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As such, the diode can be viewed as an electronic version of a check valve. This unidirectional behavior is called rectification , and is used to convert alternating current ac to direct current dc. Forms of rectifiers , diodes can be used for such tasks as extracting modulation from radio signals in radio receivers.
However, diodes can have more complicated behavior than this simple on—off action, because of their nonlinear current-voltage characteristics. The voltage drop across a forward-biased diode varies only a little with the current, and is a function of temperature; this effect can be used as a temperature sensor or as a voltage reference. Diodes, both vacuum and semiconductor, can be used as shot-noise generators.
History[ edit ] Thermionic vacuum-tube diodes and solid-state semiconductor diodes were developed separately, at approximately the same time, in the early s, as radio receiver detectors. Fleming patented the first true thermionic diode, the Fleming valve , in Britain on November 16,  followed by U. Patent , in November Throughout the vacuum tube era, valve diodes were used in almost all electronics such as radios, televisions, sound systems and instrumentation.
They slowly lost market share beginning in the late s due to selenium rectifier technology and then to semiconductor diodes during the s.
Today they are still used in a few high power applications where their ability to withstand transient voltages and their robustness gives them an advantage over semiconductor devices, and in musical instrument and audiophile applications.
Solid-state diodes[ edit ] In , German scientist Karl Ferdinand Braun discovered the "unilateral conduction" across a contact between a metal and a mineral. Semiconductor principles were unknown to the developers of these early rectifiers.
During the s understanding of physics advanced and in the mid s researchers at Bell Telephone Laboratories recognized the potential of the crystal detector for application in microwave technology. In , Sylvania began offering the 1N34 crystal diode. Etymology[ edit ] At the time of their invention, asymmetrical conduction devices were known as rectifiers. The word diode, however, as well as triode , tetrode , pentode , hexode , were already in use as terms of multiplex telegraphy.
Vacuum tube diodes[ edit ] The symbol for an indirectly heated vacuum tube diode. From top to bottom, the element names are: plate, cathode, and heater. A thermionic diode is a thermionic-valve device consisting of a sealed, evacuated glass or metal envelope containing two electrodes : a cathode and a plate.
The cathode is either indirectly heated or directly heated. If indirect heating is employed, a heater is included in the envelope. A directly heated cathode is made of tungsten wire and is heated by current passed through it from an external voltage source. An indirectly heated cathode is heated by infrared radiation from a nearby heater that is formed of Nichrome wire and supplied with current provided by an external voltage source.
The operating temperature of the cathode causes it to release electrons into the vacuum, a process called thermionic emission. The cathode is coated with oxides of alkaline earth metals , such as barium and strontium oxides. These have a low work function , meaning that they more readily emit electrons than would the uncoated cathode. The plate, not being heated, does not emit electrons; but is able to absorb them.
The alternating voltage to be rectified is applied between the cathode and the plate. When the plate voltage is positive with respect to the cathode, the plate electrostatically attracts the electrons from the cathode, so a current of electrons flows through the tube from cathode to plate.
When the plate voltage is negative with respect to the cathode, no electrons are emitted by the plate, so no current can pass from the plate to the cathode. Semiconductor diodes[ edit ] Close-up of an EFD germanium point-contact diode in DO7 glass package, showing the sharp metal wire cat whisker that forms the semiconductor junction.
Point-contact diodes[ edit ] Point-contact diodes were developed starting in the s, out of the early crystal detector technology, and are now generally used in the 3 to 30 gigahertz range.
Non-welded contact construction utilizes the Schottky barrier principle. The metal side is the pointed end of a small diameter wire that is in contact with the semiconductor crystal.
1N4448 Universal Diode bis 75V 150mA DO35
1N4448: High Conductance Fast Diode