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 .