Gain medium

Incoherent Light

Coherent Light

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Transistor/switch/amplifier – a 3 terminal device

Source

Drain

Gate

Valve

ArteryVein

Emitter Collector

Base

Ion Channel

Dam Laser Heart

Axonal conductionMOSFETBJT

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-+

N P

W

Vappl < 0-+

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 awayfrom depletion region

I

V

I

V

So if we combine these by fusing their terminals…

P N

W

Vappl > 0-+

N P

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 of2nd 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 IBIC

IE = IB + IC ………(KCL)

VEC = VEB + VBC ……… (KVL)

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BJT configurations

GAIN CONFIG

+

- +

-

IE IBIC

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Bipolar Junction Transistors: Basics

VEB, VBC > 0 VEC >> 0IE, IC > 0 IB > 0

VEB >-VBC > 0 VEC > 0 but smallIE > -IC > 0 IB > 0

VEB < 0, VBC > 0 VEC > 0IE < 0, IC > 0 IB > 0 but small

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Bipolar Junction Transistors: Basics

Bias Mode E-B Junction C-B JunctionSaturation Forward Forward

Active Forward ReverseInverted Reverse Forward

Cutoff Reverse Reverse

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BJT Fabrication

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PNP BJT Electrostatics

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PNP BJT Electrostatics

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NPN Transistor Band Diagram: Equilibrium

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PNP Transistor Active Bias Mode

Most holesdiffuse tocollector

Large injectionof Holes

Collector Fields drive holesfar away where they can’t return thermionically

Few recombinein the base

VEB > 0 VCB > 0

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P+ N P

nE(x’)

nE0

pB0

pB(x)

nC0

nC(x’’)

Forward Active minority carrier distribution

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PNP Physical Currents

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PNP transistor amplifier action

IN (small)

OUT (large)

Clearly this works in common emitterconfiguration

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Emitter Injection Efficiency - PNP

EnEp

Ep

E

Ep

III

II

10

ECIEp ICp

IEn ICn

IB

IE IC

Can we make the emittersee holes alone?

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Base Transport Factor

ECIEp ICp

IEn ICn

IB

IE IC

Ep

CpT I

I

10 T Can all injected holesmake it to the collector?

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Common Base DC current gain - PNP

Common Base – Active Bias mode:

IC = DCIE + ICB0

ICp = TIEp = TIEIC = TIE + ICn

DC = T

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Common Emitter DC current gain - PNP

Common Emitter – Active Bias mode:

IE = DCIB + ICE0

DC = DC /(1-DC)

IE

IB

IC

IC = DCIE + ICB0

= DC(IC + IB) + ICB0

IC = DCIB + ICB0 1-DC

GAIN !!

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Common Emitter DC current gain - PNP

T

Tdc

1

Thin base will make T 1Highly doped P region will make 1

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PNP BJT Common Emitter Characteristic