Diode - Wikiparency

# Diode A **diode** is a two-terminal component that conducts current far more readily in one direction than in the other. Nearly all modern diodes are semiconductors, built around a [[P–n_junction|p–n junction]] or a metal-semiconductor contact, and their forward current rises exponentially with applied voltage. That exponential is the reason a diode has no single forward voltage: the familiar figure of about 0.7 volts for silicon is not a property of the device but the voltage at which its curve happens to meet the rest of the circuit. The microsim on this page makes that explicit. It draws the diode's exponential curve together with the load line of the resistor feeding it, and marks the crossing. Changing the resistor or the supply moves the crossing along the curve, and heating the junction slides the whole curve sideways, so the forward drop falls by roughly two millivolts for every kelvin. ## Main functions The asymmetry is the function. A diode passes current one way and blocks it the other, which makes it a [[Rectifier|rectifier]]: it turns alternating current into a unidirectional flow, the first step in almost every mains power supply. Because it blocks reverse current it also serves to protect, standing across an inductive load to carry the current a collapsing magnetic field insists on delivering, or in series to prevent damage if a supply is connected backwards. The exponential relation gives a second family of uses. A device whose current depends exponentially on voltage is a device whose voltage depends logarithmically on current, which is the basis of logarithmic amplifiers and of analogue multiplication. The same exponential, with its strong and predictable temperature dependence, makes a plain diode a usable thermometer, and a matched pair of junctions the reference at the heart of a bandgap [[Voltage_reference|voltage reference]]. Diodes also clamp and limit. Placed between a signal and the supply rails they conduct only when the signal strays outside them, protecting an input; placed in series with a capacitor they shift a waveform's average level; arranged in ladders they approximate curved transfer functions from straight segments. ## History The rectifying behaviour of a point contact between a metal wire and a crystal was noticed by Ferdinand Braun in 1874, well before any theory could explain it. Such [[Crystal_detector|crystal detectors]] — a fine wire pressed against galena or carborundum — became the standard detector of early radio, cheap and effective but mechanically delicate and unrepeatable, since the operator had to hunt for a sensitive spot on the crystal. The thermionic route arrived in parallel. Thomas Edison observed in 1880 that current would flow from a heated filament to a nearby electrode in an evacuated bulb but not the other way, an effect that took his name without his finding a use for it. John Ambrose Fleming turned it into a component in 1904, and the thermionic valve displaced the crystal detector for four decades because it was reliable and could be manufactured to a specification. Semiconductors returned with an understanding of why they worked. Wartime radar demanded detectors at frequencies valves handled badly, which funded serious work on silicon and germanium point contacts; the theory of the p–n junction followed from that work, and with the [[Transistor|transistor]] it established semiconductor electronics. By the 1960s silicon junction diodes had displaced valves in all but a few high-power and high-voltage roles. ## Etymology The name was coined by William Henry Eccles around 1919 from Greek roots meaning two paths or two electrodes, following the pattern that also gives triode and tetrode for valves with more electrodes. It described the thermionic valve, and passed to the semiconductor device because the function was the same even though the mechanism was not. The terminal names are older and come from electrochemistry by way of Michael Faraday: the [[Anode|anode]] is the terminal into which conventional current flows and the [[Cathode|cathode]] the one from which it leaves, so a diode conducts when its anode is positive with respect to its cathode. On a physical part the cathode is marked, conventionally with a band. ## Vacuum tube diodes A thermionic diode is a heated cathode and a cold anode in an evacuated envelope. The hot cathode emits electrons, which are collected by the anode when it is positive; when the anode is negative it emits nothing, because it is not hot, and no current flows. Conduction is therefore a [[Thermionic_emission|thermionic emission]] process, and the current is limited not by the supply of electrons but by the space charge of the electrons already in flight. Valve diodes have a much larger forward drop than semiconductors, tens of volts under load, and waste continuous power heating the cathode, so they are obsolete for rectification. They survive in a few niches: very high voltage rectification, where their tolerance of momentary overload and their absence of reverse recovery charge still count, and in audio equipment maintained for its own sake. ## Semiconductor diodes The p–n junction diode is the standard device. Joining p-type and n-type material produces a [[Depletion_region|depletion region]] at the boundary with a built-in potential across it. Forward bias lowers that barrier and lets carriers diffuse across, giving a current that rises exponentially with the applied voltage; reverse bias raises the barrier and leaves only a small saturation current. Three real departures matter. The exponential is the [[Shockley_diode_equation|Shockley equation]], but its saturation current climbs steeply with temperature, so the forward drop at constant current falls as the junction heats — the effect the microsim shows, and the reason paralleled diodes do not share current, since the warmest takes more and grows warmer. At high current an ohmic series resistance takes over and the curve straightens. And on switching off, a junction that has been conducting holds stored charge that must be removed before it blocks, the reverse recovery time that limits how fast a junction diode can rectify. A [[Schottky_diode|Schottky diode]] replaces one side of the junction with a metal contact. Conduction is by majority carriers only, so there is no stored minority charge and effectively no reverse recovery, and the forward drop is markedly lower. The costs are a larger reverse leakage and a lower reverse voltage rating. Where switching speed or forward loss dominates, this is the usual choice. ## Related devices Several devices are diodes operated in regimes the plain rectifier avoids. A [[Zener_diode|Zener diode]] is designed to break down at a specified reverse voltage in a controlled, non-destructive way, and is used as a voltage reference or a clamp. A [[Varicap|varactor]] exploits the fact that the depletion region's width, and so the junction capacitance, varies with reverse bias, giving a voltage-controlled capacitance for tuning. Others convert between current and light. A [[Light-emitting_diode|light-emitting diode]] emits a photon when a carrier recombines across the junction, in a direct-bandgap material chosen for the wavelength wanted; a [[Laser_diode|laser diode]] adds an optical cavity to the same structure. Run in reverse, a junction absorbs photons and produces current, which is the [[Photodiode|photodiode]] and, at large area and optimised for power, the [[Solar_cell|solar cell]]. A few devices exploit still other mechanisms: the tunnel diode, whose current falls as voltage rises over part of its range, giving the negative differential resistance used in oscillators; and the PIN diode, whose wide undoped middle layer makes it useful as a radio-frequency switch and as a high-voltage rectifier. ## Applications Rectification is the largest application by volume: a single diode for half-wave, four in a bridge for full-wave, feeding a [[Capacitor|capacitor]] that smooths the result. Clamping and protection are next, from the flyback diode across every relay coil to the transient suppressors on exposed inputs. Signal roles include detection — recovering the envelope of a modulated carrier, the direct descendant of the crystal detector — and mixing, where the non-linearity of the curve produces sum and difference frequencies. Logic was once built from diodes and resistors alone, and although diode logic cannot amplify and so cannot be cascaded indefinitely, diode-resistor arrangements still appear where a simple OR of several sources is wanted. Diodes also serve as references and sensors. The predictable temperature coefficient makes a junction a temperature sensor accurate enough for on-chip thermal monitoring, and the same physics, arranged so that two junctions run at different current densities, yields a voltage reference that is stable against temperature. ## Abbreviations Diodes are designated D or CR on schematics, the latter from crystal rectifier and still common in older and in aerospace documentation. Part numbering follows regional series: the 1N prefix of the American JEDEC scheme, of which the 1N4001 rectifier family and the 1N4148 small-signal diode are the most widely used; the BA and BY prefixes of the European Pro Electron scheme; and the 1S prefix of the Japanese JIS scheme. ## Microsim The microsim draws the Shockley curve for the selected material at the junction temperature set, with cooler and hotter curves fifty kelvin either side of it, and crosses them with the load line of the chosen supply and series resistor. The white marker sits at the operating point, found by solving the two together. Three things are worth trying. Change the series resistor over its range and watch the operating point slide along the curve while the forward drop moves by only a little: this is why the drop can be quoted as a single figure at all, and the readout showing about sixty millivolts per decade of current is the quantitative version of the same fact. Then raise the temperature and watch the drop fall, with the readout confirming roughly minus two millivolts per kelvin. Finally, switch the device from silicon to Schottky: the curve moves left by several hundred millivolts, which in a rectifier carrying amperes is the difference between a warm part and a hot one.  **Microsim — three.js (Wikitube framework):** *The diode: the Shockley curve, the load line and temperature* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/electronics/Diode.html" data-title="Diode"></div> *Built from `MICROSIM_GUIDE/specs/sims/Diode.json`; part of the [[Electronics]] set ([[PORTAL_Electronics]]).*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Diode) : [Wikitube](https://en.wikitube.io/wiki/Diode) Skeleton mirrored at revision 1373727382. Prose, emphasis and the microsim are Wikitube's own. ## See also - [[P–n_junction]] - [[Rectifier]] - [[Schottky_diode]] - [[Zener_diode]] - [[Light-emitting_diode]] - [[Transistor]] ## References The device physics and circuit theory here — the Shockley diode equation, the thermal voltage, the temperature dependence of saturation current and the resulting drift of forward voltage at constant current, the roughly sixty millivolts per decade slope, space-charge-limited thermionic conduction, depletion-region capacitance under reverse bias, and the load-line construction — is standard textbook material and is not separately footnoted, per the Wikitube style guide §6.1. *Citation needed:* the historical dates and attributions in "History" and "Etymology" (Braun 1874, Edison 1880, Fleming 1904, Eccles about 1919), and the part-numbering series named in "Abbreviations", are given as generally reported history and industry practice rather than from a pinned source. Pinning these to primary and history-of-technology sources, and to the JEDEC, Pro Electron and JIS registries, is queued for the next pass; no source is asserted for them here. ## Further reading - Standard semiconductor device physics texts covering the junction diode, to be pinned with the citations above. ## External links - JEDEC, Pro Electron and JIS part-number registries, to be pinned with the citations above.