Transistors and amplifiers

Transistor is a three terminal two junction semiconductor device. Actually for manufacturing a transistor first step is to select an intrinsic semiconductor, say, pure silicon. The silicon substrate is treated with the process, doping. Doping is nothing but adding of pentavalent and trivalent impurities to pure crystal. If we add pentavalent impurity, that will result in n-type semiconductor. If we add trivalent impurity, that will result in p-type impurity. Two p-type semiconductor and one n-type semiconductor are sandwiched to form p-n-p transistor. Two n-type semiconductor and one p-type semiconductor are sandwiched to form n-p-n transistor. Usually common emitter configuration yields high voltage gain, current gain and good characteristics. Hence this configuration is used for making amplifiers.