Transistor is an active component and that is establishing in all over electronic circuits. They are used as amplifiers and switching apparatus. As the amplifiers, they are used in high and low level, frequency stages, oscillators, modulators, detectors and in any circuit need to perform a function. In digital circuits they are used as switches. There are a huge number of manufacturers approximately the world who produces semiconductors (transistors are members of this family of apparatus), so there are exactly thousands of different types. There are low, medium and high power transistors, for functioning with high and low frequencies, for functioning with very high current and or high voltages.