TRANSISTOR

TRANSISTORBackground and Introduction

A semiconductor device that Amplifies, Oscillates, or Switches the flow of current between two terminalsInvention of the Transistor

American physicists John Bardeen, Walter H. Brattain, and William Shockley(later jointly awarded a Nobel Prize)

It was also independently developed nearly simultaneously by Herbert Mataré and Heinrich Welker, German physicists working at Westinghouse Laboratory in Paris. First Transistor

Model, 1947

Transistor Material

The transistor is an arrangement of semiconductor materials that share common physical boundaries.Materials most commonly used are

Silicon gallium-arsenideand germaniumImpurities have been introduced by a process called “doping”

TYPES OF TRANSISTOR

BJTBi Polar Junction transistor

FETField effect transistor

BIPOLAR JUNCTION TRANSISTORS

BJTA BJT (Bipolar Junction Transistor) transistor has inside two similar semi conductive materials, and between them there is a third semi conductive material of different typeif the two similar materials are P and the middle one is N, then we have a P-N-P or PNP transistor.if the two materials are N and the middle one is P, then we have a N-P-N material or NPNEach transistor has 3 leads which we call base, collector and emitter, and we use the symbols b, c and e respectivelyThe symbol of the transistor has an arrow on the emitter.If the transistor is a PNP, then the arrow points to the base of the transistor, otherwise it points to the output.

Types Of BJTTwo basic types of bipolar junction transistor construction,

1. PNP 2. NPN,

which basically describes the physical arrangement of the P-type and N-type semiconductor materialsn-type semiconductors the impurities result in an excess of electrons, or negative chargesp-type semiconductors the material lead to a deficiency of electrons and therefore an excess of positive charge carriers or “holes.”current regulating devices that control the amount of current flowing through them

NPN and PNP Transistor

BJT

Basic construction The BJT is a three terminal device that produce two PN junctionsEmitter ( E )Base ( B )Collector ( C )

Principle of operation of the two transistor types PNP and NPNBiasingpolarity of the power

supply for each typeEmitter

Base

Collector

EMITTERIt is highly dopedTo inject a large number of charge carriersMain function is to supply the majority

carriers to the base.It is always forward biased with respect to

base

BASE

It is a middle section of a transistorIt is lightly dopedSo that most of the charge carriers pass to

the collector.It controls flow of chargesIt forms two PN junctions with Emitter and

Collector

COLLECTORIT IS situated opposite to the emitterIt is always reversed biasedSo that it can collect the majority carriers Size of collector is larger than emitterIts doping level is in the middle of base and

emitter

NPN TRANSISTORIt is constructed by two N type and One P

type material.Emitter and collector are of N type materialBase is of P type materialIt consist of two PN junctionsSandwiching a P-type layer between two n-

type layers.

OPERATION OF NPN TRANSISTOR

ForwardbackwardC B E

•The base-emitter diode is forward biased•The base-collector diode is reverse biased•VBE injects the electron to the Emitter.•Emitter is highly doped•Base is lightly doped•Collector creates electrostatic field which Attracts the electrons•95 to 99% electrons diffuses in collector region

VCB VBE

Transistor operation

With no power applied to the transistor areas

There are two depletion zones between the two P-N contacts.

Power source Connected b/w base and collector in reverse-bias

With the positive of the source connected to the collector and the negative to the base.

The depletion zone of the P-N contact between the base and the collector will be widened.

A slight current will flow

within this contact (due to impurities).

This current is the reverse contact

current symbol ICBO

Transistor operation

Voltage supply between the emitter and the base in forward bias

With the positive of the source connected to the base and the negative connected to the emitter.

The depletion zone between the emitter and the base will be shortened

current (electrons) will flow when the voltage exceeds a specific level.

This level depends on the material that the transistor is made of.

Some of the electrons that go through the e-b depletion zone,

will re-connect with holes in the base.

This is the base current IB symbol for reference.

In real life, this current is at the scale

of micro-amperes (μA ):

Cont’dMost of the electrons will flow through the base (due to spilling) and will be directed to the collector. When these electrons reach the depletion area between the base and the collector, they will experience a force from the electric field which exists in this zone,The electrons will pass through the depletion zone. The electrons will then re-connect with holes in the collector. The re-connected holes will be replaced with holes coming from the base-collector power supply (VCC). The movement of these holes equals to a movement of electrons in the opposite direction, from the collector to the supply. In other words, the current that flows to the emitter will be divided into the small base current and the larger collector current: IE = IB + IC

Cont’d

Generally, the number of electrons that arrive at the collector is the 99% of the total electrons, and the rest 1% causes the base current.

At the collector, except the electrons that come from the emitter, there is also the reverse current from the base-collector contact

Both currents flow at the same direction, so they are added

IC' = IC + ICBO

Transistor Parameters and Ratings

The ratio of the dc collector current (IC) to the dc base current (IB) is the dc beta (bDC).

bDC is called the gain of a transistor:

bDC = IC/IB Typical values of bDC range from less than 20 to 200 or higher.

bDC is usually designated as an equivalent hybrid (h) parameter:

hFE = bDC

The ratio of the collector current (IC) to the dc emitter current (IE) is the dc alpha (aDC). This is a less-used parameter than beta.

aDC = IC/IETypical values range from 0.95 to 0.99 or greater.

aDC is always less than 1.

This is because IC is always slightly less than IE by the amount of IB.

Cont’dThere are a number of standard parameters that are used to define

the performance of a transistor. Some of them are given below

Type number

Case

Material

Polarity

VCEO Collector emitter voltage with base open circuit

VCBO Collector base voltage with the emitter open circuit

VEBO Emitter base voltage with collector open circuit I

C Collector current

ICM Peak collector current

IBM Peak base current

PTOT Total power dissipation

Tamb Ambient temperature T

Cont’d

Stg Storage temperature.

ICBO Collector base cut-off current

IEBO Emitter base cut-off current

hFE Forward current gain

VCEsat Collector emitter saturation voltage

VBEsat Base emitter saturation voltage

Cc Collector capacitance

Ce Emitter capacitance

Ft Frequency Transition

Introduction to AmplifiersThe BJT is an excellent amplifier when biased in the forward-active region.The FET can be used as an amplifier if operated in the saturation region.In these regions, the transistors can provide high voltage, current and power gains.DC bias is provided to stabilize the operating point in the desired operation region.The DC Q-point also determines

The small-signal parameters of the transistorThe voltage gain, input resistance, and output resistanceThe maximum input and output signal amplitudesThe overall power consumption of the amplifier