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EE 332

DEVICES AND CIRCUITS II

Lecture 3

Bipolar Junction Transistors (1)

Physical Structure

Consists of 3 alternating layers ofn- and

p-type semiconductor called emitter (E),

base (B) and collector (C).

Majority of current enters collector,

crosses base region and exits through

emitter. A small current also enters base

terminal, crosses base-emitter junction

and exits through emitter.

Carrier transport in the active base

region directly beneath the heavily

doped (n+) emitter dominates i-v

characteristics of BJT.

Transport Model fornpn Transistor

Narrow width of the base region

causes coupling between the two

back to backpnjunctions.

Emitter injects electrons into base

region, almost all of them travel

across narrow base and are

removed by collector

Base-emitter voltage vBE and

base-collector voltage vBCdetermine currents in transistor

and are said to be positive when

they forward-bias their

respectivepnjunctions.

The terminal currents are

collector current(iC ), base

current (iB) and emitter current

(iE).

Primary difference between

BJT and FET is that iB is

significant while iG = 0.

Goals of Lectures 3-5

Explore physical structure of bipolar transistor

Understand bipolar transistor action and importance of carrier transport

across base region

Study terminal characteristics of BJT.

Explore differences between npn andpnp transistors.

Develop Transport and Ebers-Moll models for bipolar device.

Define four operation regions of BJT.

Explore model simplifications for each operation region.

Understand origin and modeling of Early effect.

Present SPICE model for bipolar transistor.Provide examples of worst-

case and Monte Carlo analysis of bias circuits.

Discuss bipolar current sources and current mirror.

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npn Transistor: Forward Characteristics

Forward transport current is

IS is saturation current

== 1expT

VBEv

SI

Fi

Ci

A910A1810 S

I

VT= kT/q =0.025 V at room temperature

Base current is given by

== 1expT

VBEv

F

SI

F

Fi

Bi

50020 F

Emitter current is given by

=+= 1expT

VBEv

F

SI

Bi

Ci

Ei

0.11

95.0 +

=

F

FF

is forward common-emitter

current gain

is forward common-

base current gain

In this forward active operation region,

Fi

Ci

=F

Ei

Ci

=

npn Transistor: Complete TransportModel Equations for Any Bias

= 1expexpexp

TV

BCv

R

SI

TVBCv

TVBEv

SI

Ci

+= 1expexpexp

TV

BEv

F

SI

TVBCv

TVBEv

SI

Ei

+= 1exp1exp

TV

BCv

R

SI

TVBEv

F

SI

Bi

First term in both emitter and collector current expressions give currenttransported completely across base region.

Symmetry exists between base-emitter and base-collector voltages inestablishing dominant current in bipolar transistor.

Transport Model Calculations: Example

Problem: Find terminal voltages and

currents.

Given data: VBB = 0.75 V, VCC = 5.0

V,IS=10-16 A, F=50, R =1

Assumptions: Room temperature

operation, VT=25.0 mV.

Analysis: V =0.75 V,BE

V = VBC BB

- VCC

=0.75 V-5.00V=-4.25 V

Evaluating the expressions for

terminal currents,

mA07.1=C

I

mA09.1=EI

A04.21 =BI

982.0mA09.1

mA07.1

50mA0214.0

mA07.1

===

===

EI

CI

F

BI

CI

F

npn Transistor: Reverse Characteristics

Reverse transport current is

== 1expT

VBCv

SI

Ei

Ri

== 1expT

VBCv

R

SI

R

Ri

Bi

200 R


= 1exp

TVBCv

R

SI

Ci

95.01

0 +

=

R

RR

is reverse common-emitter

current gain

is reverse common-

base current gainBase current is given by

Base currents in forward and reverse modes

are different due to asymmetric doping levels

in emitter and collector regions.

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pnp Transistor: Structure

Voltages vEB and vCB are positive when they forward bias their

respectivepnjunctions.

Collector current and base current exit transistor terminals and

emitter current enters the device.

pnp Transistor: Reverse Characteristics

Reverse transport current is

== 1exp

T

VCB

SI

Ei

Ri

v


== 1expT

VCBv

R

SI

R

Fi

Bi


+= 1exp1

1VCBv

TR

SI

Ci

pnp Transistor: Complete Transport

Model Equations for Any Bias

= 1expexpexp

T

V

CBv

R

SI

T

VCBv

T

VEBv

SI

Ci

+= 1expexpexp

TV

EBv

F

SI

TVCBv

TVEBv

SI

Ei

TV

RT

V

F

B

+= 1exp1exp CBv

SI

EBv

SI

i

pnp Transistor: Forward Characteristics

Forward transport current is

== 1expT

VEBv

SI

Fi

Ci


== 1expT

VEBv

F

SI

F

Fi

Bi


+=+= 1exp1

1T

VEBv

F

SI

Bi

Ci

Ei

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Circuit Representation for Transport

Models

In npn transistor (expressions analogous forpnp transistors), total current

traversing base is modeled by current source given by:

==T

VBCv

TVBEv

SI

Ri

Fi

Ti expexp

+= 1exp1expT

VBCv

R

SI

TVBEv

F

SI

Bi

Diode currents correspond directly to 2 components of base current.

Ebers-Moll Model (contd.)

Complete Ebers-Moll equations (npn transistor) are given by combining

forward and reverse characteristics:

= 1exp1expT

V

BCv

CSIRT

VBE

v

ESIEi

= 1exp1exp

TV

BCv

CSI

TVBEv

ESI

FCi

+== 1exp11exp1

T

V

BCCS

IR

TVBE

ESI

FCi

Ei

Bi

vv

CSI

RESI

F =

Ebers-Moll Model (contd.)

Complete Ebers-Moll equations (pnp transistor) are given by:

= 1exp1expT

VCB

v

CSIRT

VEB

v

ESIEi

= 1exp1exp

TV

CBv

CSI

TVEBv

ESI

FCi

TVCSRT

VESFB

+= 1exp11exp1 CBv

IEBv

Ii

Ebers-Moll Model

Forward characteristics (npn transistor)

== 1exp1expT

VBEv

ESI

TVBEv

F

SI

Ei

== 1exp1expT

VBEv

ESI

FT

VBEv

SI

Ci

F

SI

ESI

=where

Reverse characteristics (npn

transistor)

== 1exp1expT

VBCv

CSI

TVBCv

R

SI

Ci

== 1exp1expT

VBCv

CSI

RT

VBCv

SI

Ei

R

SI

CSI

=where

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Operation Regions of Bipolar Transistor

Reverse Bias Forward Bias

Forward Bias Forward active region

(Normal active region)

(Good Amplifier)

Saturation region

(Not same as FET

saturation region)(Closed switch)

Reverse Bias Cutoff region

(Open switch)

Reverse-active region

(Inverse active region)(Poor amplifier)

Base-emitter junction Base-collector junction

i-v Characteristics of Bipolar Transistor:

Common-Base Output Characteristics

ForvCB > 0, npn transistor is in forward active

region, iC= iEis independent of and vCE.

ForvCB< 0, base-collector diode becomes

forward-biased and iCgrows exponentially (in

negative direction) as base-collector diode

begins to conduct.

End of Lecture 3

i-v Characteristics of Bipolar Transistor:

Common-Emitter Output Characteristics

ForiB=0, transistor is cutoff. IfiB >0, iCalso

increases.

ForvCE> vBE, npn transistor is in forward activeregion, iC= FiB is independent of and vCE.ForvCE< vBE, transistor is in saturation.

ForvCE< 0, roles of collector and emitter

reverse.

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