Bipolar junction transistor - Introduction

INTRODUCTION :

A bipolar junction transistor (BJT or bipolar transistor) is a type of transistor that relies on the contact of two types of semiconductor for its operation. BJTs can be found either as individual discrete components, or in large numbers as parts of integrated circuits.

BJTs come in two types, PNP and NPN, based on the doping types of the three main terminal regions.

In typical operation, the base–emitter junction is forward biased, which means that the p-doped side of the junction is at a more positive potential than the n-doped side, and the base–collector junction is reverse biased. 

In an NPN transistor, when positive bias is applied to the base–emitter junction, the equilibrium is disturbed between the thermally generated carries and the repelling electric field of the n-doped emitter depletion region. The electrons injected from he emitter into the base region, diffuse through the base from the region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called 'minority carriers' because the base is doped p-type, which makes holes the 'majority carriers' in the base.