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

2. 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’ (3 rd terminal) creates Large change in output between 1 st and 2 nd terminal (Amplification  Analog applications; Turn 0.5  50)

3. Example: BJT common emitter characteristics Gain = 300

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

5. 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?

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

7. So if we combine these by fusing their terminals… P N W V appl > 0 - + N P W V appl < 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

8. Bipolar Junction Transistors: Basics + - + - IEIE IBIB ICIC I E = I B + I C ………(KCL) V EC = V EB + V BC ……… (KVL)

9. ECE 663 BJT configurations GAIN CONFIG

10. + - + - IEIE IBIB ICIC ECE 663 Bipolar Junction Transistors: Basics V EB, V BC > 0  V EC >> 0 I E, I C > 0  I B > 0 V EB >-V BC > 0  V EC > 0 but small I E > -I C > 0  I B > 0 V EB 0  V EC > 0 I E 0  I B > 0 but small

11. ECE 663 Bipolar Junction Transistors: Basics Bias ModeE-B JunctionC-B Junction SaturationForward ActiveForwardReverse InvertedReverseForward CutoffReverse

12. ECE 663 BJT Fabrication

13. ECE 663 PNP BJT Electrostatics

14. ECE 663 PNP BJT Electrostatics

15. ECE 663 NPN Transistor Band Diagram: Equilibrium

16. 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 Few recombine in the base V EB > 0 V CB > 0

17. ECE 663 P+ N P n E (x’) n E0 p B0 p B (x) n C0 n C (x’’) Forward Active minority carrier distribution

18. ECE 663 PNP Physical Currents

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

20. ECE 663 Emitter Injection Efficiency - PNP E C I Ep I Cp I En I Cn IBIB IEIE ICIC Can we make the emitter see holes alone?

21. ECE 663 Base Transport Factor E C I Ep I Cp I En I Cn IBIB IEIE ICIC Can all injected holes make it to the collector?

22. ECE 663 Common Base DC current gain - PNP Common Base – Active Bias mode: I C =  DC I E + I CB0 I Cp =  T I Ep =  T  I E I C =  T  I E + I Cn  DC =  T 

23. ECE 663 Common Emitter DC current gain - PNP Common Emitter – Active Bias mode: I E =  DC I B + I CE0  DC =  DC /(1-  DC ) IEIE IBIB ICIC I C =  DC I E + I CB0 =  DC (I C + I B ) + I CB0 I C =  DC I B + I CB0 1-  DC GAIN !!

24. ECE 663 Common Emitter DC current gain - PNP Thin base will make  T  1 Highly doped P region will make   1

25. ECE 663 PNP BJT Common Emitter Characteristic