Bipolar junction transistors are three-terminal semiconductor devices (collector, emitter, base) in which the voltage-current characteristics of the output port (collector,emitter) are controlled by the current flowing into the input port (base, emitter). BJT transistors may be n-type (npn transistor) or p-type (pnp transistor) both types having similar characteristics except for the terminal polarization. As with most transistors they may be used as amplifiers (linear region) or switches (saturation region) depending on the mode of operation.