Bipolar Junction Transistor (BJT) is a three layers and three terminal device having two PN junction. Bipolar junction transistor is made up of two N-type and a P-type material in case of NPN or two P-type and single N-type material making three two junctions. Transistors are mainly used as a switching device or as an amplifier.
Construction of BJT Transistor
A Bipolar Junction Transistor is three-layer device where every layer is connected by an electrical contact called Emitter E, Base B and Collector C. An NPN transistor can be formed by doping a sandwich of P-type between two N-types. Similarly, a PNP transistor is formed by doping a sandwich of N-type between two P-type materials. There are two PN junctions at a transistor first called base-emitter BE junction and the second one is called base-collector BC junction.
In either NPN or PNP, the emitter E is heavily doped, base B is lightly doped and the collector C is moderately doped. The outer layers are wide as compare to the central layer. The ratio of total size of transistor to the base is 150:1. The doping of the central layer is also less as compare to the outer layers by 10:1 ratio which increases the resistance of the layer by limiting the number of carriers. The transistor is called Bipolar due to the fact that transistor current is made up of holes and electron current.
Symbol of BJT Transistor
The symbol of transistor contain three terminal representing Emitter, Base and Collector. The Emitter terminal is differentiated by an arrow head from the Collector terminal. The direction of arrow head shows the type of transistor (NPN or PNP). The direction of arrow head shows the conventional current flow. In case PNP transistor the arrow points inside and for NPN the arrow points outside the transistor.
The transistor equivalent circuit contains two back to back diode. In case of NPN transistor, the anode terminals of both diode is shorted and named as base. Where in case of PNP transistor, the cathode terminal is shorted and named as base terminal of diode.
Working principles of BJT transistor
For a properly working transistor, a transistor should be connected to dc voltages at all the three terminals such that both of the PN junctions are correctly biased. The base emitter junction should be forward biased and base collector junction should be reversed biased. The working of NPN and PNP transistor is the same except that the role of electrons and holes, biasing voltage polarities and direction current flow.
As the BE junction is forward biased and BC junction is reversed biased so the BE depletion region become narrow and BC depletion region widens. The heavily doped emitter can easily inject electrons (in NPN) to baes and called emitter current IE. As the base is lightly doped, only few electrons recombines with hole at lightly doped base and become base current IB. Where the reset of the electrons cross the BC junction due the electric field developed by the positive and negative ions. It become collector current IC.