Transistor, which is a type of semiconductor device, is used both as a switch, and an amplifier, to control the flow of electronic signals. Switching means choosing between available options, while amplification means increasing the signal strength. A transistor can act as a switch to turn on and off the current flow, and also adjust voltage for efficient use of power in electronic devices. As an amplifier, an electronic transistor can amplify weak signals, such as the output from microphone, to the levels that can drive a loudspeaker. A typical transistor is composed of three layers of semiconductors, more specifically terminals, to make a connection to an external circuit. A voltage or current applied to a pair of terminals controls the flow of current to other pair of terminals. An electronic transistor consists of three types of terminals viz.