Chater 6 Bipolar Junction Transistor...

Chater 6

Bipolar Junction Transistor

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

ECE 663

BJT Operation Principle

I gcontrollin I

up scaling I I Increasing

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-

CnBEnBBBB

EEpCnEpC

CpCnCnEpCnCpC

EnEpEnEpE

IIIIIII

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)

CnEpTCnCpC

ETEpTCp

IIαII I

IαIα I

0 as current Collector-

gain DC Emitter Common

:gain DC Emitter Common (4)

0dc0dc

CBCEdc

CBBCCBEC

CEBdcC

IIIIII

For npn transistor, similar analysis can be carried out.

However, the emitter current is mainly carried by

electrons.

.etc , T

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:

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

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

–IEn

–IBR –ICn

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

lifetimes, diffusion lengths, etc.

Current in a Forward PN Junction

pqADIII

pdqADI

ndqADI

expexp

1exp1exp

)0()0(

:Current Total

Collector Current

dpqADI

exp1exp

)0(0)0(

junction B-C biased reverse

of current saturation reverse small the neglect weif current hole only

:Current Collector

Emitter Current

pqAWIIII

BBCBRCEp

exp1exp

Base Current

qVWqApI

EEBBBB

expexp2

emitter. to injection for electrons supplies-

base in ionrecombinat for electrons supplies-

Current, Base

BJT Parameters

:gain DC Base Common

Base in doping- N ,

Emitter in doping- N , where

:efficiency injection Emitter

:factor transport Base

EnEnEp

:gain DC Emitter Common

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.

smaller IC than ideal , BBEBE WxV

IB unchanged

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

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.

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

Chater 6 Bipolar Junction Transistor...

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