Chater 6 Bipolar Junction Transistor (BJT)ic.sjtu.edu.cn/ic/spd/wp-content/uploads/sites/11/2013/... · 2013-06-05 · Basic about BJT Invention Invented in 1948 by Bardeen, Brattain

Chater 6

Bipolar Junction Transistor

(BJT)

Xiulan Cheng/Shirla Cheng

2012-05-20

Prepared by Xiulan Cheng/程秀兰

Basic about BJT

Invention

Invented in 1948 by Bardeen, Brattain and Shockley

in Bell Lab (First Transistor)

Bipolar

Both types of carriers (electron and hole) play

important roles in operation of BJT

Field Effect Transistor (FET) is unipolar minority

device.

Application

linear amplifier circuits (linear means that the output is

proportional to input).

switch ( for example, logic circuits).


Application of BJT

Main applications

Uniqueness of BJT: high current drivability per input

capacitance fastexcellent for analog and front-

end communications applications.

Power application and analog application due to high

current drive capability and perfect analog

performance.

High speed and RF application (e.g., Emitter couple

logic, ECL)

BiCMOS: combine the best of MOSFET and BJT

In most application areas, BJT is gradually substituted

by instead of MOSFET.


BJT Structure Basic structure

A semiconductor device constructed with three doped regions which

form two ‘back-to-back’ p-n junctions in the same block of

semiconductor material (silicon). Close enough that minority carriers interact (negligible recombination in base)

For apart enough that depletion regions don’t interact (no “punchthrough”)

Three regions: emitter (E), base (B) and collector (C) regions

Emitter: heavy doping, ~1019 /cm3

Base: narrow and little heavy doping , ~1017 /cm3;

Collector: light doping, , ~1015 /cm3

Basic is very thin compared to the diffusion length of minority carriers (.

If the base is much larger, then this will behave like back-to-back

diodes.


Doping


Structure of BJT


BJT Structures and Symbols


BJT Operation Principle Principle

The majority of current enters

collector, crosses the base

region and exits through the

emitter. A small current also

enters the base terminal,

crosses the base-emitter

junction and exits through the

emitter.

Carrier transport in the active

base region directly beneath

the heavily doped (n+)

emitter dominates the i-v

characteristics of the BJT.


Operation depends on the bias

condition

ECE 663

BJT configurations

(1) Common emitter is the most common configuration

(2) Common base is occassionally used.

(3) Common collector is barely used.


Bias Modes of BJT


Bias Mode E-B Junction C-B Junction

Saturation Forward Forward

Active Forward Reverse

Inverted Reverse Forward

Cutoff Reverse Reverse

ECE 663

BJT Fabrication

Discrete BJT

BJT in IC

Electrostatic properties (pnp)


PNP Transistor Active Bias Mode

Minority distribution

--Active bias mode: VEB>0(EB positive bias)

, VBC>0(BC negative bias)

--Majority holes inject into Base from Emitter(IEp)

and electrons inject into Emitter from Base (IEn)

--if Base width W<<Lp, most of holes injected

(minority in Base) diffuse in Base and then

sweep into Collector by electrical filed of BC

(ICp≈ICn).

-- Emitter: emitting carrier into Base

as source.

--Collector: collecting carrier from

Base as drain.

--Base: providing a path of

carrier ,IB≠0

CnI

EpI

EnI

ECE 663

BJT Operation Principle

CB

CB

B

CDC

E C

B

2

2321

I gcontrollin I

up scaling I I Increasing

, I

I :gain DC

I and current hole isI

small very and current electron is I

:(pnp) modeEmitter common for principle Amplyfing

Base in current nCombinatio-

(small)

current bias reverse junction BC-

,

)(

BRB

CnBEnBBBB

CEB

EEpCnEpC

Cn

CpCnCnEpCnCpC

EnEpEnEpE

II

IIIIIII

III

IIIII

I

IIIIIII

IIIII

BJT Parameters (PNP)


gain current BJT high 1 ,10

,

0I as current Collector

gain DC Base Common

:gain DC Base Common (3)

gain. current BJT high 1 ,10

BJT) (pnp

:Base of tCoefficien Transport (2)

gain. current BJT high 1 ,10

BJT) (pnp

:efficiency Emitting (1)

DCdc

0dc

E0

dc

0dc

TT

T

CnCBT

CnEpTCnCpC

ETEpTCp

CB

CBEC

Ep

Cp

EnEp

Ep

E

Ep

IIα

IIαII I

IαIα I

I

III

I

I

II

I

I

I

0

1

1 ,

1

1

1

1

)(

0 as current Collector-

gain DC Emitter Common

:gain DC Emitter Common (4)

0

0

dc

0

dc

dc

0

dcdc

dc

0dc0dc

0

0

B

Cdc

CE

CBCEdc

CBBC

CBBCCBEC

BCE

dc

CEBdcC

I

Iβ

I

IIβ

III

IIIIII

II

β

IIβI

For npn transistor, similar analysis can be carried out.

However, the emitter current is mainly carried by

electrons.

.etc , T

EpEn

En

En

Cn

I

I

II

I

Detailed Quantitative Analysis


Assumptions and Diffusion Equations

Assumptions

pnp transistor, steady state, low-level injection.

Only drift and diffusion, no external generations

One dimensional etc.

General approach is to solve minority carrier diffusion equations for each of the

three regions:

LGp

x

pD

t

p

p2

2

p

Ln

2

2

n Gn

x

nD

t

n

For steady state and when GL= 0, and for Base of pnp

p2

2

p

p

x

pD

t

p

Review: Operational Parameters

Base transport factor : T = IC / IEp Collector to emitter current gain: dc = T

Collector to base current gain: dc = dc / (1 – dc)

Injection Efficiency : )/( EnEpEp III

EICI

BI

IEp

–IEn

IBR

–IBR –ICn

These parameters can be related to device parameters such as doping,

lifetimes, diffusion lengths, etc.

20

Current in a Forward PN Junction

kT

qVn

L

qAD

kT

qVp

L

qAD

kT

qVn

L

qAD

kT

qVp

L

qAD

L

nqAD

L

pqADIII

L

pqAD

dx

pdqADI

L

nqAD

dx

ndqADI

EBE

E

EEBB

B

B

EBE

E

EEBB

B

B

E

EE

B

BBnp

B

BBBp

E

EEEn

expexp

1exp1exp

)0()0(

:Current Total

)0(

,)0(

00

00

21

Collector Current

kT

qVp

W

qAD

kT

qVp

W

qAD

W

pqAD

W

pqAD

dx

dpqADI

EBB

B

BEBB

B

B

B

BB

B

BB

B

BC

exp1exp

)0(0)0(

junction B-C biased reverse

of current saturation reverse small the neglect weif current hole only

:Current Collector

00

Emitter Current


kT

qVn

L

DqA

kT

qVn

L

DqAI

kT

qVp

WqA

kT

qVp

W

DqA

pqAWIIII

III

EBE

E

EEBE

E

EEn

EBB

BEBB

B

B

B

BBCBRCEp

EnEpE

exp1exp

exp2

exp

2

)0(

00

00

23

Base Current

kT

qVn

L

DqA

kT

qVWqApI

I

EBE

E

EEBBBB

B

expexp2

emitter. to injection for electrons supplies-

base in ionrecombinat for electrons supplies-

Current, Base

00

24

BJT Parameters

2

2Tdc

B

2

0

E

2

0

0

0

0

0

2

2EB

0B

B

BEB0B

B

B

EB0B

B

B

21

1

:gain DC Base Common

Base in doping- N ,

Emitter in doping- N , where

1

1

1

1

/

/1

1

1

1

:efficiency injection Emitter

21

1

exp2

exp

exp

:factor transport Base

B

B

EEB

BBE

B

iB

E

iE

EEB

BBE

BEB

EBE

BBB

EEE

Ep

EnEnEp

Ep

B

BEp

CT

L

W

NLD

NWD

N

np

N

nn

NLD

NWD

pLD

nWD

WpD

LnD

I

III

I

L

W

kT

qVp

qAW

kT

qVp

W

qAD

kT

qVp

W

qAD

I

I

2

2dc

dcdc

2

1

1

:gain DC Emitter Common

B

B

EEB

BBE

L

W

NLD

NWD

Deviation from Ideal BJT


Deviations from the ideal

The measured characteristics deviates

slightly from the ideal characteristics

discussed.

Base-width modulation

Punch-through

Avalanche multiplication and breakdown

Others – base resistance, depletion region recombination-general


Base Width Modulation Early Effect

When the reverse bias applied to the C-B junction increases, the C-B

depletion width increases and W decreases. the collector current, IC

increase,but IB unchanged.

Reverse Early Effect

impact of VBE on WB: increase VBEWB increaseIC smaller, IB

unchanged.


kT

qV

CW

pqAD

W

pqADI

EB

e0)0(

B

B0B

B

BB

smaller IC than ideal , BBEBE WxV

IB unchanged

F


Base With Modulation--Early Effect

Punch Through Cause Reason

Punch-through can be

viewed as base width

modulation carried to the

extreme, i.e., punch-through

occurs when W 0.

For C-B voltage beyond

punch-through, the E-B

barrier lowers and results in

large increase in carrier

injection from emitter to

collector.

Large increase in collector

currents at high VCE0 occurs

due to two reasons: punch-

through or avalanche

multiplication.


High Injection Effect

VBE increasesBase injected minority carrier concentration

may approach, or even become large than the majority carrier

concentration.lowering the emitting efficiency


High Collector Current Effect

As IC↑, electron velocity in collector↑, and approaches a limit

vsat, Ic approaches .qAENCVsat

t


Current Crowding


Breakdown Effect

Sudden rise in IC for large reverse VCB.


Bipolar Issues in CMOS: Latch-up

Latch up

Interaction of two hidden BJT’s inside a CMOS pair.


In principle, no problem because in both

BJTs, VBE=0.


But there are also two parasitic resistors:

POSITIVE FEEDBACK LOOP can cause device destruction.

Suppose for some reason:

Minority carrier injection into substrate by transient forward

bias on pn junctions (typically in I/O circuits);

photogeneration by ionizing radiation;

or impact ionization by hot carriers

Current flows through RX pnp goes into FAR IC (pnp)

voltage drop in RW npn goes into FAR IC (npn) more

voltage drop in RX

Avoiding of Latch up

Reduce RX and RW

Reduce pnp and npn

Methods

Heavily doped substrate (need lower doping epi layer

on top for devices)

Sufficient transistor spacing

Guard rings at sensitive locations

SOI wafer



Guard ring: reverse-biased pn junction that collects injected holes.

38

Other Effect

:

Base series resistance

Recombination-generation current

Homework


Pierret Book:

P276:10.2,10.6

P312:11.1(b), 11.8,11.9,11.17

Documents

Chater 6 Bipolar Junction Transistor (BJT)ic.sjtu.edu.cn/ic/spd/wp-content/uploads/sites/11/2013/... · 2013-06-05 · Basic about BJT Invention Invented in 1948 by Bardeen, Brattain