1. 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

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

3. Example: BJT common emitter characteristics
Gain = 300

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 + - 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

7. 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

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

9. BJT configurations
GAIN
CONFIG
ECE 663

10. 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

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

12. BJT Fabrication
ECE 663

13. PNP BJT Electrostatics
ECE 663

14. PNP BJT Electrostatics
ECE 663

15. NPN Transistor Band Diagram: Equilibrium
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16. 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

17. Forward Active minority carrier distributionP+ N P
pB(x)
nE(x’)
nC0
pB0
nE0
nC(x’’)
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18. PNP Physical Currents
ECE 663

19. PNP transistor amplifier action
IN (small)
OUT (large)
Clearly this works in common emitter
configuration
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20. Emitter Injection Efficiency - PNP
IEp
ICp C E
IEn
ICn IB
Can we make the emitter
see holes alone?
ECE 663

21. Base Transport Factor IE IC IEp ICp C E IEn ICn IB
Can all injected holes
make it to the collector?
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22. Common Base DC current gain - PNP
Common Base – Active Bias mode:
IC = aDCIE + ICB0
ICp = aTIEp
= aTgIE
aDC = aTg
IC = aTgIE + ICn
ECE 663

23. 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

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