a semiconductor device that allows electric current to pass in one direction, but not in the other. Diodes are formed by joining two types of doped semiconductors: P type, with a deficiency of free electrons (and an excess of holes), and N type, with a surplus of free electrons. (See semiconductor). The place where the two regions are joined is called the junction.
Electrons can flow from N-type to P-type material, but not the other way. The diode is said to be forward-biased when the voltage across it is in the right direction to make it conduct, and reverse-biased when the voltage is applied in the opposite direction. The diode is forward biased when a negative voltage is applied to the N region and a positive voltage to the P region; then free electrons in the N region are driven to the junction where they combine with holes from the P region. If a positive voltage is applied to the N region, electrons are pulled away from the junction and no current can flow (the diode is reverse biased).
