Transistor is a device formed of semiconductor material that amplifies a signal or opens or closes a circuit. Invented in the year 1947 at Bell Labs, transistors have turn into the key ingredient of all digital circuits, by including computers. Today's microprocessors contain tens of millions of microscopic transistors. Prior to invention of transistors, digital circuits were formed of vacuum tubes, which had several disadvantages. They were much larger, needed more energy, dissipated more heat, and were more prone to failures. It is secure to say that without the invention of transistors, calculating, as we know it today, would not be possible. Bipolar Junction Transistor (BJT) and Field Effect Transistor (FET) are one of the most significant device components for today's semiconductor industries. BJT are bipolar semiconductor devices (holes and electrons are both charge carriers for such devices). While, FET patented by Lilienfeld in the year of 1930, is a unipolar device (using either electron or holes as a charge carrier).