Applications
Radio demodulation
The first use for diode was demodulation of the amplitude modulated (AM) radio broadcasts. History of the discovery is treated in depth in radio article. An AM signal comprises of alternating positive and negative peaks of the voltage, whose amplitude or envelope is proportional to original audio signal. The diode (initially the crystal diode) rectifies the AM radio frequency signal, leaving an audio signal which is original audio signal, minus atmospheric noise. The audio is extracted by making use of a simple filter and fed into the audio amplifier or transducer, which generates the sound waves.
Power conversion
Rectifiers are constructed from diodes, here they are used to convert alternating current (AC) electricity into direct current (DC). The automotive alternators are a common example, where the diode, which rectifies AC into DC, gives better performance than commutator of the earlier dynamo. Similarly, diodes are also used in the Cockcroft–Walton voltage multipliers to convert AC into higher DC voltages.
Over-voltage protection
The diodes are frequently used to conduct damaging high voltages away from the sensitive electronic devices. They are frequently reverse-biased under normal circumstances. When voltage rises above the normal range, the diodes become forward-biased. For instance, diodes are used in (stepper motor and H-bridge) motor controller and relay circuits to de-energize coils rapidly without the damaging voltage spikes that would otherwise occur. (Any diode used in like application is called as flyback diode). A number of integrated circuits also incorporate diodes on connection pins to prevent external voltages from damaging their sensitive transistors. The specialized diodes are used to protect from over-voltages at higher power.
Logic gates
Diodes can be combined with the other components to build OR and AND logic gates. This is referred to as a diode logic.
Ionizing radiation detectors
In addition to light, stated above, semiconductor diodes are sensitive to energetic radiation. In electronics, The cosmic rays and other sources of the ionizing radiation creates noise pulses and single and multiple bit errors. This effect is exploited sometimes by particle detectors to detect radiation. The single particle of radiation, having thousands or millions of electron volts of energy, generates several charge carrier pairs, as its energy is deposited in semiconductor material. If the depletion layer is large enough to catch whole shower or to stop a heavy particle, a fairly accurate measurement of particle‘s energy can be made, simply by measuring the charge conducted and without complexity of the magnetic spectrometer or etc. These semiconductor radiation detectors require efficient and uniform charge collection and low leakage current. They are cooled by liquid nitrogen. For longer range particles, they require a very large depletion depth and large area. For short range particles, they require any contact or un-depleted semiconductor on at least one surface to be very thin. The back- bias voltages are near breakdown (around the thousand volts per centimeter). Germanium and silicon are ordinarily used materials.
Some of the detectors sense position and energy. They have a finite life, in particular when detecting heavy particles, due to radiation damage. Silicon and germanium are different in their ability to convert gamma rays to electron showers.
Semiconductor detectors for the high energy particles are used in the large numbers. Due to energy loss fluctuations, accurate measurement of energy deposited is of less use.
Temperature measurements
A diode is used as a temperature measuring device, as the forward voltage drop across the diode depends on the temperature, as in a Silicon band gap temperature sensor. From Shockley ideal diode equation given above, it appears that the voltage has a positive temperature coefficient (at a constant current) but depends on doping concentration and operating temperature (Sze 2007). Temperature coefficient can be negative as in typical thermistors or positive for temperature sense diodes down to around 20 kelvins. Typically, silicon diodes have about −2 mV/°C temperature coefficient at the room temperature.
Current steering
Diodes will prevent currents in the unintended directions. To supply power to the electrical circuit during the power failure, circuit can draw current from the battery. An Uninterruptible power supply can use diodes in this manner to ensure that the current is only drawn from battery when required. Likewise, small boats typically have 2 circuits each with their own battery/batteries: one used for engine starting; one used for domestics. Generally both are charged from a single alternator, and a heavy duty split charge diode is used to prevent higher charge battery (the engine battery) from discharging through lower charged battery when alternator is not running.
Diodes are used in electronic musical keyboards also. To lessen the amount of wiring required in electronic musical keyboards, these instruments use keyboard matrix circuits. The keyboard controller scans rows and columns to determine which note the player has pressed. The problem with the matrix circuits is that when a number of notes are pressed at once, the current can flow backwards through circuit and trigger "phantom keys" which cause ?ghost notes to play. To avoid the triggering unwanted notes, most keyboard matrix circuits have diodes soldered with switch under each key of the musical keyboard. The same principle is used for the switch matrix in the solid state pinball machines.
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