presentation on bipolar junction transistor 1department of ece / eee

Presentation On

BIPOLAR JUNCTION TRANSISTOR

1Department of ECE / EEE

Introduction

• A bipolar junction transistor (BJT) is a three terminal semiconductor

device in which the operation depends on the interaction of both

majority and minority carriers and hence the name Bipolar.

• The BJT is analogous to a vacuum triode and is comparatively

smaller in size.

• It is used in amplifier and oscillator circuits, and as a switch in

digital circuit.

• It has wide applications in computers, satellites and other modern

communication system.


Construction

• The BJT consists of a silicon (or germanium) crystal

in which a thin layer of N-type silicon is sandwiched

between two layers of P-type silicon.

• This transistor is referred to as PNP. Alternatively, in

a NPN transistor, a layer of P-type material is

sandwiched between two layers of N-type material.


NPN and PNP

N NP

B

E C P PN

B

E C

NPN PNP


• The three portions of the transistor are Emitter, Base and Collector,

shown as E, B, and C, respectively.

• The arrow on the emitter specifies the direction of current flow

when the EB junction is forward biased.

• Emitter is heavily doped so that it can inject a large number of

charge carriers into the base.

• Base is lightly doped and very thin.

• It passes most of the injected charge carriers from the emitter into

the collector.

• Collector is moderately doped.


Symbol

B

E C

Symbol of NPN transistor Symbol of PNP transistor

B

EC


Transistor biasing

• The following figure shows, usually the emitter-base

junction is forward biased and collector-base junction is

reverse biased.

• Due to the forward bias on the emitter-base junction, an

emitter current flows through the base into the collector.

• Through the collector-base junction is reverse biased,

almost the entire emitter current flows through the collector

circuit.


NPN Transistor biasing

Department of ECE / EEE 8

PNP Transistor biasing


Active-mode NPN transistors in circuits


• The diagram opposite is a schematic representation of an NPN transistor

connected to two voltage sources. To make the transistor conduct

appreciable current (on the order of 1 mA) from C to E, VBE must be above

a minimum value sometimes referred to as the cut-in voltage.

• The cut-in voltage is usually about 600 mV for silicon BJTs at room

temperature but can be different depending on the type of transistor and its

biasing.

• This applied voltage causes the lower P-N junction to 'turn-on' allowing a

flow of electrons from the emitter into the base. In active mode, the electric

field existing between base and collector (caused by VCE) will cause the

majority of these electrons to cross the upper P-N junction into the

collector to form the collector current IC. Department of ECE / EEE 11

• The remainder of the electrons recombine with holes, the majority carriers

in the base, making a current through the base connection to form the base

current, IB.

• As shown in the diagram, the emitter current, IE, is the total transistor

current, which is the sum of the other terminal currents (i.e., IE = IB + IC ).

• The arrows representing current point in the direction of conventional

current – the flow of electrons is in the opposite direction of the arrows

because electrons carry negative electric charge.

• In active mode, the ratio of the collector current to the base current is

called the DC current gain.


• This gain is usually 100 or more, but robust circuit

designs do not depend on the exact value (for example

- op-amp).

• The value of this gain for DC signals is referred to as

hFE, and the value of this gain for AC signals is referred

to as hfe.

• However, when there is no particular frequency range

of interest, the symbol β is used


Active-mode PNP transistors in circuits


• The diagram opposite is a schematic representation of a PNP transistor

connected to two voltage sources. To make the transistor conduct

appreciable current (on the order of 1 mA) from E to C, VEB must be above

a minimum value sometimes referred to as the cut-in voltage.

• The cut-in voltage is usually about 600 mV for silicon BJTs at room

temperature but can be different depending on the type of transistor and its

biasing.

• This applied voltage causes the upper P-N junction to 'turn-on' allowing a

flow of holes from the emitter into the base.

• In active mode, the electric field existing between the emitter and the

collector (caused by VCE) causes the majority of these holes to cross the

lower P-N junction into the collector to form the collector current IC. Department of ECE / EEE 15

• The remainder of the holes recombine with electrons, the majority

carriers in the base, making a current through the base connection to

form the base current, IB.

• As shown in the diagram, the emitter current, IE, is the total

transistor current, which is the sum of the other terminal currents

(i.e., IE = IB + IC).

• The arrows representing current point in the direction of

conventional current – the flow of holes is in the same direction of

the arrows because holes carry positive electric charge. In active

mode, the ratio of the collector current to the base current is called

the DC current gain. Department of ECE / EEE 16

• This gain is usually 100 or more, but robust

circuit designs do not depend on the exact value.

• The value of this gain for DC signals is referred to

as hFE, and the value of this gain for AC signals is

referred to as hfe.

• However, when there is no particular frequency

range of interest, the symbol β is used


Types of configuration

• When a transistor is to be connected in a circuit, one terminal is used

as an input terminal, the other terminal is used as an output terminal

and the third terminal is common to the input and output.

• Depending upon the input, output and common terminal, a transistor

can be connected in three configurations.

– Common base configuration

– Common emitter configuration

– Common collector configuration


CB Configuration

• This is also called grounded base

configuration. In this configuration, emitter is

the input terminal, collector is the output

terminal and base is the common terminal.


NPN common-base circuit


CE Configuration

• This is also called grounded emitter

configuration. In this configuration, base is

the input terminal, collector is the output

terminal and emitter is the common terminal.


NPN common-emitter circuit


CC Configuration

• This is also called grounded collector

configuration. In this configuration, base is the

input terminal, emitter is the output terminal

and collector is the common terminal.


NPN common-collector circuit


Comparison of CB CE and CC configuration

S .No Characteristics CB CE CC

1 Input impedance Low Medium High

2 Output impedance High medium low

3 Current gain Low High High

4 Voltage gain High High Unity

5 Power gain Medium High Low

6 Phase reversal No Yes No

7 application AF amplifiers Voltage & power

amplifiers

Impedance matching


Transistor as a switch• When a transistor is used as a switch, it is usually required

to be brought alternatively in the saturation and cut-off

conditions.

• When in saturation condition, it should carry heavy current,

so the voltage drop across the transistor is as near to zero as

possible. It may be considered as “closed switch”.

• When in cut-off condition, it should carry almost no current

so that it may be considered to be an “open switch”.


Applications

• The BJT remains a device that excels in some applications, such as

discrete circuit design, due to the very wide selection of BJT types

available, and because of its high transconductance and output

resistance compared to MOSFETs.

• The BJT is also the choice for demanding analog circuits, especially

for very-high-frequency applications, such as radio-frequency

circuits for wireless systems. Bipolar transistors can be combined

with MOSFETs in an integrated circuit by using a BiCMOS process

of wafer fabrication to create circuits that take advantage of the

application strengths of both types of transistor.


Temperature sensors

• Because of the known temperature and current

dependence of the forward-biased base–emitter

junction voltage, the BJT can be used to measure

temperature by subtracting two voltages at two

different bias currents in a known ratio


Logarithmic converters

• Because base–emitter voltage varies as the log of

the base–emitter and collector–emitter currents, a

BJT can also be used to compute logarithms and

anti-logarithms.

• A diode can also perform these nonlinear

functions, but the transistor provides more circuit

flexibility.


Vulnerabilities

• Exposure of the transistor to ionizing radiation causes radiation

damage. Radiation causes a buildup of 'defects' in the base region

that act as recombination centers.

• The resulting reduction in minority carrier lifetime causes gradual

loss of gain of the transistor.

• Power BJTs are subject to a failure mode called secondary

breakdown, in which excessive current and normal imperfections in

the silicon die cause portions of the silicon inside the device become

disproportionately hotter than the others.


• The doped silicon has a negative temperature coefficient, meaning

that it conducts more current at higher temperatures.

• Thus, the hottest part of the die conducts the most current, causing

its conductivity to increase, which then causes it to become

progressively hotter again, until the device fails internally.

• The thermal runaway process associated with secondary

breakdown, once triggered, occurs almost instantly and may

catastrophically damage the transistor package.


The End

M.S.P.V.L Polytechnic College,

Department of ECE,

Pavoorchatram.


presentation on bipolar junction transistor 1department of ece / eee

Documents

collectorbase junction

emitterbase junction

base current

type of transistor

base connection

collector circuit

collector current ic

eb junction