Transistor is a device that is extension of a semiconductor diode. These devices have brought an end to power hungry vacuum tube amplification and further amplification. On the day of 23rd December 1947, William Shockley introduced this device at the Bell Laboratories. Transistors are a combination of two similar types and one different type of semiconductor. Suppose this combination is npn (the other being pnp). The two n types are conventionally called emitter and collector. The remaining type being the base.