Jun 15, 2016




It is one of the most important semiconductor devices. It was invented by William Shockey in the year 1947. The term transistor is derived by contracting the word transfer resistor.
The transfer of current here takes place from a low resistance to high resistance circuit. The transistor created a revolution in the field of electronic leading to other inventions like integrated circuits IC, opto–electronic devices and microprocessorss. Transistor has replaced vacuum tubes in almost all applications.
The basic function of a transistor is to amplify a weak signal, used in analogue circuits. It is also used as a switch or a gate in digital circuit.

Bipolar Junction Transistor

An electronic circuit device that consists of two P-N junctions formed by sand wiching either P-type or N-type semiconductor between a pair of opposite layers is known as transistor.
  • The junction transistor is a three layer (terminal) semiconductor device consisting of either two p-type and one N-type regions or two N-type and one P-type regions. These formations are called as PNP transistor or NPN transistor.
  • Thus there are two types of junction transistors. The emitter and base P-N regions from the diode while the collector and the base P-N regions from the collector diode.
  • In general the transistor can be regarded as two P-N junctions connected back to back is series as shown in the fig. The middle section a very thin layer called base acts as the most important region in the function of a transistor.
  • The abbreviation BJT, from bipolar junction transistor, is often applied to this three terminal device. The term bipolar reveal the fact that holes and electrons participate in current flow.

Transistor terminals

A transistor has three doped regions known as emitter, base and collector. All these regions are provided with the terminals.


The left hand region of transistor shown in the fig is called the emitter and it is heavily doped than the other two regions of a transistor.
Its main function is to supply majority carriers either electrons or holes to the base. The emitter is always forward biased with respect to base so that it can supply large number of majority carriers.


The narrow central region of the transistor is called the base. The thickness of this region is very thin and is very lightly doped. As the base emitter junction is forward biased, it offers low resistance for the emitter current.


The right hand region of the transistor as shown in the figure is called collector. It is moderately doped. The collector region is always reverse biased with respect to the base. The collector region of physically larger than the emitter region because it has no dissipates much greater power.
schematic symbols of pnp transistor npn transistor
The junction between emitter and base regions as emitter-base junction. Similarly, the junction between collector and base regions is known as collector base junction. The schematic symbol of PNP and NPN transistor are shown in the figure.
The arrow in the circuit symbol of the transistor indicates the direction of conventional current flow and can be used to differentiate and NPN transistor from a PNP transistor. It is represented at the emitter terminal.


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