BIPOLAR TRANSISTORS

Before  transistor,  the  amplification  was  achieved  by  using  vacuum  tubes  as  an amplifier. Now-a-days vacuum tubes are replaced by transistors because of  following:’ advantages of transistors.

1. Low operating voltage
2. Higher efficiency
3. Small size and ruggedness and
4. Does not require any filament power

• A  Bipolar  Junction  Transistor  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 vacuum triode and is comparatively smaller in size. It is used in amplifiers and oscillator circuits and as a switch in digital circuits.

• It has wide application s in computer, satellites and other modern communication systems.

• There  are  two  basic  types  of  transistors  unipolar  junction  transistor  and bipolar junction transistor.

• In Unipolar transistor the current conduction is only due to one type carriers, majority carriers.

• The  current  conduction in  bipolar  transistor  is  because  of  both  the  types  of charge  carriers,  holes  and  electrons.  Hence  this  is  called  bipolar  junction transistor, hereafter referred to as BJT.

The BJTs are of two basic types

1.n-p-n type

2.p-n-p type

JUNCTION TRANSISTOR

• The BJT consists of a Silicon or Germanium crystals 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.

• 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 to the collector.

• Collector is moderately doped.

STANDARD TRANSISTOR SYMBOLS

TWO-DIODE TRANSISTOR ANALOGY

• A  transistor has  two p-n  junctions.  One  junction is  between the  emitter  and the  base,  and  is  called  the  emitter  base  junction,  or  simply  the  emitter junction JE.

• The  other  junction  is  between  the  base  and  the  collector,  and  is  called collector-base junction, or simply junction diodes connected back-to-back

• Another   important   point   is   that,   the   emitter   area   in   the   transistor   is considerably  smaller  than  the  collector  area.  This  is  because  the  collector region has to handle more power than the emitter and more surface area is  required for heat dissipation.