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In Electronics , a diode is a Component that restricts the direction of movement of Charge Carrier s. It allows an Electric Current to flow in one direction, but essentially blocks it in the opposite direction. Thus the diode can be thought of as an electronic version of a Check Valve . Circuits that require current flowing in only one direction will typically consist of one or more diodes in the circuit design. Early diodes included "cat's Whisker" Crystals and Vacuum Tube devices (called '' Thermionic Valves '' in British English ). Today the most common diodes are made from ultrapure Semiconductor materials such as Silicon or Germanium . HISTORY Thermionic and solid state diodes developed in parallel. The principle of operation of {Link without Title} . Thermionic diode principles were rediscovered by , 1904 ( in November 1905). Pickard received a patent for a silicon crystal detector on November 20 , 1906 {Link without Title} (). At the time of their invention such devices were known as Rectifiers . In 1919 William Henry Eccles coined the term diode from Greek Roots ; ''di'' means 'two', and ''ode'' means 'path'. THERMIONIC OR GASEOUS STATE DIODES Thermionic diodes are Vacuum Tube devices (also known as Thermionic Valve s), which are arrangements of electrodes surrounded by a vacuum within a glass envelope, similar in appearance to incandescent Light Bulb s. In vacuum tube diodes, a current is passed through the Cathode , a Filament treated with a mixture of Barium and Strontium Oxide s, which are oxides of Alkaline Earth Metal s. The current heats the filament, causing Thermionic Emission of electrons into the vacuum envelope. In forward operation, a surrounding metal electrode, called the Anode , is positively charged, so that it Electrostatically attracts the emitted electrons. However, electrons are not easily released from the unheated anode surface when the voltage polarity is reversed and hence any reverse flow is a very tiny current. For much of the 20th century vacuum tube diodes were used in analog signal applications, and as rectifiers in power supplies. Today, tube diodes are only used in niche applications, such as rectifiers in tube guitar and hi-fi amplifiers, and specialized high-voltage equipment. SEMICONDUCTOR DIODES Most modern diodes are based on Semiconductor P-n Junction s. In a p-n diode, Conventional Current can flow from the p-type side (the Anode ) to the n-type side (the Cathode ), but not in the opposite direction. Another type of semiconductor diode, the Schottky Diode , is formed from the contact between a metal and a semiconductor rather than by a p-n junction. A semiconductor diode's current- Voltage , or I-V, characteristic curve is ascribed to the behavior of the so-called '' Depletion Layer '' or ''depletion zone'' which exists at the P-n Junction between the differing semiconductors. When a p-n junction is first created, conduction band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (places for electrons in which no electron is present) with which the electrons "recombine". When a mobile electron recombines with a hole, the hole vanishes and the electron is no longer mobile. Thus, two charge carriers have vanished. The region around the p-n junction becomes depleted of Charge Carrier s and thus behaves as an Insulator . However, the Depletion Width cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a 'built-in' potential across the depletion zone. If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator preventing a significant electric current. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed resulting in substantial electric current through the p-n junction. For silicon diodes, the built-in potential is approximately 0.6 V. Thus, if an external current is passed through the diode, about 0.6 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be 'turned on'. A diode's I-V characteristic can be approximated by two regions of operation. Below a certain difference in potential between the two leads, the depletion layer has significant width, and the diode can be thought of as an open (non-conductive) circuit. As the potential difference is increased, at some stage the diode will become conductive and allow charges to flow, at which point it can be thought of as a connection with zero (or at least very low) resistance. More precisely, the Transfer Function is Logarithm ic, but so sharp that it looks like a corner on a zoomed-out graph (''see also'' Signal Processing ). In a normal silicon diode at rated currents, the voltage drop across a conducting diode is approximately 0.6 to 0.7 Volt s. The value is different for other diode types - Schottky Diode s can be as low as 0.2 V and Light-emitting Diode s (LEDs) can be 1.4 V or more (Blue LEDs can be up to 4.0 V). Referring to the I-V characteristics image, in the reverse bias region for a normal P-N rectifier diode, the current through the device is very low (in the µA range) for all reverse voltages upto a point called the peak-inverse-voltage (PIV). Beyond this point a process called reverse breakdown occurs which causes the device to be damaged along with a large increase in current. For special purpose diodes like the avalanche or zener diodes, the concept of PIV is not applicable since they have a deliberate breakdown beyond a known reverse current such that the reverse voltage is "clamped" to a known value (called zener voltage). The devices however have a maximum limit to the current and power in the zener or avalanche region. Shockley diode equation The ''Shockley ideal diode equation'' (named after William Bradford Shockley ) can be used to approximate the p-n diode's I-V characteristic in the forward-bias region. :, where I I V V :and ''n'' is the ''emission coefficient''. The emission coefficient ''n'' varies from about 1 to 2 depending on the fabrication process and semiconductor material and in many cases is assumed to be approximately equal to 1 (thus ommitted). The ''thermal voltage'' ''V''T is approximately 25.2 mV at room temperature (approximately 25oC or 298K) and is a known constant. It is defined by: : , where q k T TYPES OF SEMICONDUCTOR DIODE
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