Schottky diode
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The Schottky diode (named after German physicist Walter H. Schottky; also known as hot carrier diode) is a semiconductor diode with a low forward voltage drop and a very fast switching action.
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[edit] Applications
Typical applications include discharge-protection for solar cells connected to lead-acid batteries and in switched-mode power supplies; in both cases the low forward voltage leads to increased efficiency. While standard silicon diodes have a forward voltage drop of about 0.7 volts and germanium diodes 0.3 volts, Schottky diodes voltage drop at forward biases of around 1 mA is in the range 0.15 V to 0.46 V, which makes them useful in voltage clamping applications and prevention of transistor saturation. This is due to the higher current density in the Schottky diode.
[edit] Construction
A Schottky diode uses a metal-semiconductor junction as a Schottky barrier (instead of a semiconductor-semiconductor junction as in conventional diodes). This Schottky barrier results in both very fast switching times and low forward voltage drop.
The most important difference between P-N and Schottky diode is reverse recovery time, when the diode switches from non-conducting to conducting state and vice versa. Where in a P-N diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than 100ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from. The switching time is ~100 ps for the small signal diodes, and up to tens of nanoseconds for special high-capacity power diodes. With P-N junction switching, there is also a reverse recovery current, which in high-power semiconductors brings increased EMI noise. With Schottky diodes switching instantly with only slight capacitive loading, this is much less of a concern.
It is often said that the Schottky diode is a "majority carrier" semiconductor device. This means that if the semiconductor body is doped N-type, only the N-type carriers (mobile electrons) play a significant role in normal operation of the device. The majority carriers are quickly injected into the conduction band of the metal contact on the other side of the diode to become free moving electrons. Therefore no slow, random recombination of N- and P- type carriers is involved, so that this diode can cease conduction faster than an ordinary PN rectifier diode. This property in turn allows a smaller device area, which also makes for a faster transition. This is another reason why Schottky diodes are useful in switch-mode power converters; the high speed of the diode means that the circuit can operate at frequencies in the range 200 kHz to 2 MHz, allowing the use of small inductors and capacitors with greater efficiency than would be possible with other diode types. Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate up to 5 GHz.
The most evident limitations of Schottky diodes are the relatively low reverse voltage rating for silicon-metal schottky diodes, 50 V and below, and a relatively high reverse leakage current. The reverse leakage current, increasing with temperature, leads to a thermal instability issue. This often limits the useful reverse voltage to well below the actual rating. Luckily, times are changing and the diodes are becoming better and better. The voltage ratings are now up at 200 V.
Since 2001 another important invention was presented by Siemens Semiconductor (now Infineon): a silicon carbide Schottky diode. SiC Schottky diodes have about 40 times lower reverse leakage current compared to silicon schottky diodes and are available in 300V and 600V variants. As of 2007 a new 1200 volt 7.5A variant is sold as 2x2mm chip for power inverter manufacturers.
Silicon carbide has high a thermal conductivity and temperature has little influence on its switching and thermal characteristics. With special packaging it is possible to have operating junction temperatures of over 500K, which allows passive radiation cooling in aerospace applications.
[edit] Applications
Schottky diodes can be used in power supply "OR"ing circuits in products that have both an internal battery and a mains adaptor input, or similar. However, the high reverse leakage current presents a problem in this case, as any high-impedance voltage sensing circuit (e.g. monitoring the battery voltage or detecting whether a mains adaptor is present) will see the voltage from the other power source through the diode leakage.
Commonly encountered Schottky diodes include the 1N5817 series 1 A rectifiers. Schottky metal-semiconductor junctions are featured in the successors to the 7400 TTL family of logic devices, the 74S, 74LS and 74ALS series, where they are employed as clamps in parallel with the collector-base junctions of the bipolar transistors to prevent their saturation, thereby greatly reducing their turn-off delays.
[edit] Alternatives
When an even lower forward voltage is desired, or if the reverse-leakage is problematic, a so-called "ideal diode", combining a MOSFET switch and a control circuit, can be used, in an operation mode known as synchronous rectification.
