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Diodes are double-terminal electronic components that conduct current mainly in one direction (asymmetric conductance); It has low resistance in one direction (ideally zero) and high resistance in the other direction (ideally infinite). A diode vacuum tube or thermoelectron diode is a vacuum tube with two electrodes, a heated cathode and a plate in which electrons can flow from the cathode to the plate in only one direction. A semiconductor diode, the most commonly used type today, is a crystalline semiconductor material with a pn junction connected to two electrical terminals.

The most common function of a diode is to allow current to pass in one direction (called the forward direction of the diode), while blocking it in the opposite direction (the reverse). In this way, the diode can be viewed as an electronic version of the return valve. This one-way behavior is called rectification and is used to convert alternating current (ac) to direct current (dc). Rectifiers, in the form of diodes, can be used for tasks such as extracting modulation from radio signals in a radio receiver.

However, due to the nonlinear current-voltage characteristics of the diode, its behavior can be more complex than this simple switching action. A semiconductor diode conducts electricity only when there is a threshold voltage or an input voltage in the forward direction (the diode is said to be in the forward biased state). The voltage drop at both ends of the forward-biased diode varies only slightly with the current and is a function of temperature. This effect can be used as a temperature sensor or reference voltage. In addition, when the reverse voltage at both ends of the diode reaches a value called the breakdown voltage, the high resistance of the diode to the reverse flow suddenly drops to a low resistance.

The current-voltage characteristics of semiconductor diodes can be customized by selecting the semiconductor material and introducing doping impurities in the material during the manufacturing process. These techniques are used to create specialized diodes that perform many different functions. For example, diodes are used to regulate voltage (Zener diodes), protect circuits from high-voltage surges (avalanche diodes), electronically tune radio and television receivers (varator diodes) to produce RF oscillations (tunnel diodes), Gunn diodes, IMPATT diodes, and produce light (light-emitting diodes). Tunnel diodes, Gunn diodes, and IMPATT diodes have negative resistance, which is useful in microwave and switching circuits.

Both vacuum diodes and semiconductor diodes can be used as scatter noise generators.