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In Electronics , a diode is a Component that restricts the directional flow of Charge Carrier s. Essentially, a diode allows an Electric Current to flow in one direction, but 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 flow in only one direction typically include 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 Semiconductor materials such as Silicon or Germanium . HISTORY Thermionic and solid state diodes developed in parallel. The principle of operation of . Historical lecture on Karl Braun Thermionic diode principles were rediscovered by into a useful device for radio detection. The first radio receiver using a crystal diode was built around , 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'' (from ''odos'') means 'path'. THERMIONIC OR GASEOUS STATE DIODES Thermionic diodes are Thermionic Valve devices (also known as Vacuum Tube s), which are arrangements of Electrode s surrounded by a vacuum within a glass envelope, similar in appearance to Incandescent Light Bulb s. In thermionic valve diodes, a current is passed through the heater Filament . This indirectly heats the Cathode , another filament treated with a mixture of Barium and Strontium Oxide s, which are Oxide s of Alkaline Earth Metal s; these substances are chosen because they have a small Work Function . (Some valves use direct heating, in which the heating current is passed through the cathode itself.) The heat causes 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 thermionic valve diodes were used in analog signal applications, and as rectifiers in power supplies. Today, valve diodes are only used in niche applications, such as rectifiers in guitar and hi-fi valve 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 cannot flow 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. This is the '' Reverse Bias '' phenomenon. 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' as it has a '' Forward Bias ''. 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 up to 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 Diode s, 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 the ''zener voltage'' or Breakdown Voltage ). These 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'' or the ''diode law'' (named after Transistor co-inventor William Bradford Shockley , not to be confused with Tetrode inventor Walter H. Schottky ) is the I-V characteristic of an ideal diode in either forward or reverse bias (or no bias). It is derived with the assumption that the only processes giving rise to current in the diode are drift (due to electrical field), diffusion, and thermal recombination-generation. It also assumes that the recombination-generation (R-G) current in the depletion region is insignificant. This means that the Shockley equation doesn't account for the processes involved in reverse breakdown and photon-assisted R-G. Additionally, it doesn't describe the "leveling off" of the I-V curve at high forward bias due to internal resistance, nor does it explain the practical deviation from the ideal at very low forward bias due to R-G current in the depletion region. : where I I V V :and ''n'' is the '' Emission Coefficient '', also known as the ''ideality factor''. 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 omitted). The ''thermal voltage'' ''V''T is approximately 25.7 mV at room temperature (25 °C or 298 K) and is a known constant. It is defined by: : where q k T Hydrodynamic analogy The diode, in the manner of a valve, allows the passage of the current only in one direction. It is a polarized dipole, the anode and cathode is thus located on the component. |
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| "http://wwwinformationdelightinfo/information/entry/Zener_diode" class="copylinks">Zener<br/> Diode |
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| "http://wwwinformationdelightinfo/information/entry/Schottky_diode" class="copylinks">Schottky<br/> Diode |
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| "http://wwwinformationdelightinfo/information/entry/Tunnel_diode" class="copylinks">Tunnel<br/> Diode |
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| "http://wwwinformationdelightinfo/information/entry/light-emitting_diode" class="copylinks">Light-emitting<br/> Diode |
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| "http://wwwinformationdelightinfo/information/entry/Photodiode" class="copylinks">Photodiode |
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| "http://wwwinformationdelightinfo/information/entry/Varicap" class="copylinks">Varicap |
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| "http://wwwinformationdelightinfo/information/entry/Silicon_controlled_rectifier" class="copylinks">SCR |
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