* The transistor operate at low voltage with sufficient amount of current. Hence they are known as current operated devices . * The power handling capacity of a semiconductor is relatively lower than that of valves . * In complementary amplifier circuit — one PNP &one NPN  transistor in series is used * The frequency on which the gain of a transistor is reduced to 70.7 %  of its value at low frequency is called alpha cut off frequency of that transistor. * UJT — uni junction  transistor  ( Voltage control device )       - It has three terminal (emitter , base 1 , base 2 )      -   It has stable negative resistance characteristics       - It is used at low frequency, relaxisation oscillator & timer      - It is also called double base transistor  * SCR — Silicon controlled rectifier ( Unidirectional switch )     - It has 4 Layer , 3 junction , 3 terminals...