In addition, most receiving sets had vacuum tubes for amplification that could easily have the thermionic diodes included in the tube (for example the 12SQ7 double diode triode), and vacuum-tube rectifiers and gas-filled rectifiers were capable of handling some high-voltage/high-current rectification tasks better than the semiconductor diodes (such as selenium rectifiers) that were available at that time. Until the 1950s, vacuum diodes were used more frequently in radios because the early point-contact semiconductor diodes were less stable. Thermionic ( vacuum-tube) diodes and solid-state (semiconductor) diodes were developed separately, at approximately the same time, in the early 1900s, as radio receiver detectors.
Tunnel, Gunn and IMPATT diodes exhibit negative resistance, which is useful in microwave and switching circuits.ĭiodes, both vacuum and semiconductor, can be used as shot-noise generators.įurther information: Vacuum tube § History and development For example, diodes are used to regulate voltage ( Zener diodes), to protect circuits from high voltage surges ( avalanche diodes), to electronically tune radio and TV receivers ( varactor diodes), to generate radio-frequency oscillations ( tunnel diodes, Gunn diodes, IMPATT diodes), and to produce light ( light-emitting diodes). These techniques are used to create special-purpose diodes that perform many different functions. Semiconductor diodes in the forward direction also need to surpass a threshold voltage, or cut-in voltage, before being able to conduct electricity.Ī semiconductor diode's current–voltage characteristic can be tailored by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture. And its high resistance to current flowing in the reverse direction suddenly drops to a low resistance when the reverse voltage across the diode reaches a value called the breakdown voltage. For instance, a diode's forward-direction voltage drop varies only a little with the current, and is more so a function of temperature this effect can be used as a temperature sensor or as a voltage reference. However, diodes can have more complicated behavior than this simple on–off action, because of their nonlinear current-voltage characteristics. As rectifiers, diodes can be used for such tasks as extracting modulation from radio signals in radio receivers. This unidirectional behavior is called rectification and is used to convert alternating current (ac) to direct current (dc). As such, the diode can be viewed as an electronic version of a check valve. The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking it in the opposite direction (the reverse direction). Diodes may be combined with other components to form logic gates. A common variant of a diode is a light emitting diode, which is used as electric lighting and status indicators on electronic devices. Īmong many uses, diodes are found in rectifiers to convert AC power to DC, demodulation in radio receivers, and can even be used as temperature sensors. Today, most diodes are made of silicon, but other semiconducting materials such as gallium arsenide and germanium are also used. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Semiconductor diodes were the first semiconductor electronic devices.
The filament itself may be the cathode, or more commonly (as shown here) used to heat a separate metal tube which serves as the cathode.Ī diode is a two- terminal electronic component that conducts current primarily in one direction (asymmetric conductance) it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.Ī diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate.Ī semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals.
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