TRANSISTOR

TRANSISTOR

A transistor is a semiconductor device used to

amplify and switch electronic signals and electrical

power.

The transistor is the fundamental building block of

modern electronic devices

There are two types of transistors

Bipolar Junction Transistor Field-effect Transistor

Bipolar Junction Transistor

Bipolar junction transistor (BJTs) is made by three doped semiconductor region

separated by two pn junctions sown in figure 1.

The term bipolar refers to use both the holes and electrons as carriers in the

transistors structure.

There are two types of BJTs

npn (two n-regions separated by a p region)

pnp (two p-regions separated by a n region)

Figure-1 Basic BJTs construction

BASIC BJTs CONSTRUCTION

The pn-junction joining the base region and emitter region is called base

emitter junction

The pn-junction joining the base region and collector region is called base-

collector junction

A wire lead is connected to each of three region. These leads are labeled

E, B and C for emitter, base and collector respectively as shown in figure-1

(b).

The base region is highly doped as compare to heavily doped emitter and

moderately doped collector regions

Figure-2 standard BJTs symbols

BASIC BJTS OPERATION

Base- emitter junction is forward bias Base collector junction is reverse bias

BJTs Currents and Voltages

Voltage across RB is

By Ohms law

The value of IB is then

Voltage at collector

Voltage across RC

Then the voltage at collector

The voltage at reverse bias collector base junction

DC Beta(βDC)b

The ratio of collector current to base current is the current gain of transistors and is given by

Typical values of βDC ranges from 20 to 200.

The ratio of collector current to emitter current is the dc αDC and is given by

The values of αDC range from 0.95- 0.99 but always less than 1.

Dc Alpha (αDC)

Problem-1

Determine IB, IC , IE, VBE, VCE, and VCB in the following circuit (βDC = 150)

Solution-1

Problem-2

Determine IB, IC , IE, VBE, VCE, and VCB for the following circuit values (βDC = 90)

RC = 220 Ω

RB = 22 k Ω

VCC = 9V

VBB = 6V

Problem-3 Determine values of IE and βDC for a transistor whereIC = 3.65 mA IB = 50 µA