Two-terminal device having a low Resistance to electric Current in one direction and a high Resistance in the reverse direction. Diodes are thus useful as Rectifier s, converting Alternating Current (AC) into Direct Current (DC). Although Electron Tube diodes were once common, almost all diodes today are Semiconductor devices. In general, current flowing through a diode is not proportional to
the Voltage between its terminals. When the voltage applied in the reverse direction
exceeds a certain value, a semiconductor diode breaks down and conducts heavily
in the direction of normally high resistance; this effect can be exploited to
regulate voltage. In another type of diode, the tunnel diode, the current
through the device decreases as the voltage is increased within a certain range; this
property, known as negative resistance, makes it useful as an Amplifier. Some diodes are sensitive to Light (see Photoelectric Cell; Photovoltaic Cell). A light-emitting diode (LED) produces light as current passes through it;
some LEDs can act as Lasers. A thermistor is a special semiconductor diode whose Conductivity increases with the diode Temperature.