Bias is direct current ( DC ) deliberately made to flow, or DC voltage deliberately applied, between two points for the purpose of controlling a circuit . In a bipolar transistor , the bias is usually specified as the direction in which DC from a battery or power supply flows between the emitter and the base. In a field-effect transistor ( FET ), the bias is DC voltage from a battery or power supply deliberately applied between the source and the gate.
Certain bias conditions are used for specified purposes. In a semiconductor P-N junction, forward bias occurs when the P-type material is positive with respect to the N-type material; in reverse bias, the P-type material is negative with respect to the N-type material. When two electrodes are at the same potential, they are said to be at zero bias.
A semiconductor junction normally conducts when it is forward-biased beyond a certain point called the forward breakover. The junction normally does not conduct when it is reverse-biased or is at zero bias. Because of the way semiconductor devices behave as the bias varies, they can be used as amplifier s, oscillator s, and high-speed digital switch es.