SEU2012 Transistor Introduction

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TRANSISTOR - Introduction

BIPOLAR JUNCTION TRANSISTOR

(BJT)

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Introduction Beside diodes, the most popular semiconductor devices is

transistors. Eg: Bipolar Junction Transistor (BJT)

Transistors are more complex and can be used in many ways

Most important feature: can amplify signals and as switch

Amplification can make weak signal strong (make sounds louderand signal levels greater), in general, provide function called Gain

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BJT is bipolar because both holes (+) and electrons (-) will take part in thecurrent flow through the device

N-type regions contains free electrons (negative carriers)

P-type regions contains free holes (positive carriers)

2 types of BJT NPN transistor

PNP transistor

The transistor regions are:

Emitter (E) send the carriers into the base region and then on to the

collector

Base (B) acts as control region. It can allow none,some or manycarriers to flow

Collector (C) collects the carriers

Transistor Structure

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P

P

P

N

N

N

PNP and NPN transistor structure

IB(A) IB(A)

Ic(mA)

IE(mA) IE(mA)

IC(mA)

Arrow shows the current flows

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N

P

NThe collector is lightly doped. C

The base is thin and

is lightly doped.B

The emitter is heavily doped. E

NPN Transistor Structure

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Transistor configuration

Transistor configurationis a connection of transistor to get varietyoperation.

3 types of configuration:

Common Collector.

Common Base. Common Emitter

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Common-Collector Configuration The input signal is applied to the base terminal and the output is

taken from the emitter terminal.

Collector terminal is common to the input and output of the circuit

Input BC

Output EC Input = Output

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Common-Base Configuration Base terminal is a common point for input and output.

Input EB

Output CB

Not applicable as an amplifier because the relation between input currentgain (IE) and output current gain (IC) is approximately 1

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Common-Emitter Configuration Emitter terminal is common for input and output circuit

Input BE

Output CE

Mostly applied in practical amplifier circuits, since it provides good voltage,current and power gain

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The C-B junction

is reverse biased.

N

P

N

NPN Transistor Bias

C

B

E

No current flows.

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The B-E junction

is forward biased.

N

P

N

NPN Transistor Bias

C

B

E

Current flows.

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When both junctions

are biased....

N

P

N

NPN Transistor Bias

C

B

E

Current flowseverywhere.

Note that IB is smaller

than IE or IC.

IC

IB

IE

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N

P

N C

B

E

Although IB is smallerit controls IE and IC.

IC

IB

IE

Note: when theswitch opens, all

currents go to zero.

Gain is something smallcontrolling something large

(IB is small).

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N

P

C

B

E

IC = 99 mA

IB = 1 mA

IE = 100 mA

b = ICIB

The current gain frombase to collector

is called b.

99 mA1 mA

= 99

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N

P

C

B

E

IC = 99 mA

IB = 1 mA

IE = 100 mA

IE = IB + IC

99 mA= 1 mA += 100 mA

Kirchhoffscurrent law:

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C

B

E

IC = 99 mA

IB = 1 mA

IE = 100 mA

In a PNP transistor,holes flow from

emitter to collector.

Notice the PNP

bias voltages.

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Emitter

Base

Collector

Memory aid: NPN

means Not Pointing iN.

EBC

NPN Schematic Symbol

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Collector

Base

Emitter

EBC

PNP Schematic Symbol

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Recall: NPN and PNP Bias

Fundamental operation of pnp transistor and npn transistor is similar except for:

role of electron and hole,

voltage bias polarity, and

Current direction

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I-V Characteristic for CE configuration : Input

characteristic Input characteristic: input

current (IB) against inputvoltage (VBE) for several outputvoltage (VCE)

From the graph IB = 0 A VBE < 0.7V (Si) IB = value VBE > 0.7V (Si)

The transistor turned on whenVBE = 0.7V

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I-V Characteristic for CE configuration : Output

characteristic Output characteristic: output

current (IC) against output

voltage (VCE) for several

input current (IB) 3 operating regions:

Saturation region

Cut-off region

Active region

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Saturation region in which both junctions are forward-biased and ICincrease linearly with VCE

Cut-off region where both junctions are reverse-biased, the IB is verysmall, and essentially no I

C

flows, IC

is essentially zero with increasing VCE

Active region in which the transistor can act as a linear amplifier, wherethe BE junction is forward-biased and BC junction is reverse-biased. ICincreases drastically although only small changes of IB.

Saturation and cut-off regions areas where the transistor can operate as a

switch

Active region area where transistor operates as an amplifier

I-V Characteristic for CE configuration :

Output characteristic

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Current Relationships

Relations between IC and IE : = IC

IE Value of usually 0.9998 to 0.9999, 1 Relations between IC and IB :

= IC @ IC= IBIB

Value of usually in range of 50 400 The equation, IE =IC + IB can also written in

IC= IBIE

= IB

+ IB

=> IE

= ( + 1)IB The current gain factor , and is:

= @ = . + 1 - 1