Common Diode Types

Depending on their applications, diodes can be segregated into the following major divisions: Small Signal Diode. These are the semiconductor devices used most often in a wide variety of applications. In general purpose applications, they are used as a switch in rectifiers, limiters, capacitors, and in wave shaping. The common diode parameters a designer needs to know include forward voltage, reverse breakdown voltage, reverse leakage current, and recovery time. Silicon Rectifier Diode. These are the diodes that have high forward-current carrying capability, typically up to several hundred amperes. They usually have a forward resistance of only a fraction of an ohm while their reverse resistance is in the megaohm range. Their primary application is in power conversion, such as for power supplies, UPS, rectifiers=inverinverters etc. In case of current exceeding the rated value, their case temperature will rise. For stud mounted diodes, their thermal resistance is between 0.1 to 1 C=W. Zener Diode. Its primary applications are in the voltage reference or regulation. However, its ability to maintain a certain voltage depends on its temperature coefficient and impedance. The voltage reference or regulation application of Zener diodes are based on their avalanche properties. In the reverse-biased mode, at a certain voltage the resistance of these devices may suddenly drop. This occurs at the Zener voltage VX , a parameter the designer knows beforehand. Figure 2.4 shows a circuit in which a Zener diode is used to control the reference voltage of a linear power supply. Under normal operating conditions, the transistor will transmit power to the load (output) circuit. The output power level will depend on the transistor base current. A very high base current will impose a large voltage across the Zener and it may attain Zener voltage VX , at which point it will crush and limit the power supply to the load. Photodiode. When a semiconductor junction is exposed to light, photons generate hole-electron pairs. When these charges diffuse across the junction, they produce photo current. Hence this device acts as a source of current that increases with the intensity of light. Light-Emitting Diode (LED). Power diodes used in PE circuits are high-power versions of the commonly used devices employed in analog and digital circuits. They are manufactured in many varieties and ranges. The current rating can be from a few amperes to several hundreds while the voltage rating varies from tens of volts to several thousand volts.

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  1. Guru

    A two-terminal electron device exhibiting a nonlinear current-voltage characteristic. Although diodes are usually classified with respect to the physical phenomena that give rise to their useful properties, in this article they are more conveniently classified according to the functions of the circuits in which they are used. This classification includes rectifier diodes, negative-resistance diodes, constant-voltage diodes, light-sensitive diodes, light-emitting diodes, and capacitor diodes.

    A circuit element is said to rectify if voltage increments of equal magnitude but opposite sign applied to the element produce unequal current increments. An ideal rectifier diode is one that conducts fully in one direction (forward) and not at all in the opposite direction (reverse). This property is approximated in junction and thermionic diodes. Processes that make use of rectifier diodes include power rectification, detection, modulation, and switching. See also Rectifier.

    Negative-resista nce diodes, which include tunnel and Gunn diodes, are used as the basis of pulse generators, bistable counting and storage circuits, and oscillators. See also Negative-resistance circuits; Oscillator; Tunnel diode.

    Breakdown-diode current increases very rapidly with voltage above the breakdown voltage; that is, the voltage is nearly independent of the current. In series with resistance to limit the current to a nondestructive value, breakdown diodes can therefore be used as a means of obtaining a nearly constant reference voltage or of maintaining a constant potential difference between two circuit points, such as the emitter and the base of a transistor. Breakdown diodes (or reverse-biased ordinary junction diodes) can be used between two circuit points in order to limit alternating-voltage amplitude or to clip voltage peaks. See also Limiter circuit.

    Light-sensitive diodes, which include phototubes, photovoltaic cells, photodiodes, and photoconductive cells, are used in the measurement of illumination, in the control of lights or other electrical devices by incident light, and in the conversion of radiant energy into electrical energy. Light-emitting diodes (LEDs) are used in the display of letters, numbers, and other symbols in calculators, watches, clocks, and other electronic units. See also Light-emitting diode; Photoconductive cell; Photodiode; Photoelectric devices; Phototube; Photovoltaic effect.

    Semiconductor diodes designed to have strongly voltage-dependent shunt capacitance between the terminals are called varactors. The applications of varactors include the tuning and the frequency stabilization of radio-frequency oscillators. See also Junction diode; Microwave solid-state devices; Semiconductor diode; Varactor.

  2. Guru

    (1) An electronic component that acts like a one-way valve. As a discrete component or built into a chip, it is used in a variety of functions. Used as a rectifier, it is a key element in changing AC to DC by limiting current flow to a single direction. Diodes are used as temperature and light sensors and light emitters (LEDs). In communications, they filter out analog and digital signals from carriers and modulate signals onto carriers. In digital logic, they’re used as one-way valves and as switches similar to transistors. See laser diode.

    (2) A type of vacuum tube used in electronic circuits as a rectifier or radio frequency detector. Modern applications of tube diodes are generally limited to rectifiers in high-end audio amplifiers and other specialized high-voltage circuits.

    The tube diode uses two active elements (cathode and plate) and one passive element (the filament or heater). In typical operation, the cathode is heated by the filament, and the AC voltage is applied to the cathode. The heated cathode releases excited electrons that flow to the plate (anode) and become the rectified current. The diode allows current flow in only one direction. For example, if current were applied to the plate, electron flow could not occur, because the plate’s electrons are not heated by the filament.

    In some instances, the filament is also the cathode. This is accomplished by connecting the AC voltage source to one of the filament’s leads. See triode, tetrode, pentode, magnetron and klystron.