Power Transistor

A power  transistor  is a three layer  PNP  or NPN  semiconductor device  having  two  junctions. Three  terminals of power  transistors are collector emitter  and bas. The  current  flow in  the device due to the movement of both  electronic  and holes  hence  named  bipolar. Early transistor was made from germanium but most modern power transistors are made from silicon. For very  high  speed  applications a  significant  minority are also  now  made form  gallium arsenide.

                                                                 Figure NPN transistor Basic structure symbol

The  structure  symbol  of NPN  transistor is  shown in figure 1.4  most bipolar transistors used  today  are NPN  because electron  mobility is higher than hole  mobility  in semiconductors which  allows  greater current  and faster  operation. NPN transistors consist  of a layer of p doped  semiconductor  between two N doped  layers. A small  current  entering the base in common  emitter  mode is  implied in the collector output.

                                                                       Figure PNP Transistor basic structure symbol

It  means an NPN  transistor  is ON  when its  pulled  high  relative to the emitter. The arrow in the NPN  transistor  symbol  shows the  direction of the  conventional  current  flow.

PNP transistor is shown  in figure  PNP transistors  consists of a layer of  N doped  semiconductor between  two layers  of p doped  material. A small  current  leaving  the base in  common  emitter mode  is amplified in the collector output. It  means  a PNP  transistor is ON when  its base  is pulled  low  relative to the emitter. The  arrow  in the PNP  transistor  shown the  direction  of the conventional current flow.

In power  transistor  the base  physically  located between  the emitter and the collector and  is made  from light  doped high resistivity  material. A cross  section  view of a BJT indicates that  the collector base junction has  a much  lager area than the emitter vase  junction. The  reverse bias  voltage to be applied  before the collector base junction breaks  down. The collector base   junctions is reverse biased  in normal  operation. The  reason  the emitter is heavily doped  is  to increase the emitter injection efficiency. It  means the ration of  carriers injected  by the emitter to those injected by the base. For high current gain most of the  carriers injected into the  emitter base junction must come from the emitter. Power  transistors  has low  current gain.  It requires  continuous signal to the base during  ON  state  condition. Power  transistor cannot  withstand reverse voltage and application  is limited to DC  voltage for inverters and choppers. Power transistors  are medium speed switches used for  power supplies of a wide  variety of  electronic  equipments  ranging from computer peripherals to industrial equipment .