Gain medium Incoherent Light Coherent Light ECE 663 Transistor/switch/amplifier – a 3 terminal device Source Drain Gate Valve Artery Vein Emitter Collector.

Gain medium

Incoherent Light

Coherent Light

ECE 663

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)

ECE 663

BJT configurations

GAIN CONFIG

+

- +

-

IE IBIC

ECE 663

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

ECE 663

Bipolar Junction Transistors: Basics

Bias Mode E-B Junction C-B Junction

Saturation Forward Forward

Active Forward Reverse

Inverted Reverse Forward

Cutoff Reverse 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

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

ECE 663

P+ N P

nE(x’)

nE0

pB0

pB(x)

nC0

nC(x’’)

Forward Active minority carrier distribution

ECE 663

PNP Physical Currents

ECE 663

PNP transistor amplifier action

IN (small)

OUT (large)

Clearly this works in common emitterconfiguration

ECE 663

Emitter Injection Efficiency - PNP

EnEp

Ep

E

Ep

II

I

I

I

10

ECIEp

ICp

IEn ICn

IB

IE IC

Can we make the emittersee holes alone?

ECE 663

Base Transport Factor

ECIEp

ICp

IEn ICn

IB

IE IC

Ep

CpT I

I

10 T Can all injected holesmake it to the collector?

ECE 663

Common Base DC current gain - PNP

Common Base – Active Bias mode:

IC = DCIE + ICB0

ICp = TIEp = TIE

IC = TIE + ICn

DC = T

ECE 663

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 !!

ECE 663

Common Emitter DC current gain - PNP

T

Tdc

1

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

ECE 663

PNP BJT Common Emitter Characteristic