Chapter 7

Bipolar Junction Transistor (BJT)

Physical Structure and Operation of BJTs

Bipolar Junction Transistor (BJT) is constructed with three doped semiconductor regions separated by two pn junctions.

The three regions are called emitter, base and collector

n p nEmitter

Base

Collector p n pEmitter

Base

Collector

PNP transistor

Base-emitter junction

Base-collector junction

NPN transistor

Bipolar Junction Transistor (BJT)

The base region is lightly doped and very narrow compared to the heavily doped emitter and collector regions

The term Bipolar refers to the use of both holes and electrons as carriers in the transistor

In order for the transistor to function, the base-emitter and base-collector pn junctions have to be correctly biased (i.e. Forward or Reverse Bias)

BJT – DC biasing configurations and Calculation

Let us consider the NPN transistor for illustration

n p nCollector

Base

Emitter

Base-Emitter junction

Base-Collector junction

-

+

-

+

(Forward

Biased)

(Reverse

Biased)

The transistor pn junctions are forward and reverse biased

NPN - Bipolar Junction Transistor (BJT)

Let us consider the NPN transistor for illustration

CollectorEmitter

Base-Emitter junction

Base-Collector junction

-

+

-

+

(Forward

Biased)

(Reverse

Biased)

ICIE

IB

e

e

e

n p n

NPN - Bipolar Junction Transistor (BJT)

The base region has only a few free holes It is not likely that an electron coming from

the emitter will find a hole in the base with which to combine

With so few electron-hole recombination in the base, the base current is very small

The collector is n-type region but positively charged

NPN - Bipolar Junction Transistor (BJT)

Since the base is such a narrow region, the positive field of the collector is quite strong and the great majority of the electrons coming from the emitter are attracted and collected by the collector

The flow of electrons in the emitter, collector and base will cause currents IE , IC , and IB to flow respectively

Current IC can flow only if current IB exists

NPN - Bipolar Junction Transistor (BJT)

n p nCollector

Base

Emitter

Base-Emitter junction

Base-Collector junction

-

+

-

+(Forwar

d Biased)

(Reverse

Biased)

-

+-

+

e ee

ICIE

IB

Circuit Symbol (NPN) IE = IC + IB

PNP - Bipolar Junction Transistor (BJT)

p n pCollector

Base

Emitter

Base-Emitter junction

Base-Collector junction

-

+ -

+(Forwar

d Biased)

(Reverse

Biased)

-

+-

+

e ee

ICIE

IB

Circuit Symbol (PNP)

IE = IC + IB

BJTs Current Relationship-

+

-

+

IB

IE

IC

VBB VC

C

npnRB

RC

VC

C

-

+

-

+

IB

IE

IC

VBB

pnpRB

RC

BJTs Current Relationship

B

Cdc I

I

IE = IC + IBIC flows only if IB exits and their relationship is given by: dc beta

IB is considered to be Input dc current and IC is Output dc currentdc current gain =

B

C

II

InputOutput

Therefore βdc dc current gain

BJTs Current Relationship

The ratio of the collector current to the emitter current is the dc alpha (αdc) and is given by:

E

Cdc I

I

Since IC < IE, then dc alpha is always < 1

Relationship of βdc and αdc

Starting with IE = IC + IB and dividing by IC

C

B

C

B

C

C

C

E

II

II

II

II

1

dc1

dc1

dcdc 111

Relationship of βdc and αdc

Rearranging, we getdc

dc

dc

11

Therefore:dc

dcdc

1

We can calculate αdc if we know βdc

Relationship of βdc and αdc

dc

dcdc

1

dcdcdc 1

dcdcdc 1

From

dc

dcdc

1

We can calculate βdc if we know αdc

DC Analysis

-

+

-

+

IB

IE

IC

VBB VC

C

RB

RC

VBE

VC

BVCE

Ground = 0V Reference point

DC Analysis

VBB forward biases the base-emitter junction VCC reverse-biases the base-collector

junction There are three transistor Currents IB , IE , IC There are three transistor voltages: VBE , VCB

and VCE When the base-emitter pn junction is

forward-biased, it is like a forward biased diode. Therefore

VBE ≈ 0.7V (Si) or 0.3V (Ge)

DC Analysis

The voltage across RB is VRB = VBB - VBE

And VRB = IB RBTherefore IB RB = VBB - VBE

B

BEBBB R

VVI BdcC II

VRC = IC RC

VCE = VCC – IC RC VCB = VCE – VBE

DC Analysis

-

+

-

+

IB

IE

IC

VBB= 5V

VC C = 10V

RB = 10k

RC = 100Ω Determine: IB , IC , IE , αdc , VCE and VCB if βdc = 150

DC Analysis

993.0151150

1

dc

dcdc

B

BEBBB R

VVI A

kVV 430

107.05

IC = βdcIB = 150 x 430µA = 64.5mA

mAmAIIdc

CE 95.64

993.05.64

VCE = VCC – IC RC = 10V – 64.5mA x 100Ω = 3.55VVCB = VCE – VBE = 3.55V – 0.7V = 2.85V

Collector Characteristic Curves

B

BEBBB R

VVI For VBB = 1V A

kVVIB 30

107.01

VCE0

(mA)

(V)

IB = 30uA

Linear Region

IC = βdc IBVC C -+

-+

IBIC

VBB

VCE10kΩ IE

Collector Characteristic Curves

Both VBB and VCC are adjustable VBB is set to produce a specific value of IB

while VCC = 0 then IC = 0 and VCE = 0 (Base-collector is forward biased)

As VCC is gradually increased, VCE will increase and so will IC

When VCE reaches approximately 0.7V the base-collector junction becomes forward-biased

IC reaches its maximum value given by IC = βdcIB

Collector Characteristic Curves

IC

0

(mA)

VCE (V)

IB = 10µA

IB = 20µAIB = 30µA

IB = 40µA

IB = 50µA

Saturation

Linear

Breakdown

Typical Transistor operations

Transistor operation can be in one of following regions:Cut-off is when the Base-Emitter junction is not forward-biased, the transistor is basically not doing anything.Active operation is when IC =βIB condition holdsSaturation is when VCE is reduced to 0 (zero)Determining factors: How large is IB or VBE and How large is RL

CUT-OFF

Transistor operations – Cut-off

When IB = 0, the transistor is in cut-off, it does not conduct

Under this condition there is very small collector leakage current, ICE0 due to thermally generated carriers

In cut-off both the base-emitter and base-collector junctions are reverse-biased

Transistor operations – Cut-off

-+

-+

IB = 0ICE0

VBB =0

VCE

VCC

IC

0

(mA)

IB = 0

Cut-off Region

ICE0

Transistor operations – Active (linear)

Transistor operations – Active (linear)

IC (mA)

0VCE (V)

IB1

IB2

IB3

IB4

IB5

Linear(Active) RegionIC = βdcIB

Saturation Region

Cut-off RegionVCE(sat)

IB = 0ICE0

Transistor operations - Saturation

When VCC is increased from zero (the base-collector junction is forward biased), VCE will increase and IC will also increase.

But IC ≠ β IC

When VCE reaches approximately 0.7V (for Si) the base-emitter junction becomes forward biased. This collector-emitter voltage is called Saturation voltage (VCE(sat)) and IC reached its full value.

Transistor operations - Saturation

-+

-+

IB1 IC

VBB

VCE

VCC

IC

0

(mA)

VCE (V)

IB = 0ICE0IB1

IB2

IB3

IB4

Saturation Region

Transistor operations - Saturation

Maximum Transistor Ratings

The transistor, like other electronic devices, has limitations on its operation

The limitations are stated in the form of maximum ratings and normally given in Manufacturer’s data sheet

Typical maximum ratings include: maximum Collector Current, IC(max) Maximum Collector Emitter Voltage, VCE(max) Maximum Power Dissipation, PD(max)

Maximum Transistor Ratings

IC(max) Is the maximum collector

current a transistor can carry safely

VCE(max) Is the maximum voltage that

can safely be applied between collector and emitter

PD(max) Is the maximum power that a

transistor can dissipate safely

VCE

IC

PD = IC VCE

IC VCE ≤ PD(max)