Bipolar Junction TransistorCHAPTER 6LECTURE 7 & 8

COLLECTOR (medium doping)

BASE (light doping)

EMITTER (heavy doping)

N

P

N

The bipolar junction transistor has 3 doped regions.

N

P

N

VCE

VCC

RC

RB

VBB

VBE

In a properly biased NPN transistor, the emitter electrons diffuse into the base and then go on to the collector.

Bipolar Junction Transistors (BJTs)

The bipolar junction transistor is a semiconductor device constructed with three doped regions.

These regions essentially form two ‘back-to-back’ p-n junctions in the same block of semiconductor material (silicon).

The most common use of the BJT is in linear amplifier circuits (linear means that the output is proportional to input). It can also be used as a switch (in, for example, logic circuits). 

npn-BJT Structure

The ‘npn’ version of the BJT consists of two n regions separated by a p region (as the name suggests). A schematic of an npn transistor is shown.

n-type p-type n-type

BJT Structure

The three regions are known as the emitter, base and collector regions.

Electrical connections are made to each of these regions.

npn-BJT Structure

EEmitter (n-type)

Base (p-type)

Collector (n-type)

B

C

npn BJT Symbol

npn BJT Symbol

E

B

C

pnp BJT Symbol

In the symbol for a pnp BJT transistor the direction of the arrow on the emitter is reversed

E

B

C

Still remember about BJT?

CBE III

CBE iii

The emitter current (iE) is the sum of the collector current (iC) and the base current (iB)

iB << iE and iC OTHER PRAMETERS & EQUATIONS?

BJT Basic structure and schematic symbol

E C

B

E

B

C

n npE C

B

E C

B

E

B

C

p pnE C

B

npn type pnp type

approximate equivalents

transistorsymbols

Refresh.. Common-emitter

current gain, β Range: 50 < β <

300 Common-base

current gain, α Range: always

slightly less than 1 The current

relationship between these 2 parameters are as follows:

EC

EC

CE

BE

BC

BCE

ii

ii

ii

iiiiiii

1

1

1

1

But

)(

)(

Refresh.. BJT as amplifying device

B-E junction is forward-biased B-C junction is reverse-biased

BIASING OF BJT

Remember…! for normal operation

emitter-base junction is always forward-biased AND

collector-base junction is always reverse-biased

FORWARD BIASING E/B JUNCTION

REVERSE BIASING C/B JUNCTION

BIASING NPN TRANSISTOR

Common-Emitter Circuit

(a) with an npn transistor(b) with a pnp transistor(c) with a pnp transistor biased with a positive voltage source

Conventional flow Electron flow

ICIB

IE

ICIB

IE

IE = IC + IB IC @ IE IB << IC

dc = IC

IEdc =

IC

IB

VCE

VCC

RC

RB

VBB

VBE

The common emitter connection has two loops:the base loop and the collector loop.

Subscript notation When the subscripts are the same, the voltage represents

a source (VCC). When the subscripts are different, the voltage is between

two points (VCE). Single subscripts are used for node voltages with ground

serving as the reference (VC).

VCE

VCC

RC

RB

VBB

VBE

The base circuit is usually analyzed with the same approximation used for diodes.

IB =VBB - VBE

RB

0 2 4 6 8 10 121416 182468

101214

VCE in Volts

IC in mA

A graph of IC versus VCE

20 mA0 mA

100 mA80 mA60 mA40 mA

(Note that each new value of IB presents a new curve.)

This set of curves is also called a family of curves.

Regions of operation

Cutoff - - - used in switching applications Active - - - used for linear amplification Saturation - - - used in switching applications Breakdown - - - can destroy the transistor

Transistor circuit approximations

First: treat the base-emitter diode as ideal and use IB to determine IC.

Second: correct for VBE and use IB to determine IC. Third (and higher): correct for bulk resistance and other

effects. Usually accomplished by computer simulation.

dcIB VCEVBE = 0.7 V

The second approximation:

VCC

RC

RB

VBB

VBE = 0.7 V

IB =VBB - VBE

RB

IB =5 V - 0.7 V

100 kW

5 V

100 kW

= 43 mA

VCC

RC

RB

VBB 5 V

100 kW

IB = 43 mA

dc = 100

IC = dc IB

IC = 100 x 43 mA = 4.3 mA

VCC

RC

RB

VBB 5 V

100 kW

IB = 43 mA

IC = 4.3 mA

1 kW

12 V

VRC = IC x RC

VRC = 4.3 mA x 1 kW = 4.3 V

VCC

RC

RB

VBB 5 V

100 kW

IB = 43 mA

IC = 4.3 mA

1 kW

12 V

VCE = VCC - VRC

VCE

VCE = 12 V - 4.3 V = 7.7 V

Typical Breakdown Ratings

VCB = 60 V VCEO = 40 V VEB = 6 V

Note: these are reverse breakdown ratings

02468

101214

VCE in Volts

IC in mA

50

A graphic view of collector breakdown

Typical Maximum Ratings

IC = 200 mA dc PD = 250 mW (for TA = 60 oC) PD = 350 mW (for TA = 25 oC) PD = 1 W (for TC = 60 oC)

Typical “On Characteristics”

IC in mA hFE(min) hFE(max)

0.1 40 ___ 1 70 ___ 10 100 300 50 60 ___100 30 ___

Troubleshooting Look for gross voltage errors. First approximation and mental estimates will

usually suffice. Resistors don’t short but circuit boards can. Circuit boards can and do open. Junctions can and do short. Junctions can and do open.