristics and speed?

Recall p-n junction W + P N N P W + - - Vappl < 0 Vappl > 0 Forward bias, + on P, - on N (Shrink W, Vbi) Reverse bias, + on N, - on P (Expand W, Vbi) Allow holes to jump over barrier into N region as minority carriers Remove holes and electrons away from depletion region I I V V

So if we combine these by fusing their terminals… N P W + - Vappl > 0 P N W + - Vappl < 0 Holes from P region (“Emitter”) of 1 st PN junction driven by FB of 1 st PN junction into central N region (“Base”) Driven by RB of 2 nd PN junction from Base into P region of 2 nd junction (“Collector”) • 1 st region FB, 2 nd RB • If we want to worry about holes alone, need P+ on 1 st region • For holes to be removed by collector, base region must be thin

Bipolar Junction Transistors: Basics + - IE IC - + IB IE = I B + I C ………(KCL) VEC = VEB + VBC ……… (KVL)

BJT configurations GAIN CONFIG ECE 663

Bipolar Junction Transistors: Basics + - IE IC - + IB VEB >-VBC > 0 VEC > 0 but small IE > -IC > 0 IB > 0 VEB, VBC > 0 VEC >> 0 IE , I C > 0 I B > 0 VEB < 0, VBC > 0 VEC > 0 IE < 0, IC > 0 IB > 0 but small ECE 663

Bipolar Junction Transistors: Basics Bias Mode E-B Junction C-B Junction Saturation Forward Active Forward Reverse Inverted Reverse Forward Cutoff Reverse ECE 663

BJT Fabrication ECE 663

PNP BJT Electrostatics ECE 663

PNP BJT Electrostatics ECE 663

NPN Transistor Band Diagram: Equilibrium ECE 663

PNP Transistor Active Bias Mode VEB > 0 VCB > 0 Few recombine in the base Collector Fields drive holes far away where they can’t return thermionically Large injection of Holes Most holes diffuse to collector ECE 663

Forward Active minority carrier distribution P+ N P p. B(x) n. E(x’) n. C 0 n. E 0 p. B 0 n. C(x’’) ECE 663

PNP Physical Currents ECE 663

PNP transistor amplifier action IN (small) OUT (large) Clearly this works in common emitter configuration ECE 663

Emitter Injection Efficiency - PNP IE E ICp IEn IC C IB Can we make the emitter see holes alone? ECE 663

Base Transport Factor IE E ICp IEn IC C IB Can all injected holes make it to the collector? ECE 663

Common Base DC current gain - PNP Common Base – Active Bias mode: IC = a. DCIE + ICB 0 ICp = a. TIEp = a. Tg. IE a. DC = a. Tg IC = a. Tg. IE + ICn ECE 663

Common Emitter DC current gain - PNP Common Emitter – Active Bias mode: IE = b. DCIB + ICE 0 b. DC = a. DC /(1 -a. DC) IC = a. DCIE + ICB 0 = a. DC(IC + IB) + ICB 0 IC = a. DCIB + ICB 0 1 -a. DC GAIN !! IC IB IE ECE 663

Common Emitter DC current gain - PNP Thin base will make a. T 1 Highly doped P region will make g 1 ECE 663

PNP BJT Common Emitter Characteristic ECE 663