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. Typical applications include discharge-protection for solar cells connected to lead-acid batteries and in switch 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 0.6 volts and germanium diodes 0.2 volts, Schottky diodes voltage drop at forward biases of around 1 mA is in the range 0.15V to 0.45 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.
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.
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, 50 V and below, and a relatively high reverse 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.
Schottky diodes can be used in power supply "ORing" circuits in products that have both an internal battery and a mains adaptor input, or similar. But 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. (ALS stands for Advanced Low-power Schottky.)
When an even lower forward voltage is desired, or if the reverse-leakage is problematic, a so-called "ideal diode" such as the Linear Technology LTC4413 can be used. This is an integrated circuit combining a MOSFET switch and a control circuit.
