Transistor/switch/amplifier – a 3 terminal device Gain medium Incoherent Light Coherent Source Drain Gate Valve Artery Vein Dam Laser Heart Ion Channel Emitter Collector 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 0s and 1s) Small change in ‘valve’ (3rd terminal) creates Large change in output between 1st and 2nd terminal (Amplification  Analog applications; Turn 0.5  50)

Example: BJT common emitter characteristics Gain = 300

http://www.computerhistory.org/semiconductor/timeline.html#1940s

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

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

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

BJT configurations GAIN CONFIG ECE 663

Bipolar Junction Transistors: Basics + - IE IB IC VEB >-VBC > 0  VEC > 0 but small IE > -IC > 0  IB > 0 VEB, VBC > 0  VEC >> 0 IE, IC > 0  IB > 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 Reverse Inverted Cutoff 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 Most holes diffuse to collector Large injection of Holes Collector Fields drive holes far away where they can’t return thermionically VEB > 0 VCB > 0 Few recombine in the base ECE 663

Forward Active minority carrier distribution P+ N P pB(x) nE(x’) nC0 pB0 nE0 nC(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 IEp ICp C E IEn ICn IB Can we make the emitter see holes alone? ECE 663

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

Common Base DC current gain - PNP Common Base – Active Bias mode: IC = aDCIE + ICB0 ICp = aTIEp = aTgIE aDC = aTg IC = aTgIE + ICn ECE 663

Common Emitter DC current gain - PNP Common Emitter – Active Bias mode: IE = bDCIB + ICE0 bDC = aDC /(1-aDC) IC = aDCIE + ICB0 = aDC(IC + IB) + ICB0 GAIN !! IE IB IC IC = aDCIB + ICB0 1-aDC ECE 663

Common Emitter DC current gain - PNP Thin base will make aT  1 Highly doped P region will make g  1 ECE 663

PNP BJT Common Emitter Characteristic ECE 663