A Schottky diode, also known as a Schottky barrier diode or hot-carrier diode, is a diode formed by a junction of a metal with a semiconductor. This is different to a standard p-n diode which is formed by the junction between p-type and n-type semiconductor materials. This construction gives the Schottky diode some unique characteristics than can be really useful to the electronics designer.
The Schottky diode is named after Walter Schottky (1886-1976), a German physicist who, among other things invented the screen-grid vacuum tube and co-invented the ribbon microphone.
The crystal detector (or cat’s whisker) in early radios was a Schottky diode formed by the junction of a fine metallic wire and a semiconducting crystal such as Galena.
Interestingly, the very first radio detectors were Schottky diodes, using a crystal of Galena (lead sulphide) or iron pyrite with a fine wire touching its surface (Figure 1). These detectors were known as crystal detectors or “cat’s whisker” detectors. Similarly, early power supplies used “metal rectifiers” in which semiconductor (usually Selenium) disks are sandwiched with copper discs to create rectifiers as shown in Figure 2.
Early “metallic rectifiers” included Schottky diode junctions formed between discs of metallic copper and Selenium
Figure 3 shows what’s going on under the hood. The Schottky diode is formed by a junction between a metal on the left and a n-type semiconductor on the right. In its unbiased state the free electrons in the semiconductor (which has an excess of electrons) move into the metal to establish equilibrium, creating a depletion region.
The depletion layer in the metallic side of a Schottky diode is extremely thin compared to the semiconductor side. This has the advantage of reducing the forward voltage necessary for conduction, but also means the diode has higher reverse leakage and lower reverse breakdown voltage.
Unlike a p-n junction diode where the depletion region is symmetrical, the depletion region in the metal will be extremely thin since the metal is a “sea” of electrons that are free to move at will. This means the “barrier” potential that must be overcome to forward bias the diode (Figure 3b) is significantly lower than that of a p-n diode.
When the diode is forward biased a lot of free electrons are injected into the n-type semiconductor. If the applied voltage is above about 0.2V, these electrons gain enough energy to push through the depletion region and the diode conducts.
When the diode is reverse biased, the depletion region in the semiconductor expands impeding the flow of electrons. There will however be some leakage as some electrons will have sufficient thermal energy to cross the barrier.
So, Schottky diodes have typical forward bis voltages of 0.2 to 0.5V compared to p-n diodes which have forward voltages of 0.6 to 0.7V. This can be a huge advantage in rectifier circuits where Schottky rectifiers have much lower voltage drop (and therefore power dissipation) than their traditional counterparts.
Schottky diodes also have much lower reverse-recovery time than p-n diodes. Reverse recovery is the process by which a forward biased diode regains its blocking capability when reverse biased. In Schottky diodes this happens very quickly because only electrons are involved in the operation of the device, and these can move very quickly in the metallic side of the junction. There is no slow recombination of electrons and holes as occurs in p-n junctions.
Schottky diodes can have reverse recovery times in picoseconds compared to tens or hundreds of microseconds for fast p-n diodes. This is big advantage for efficiency for high-frequency operations and is why the rectifiers in switch-mode power converters use Schottky diodes almost exclusively.
It’s not all good news, however. The relatively thin barrier means that Schottky diodes have higher reverse leakage and lower reverse breakdown voltages than their p-n diode cousins. Most Schottky diodes are rated for reverse voltages below 50V, although some are available with reverse voltages up to 200V. Conventional diodes can have reverse breakdown voltages well over 1kV.
We’ve already mentioned that Schottky diodes are frequently used in switching power converters, but they have other uses. They are used in clamping applications in the transistors used to create logic circuits (the “S” in 74LS logic stands for Schottky). These clamps prevent the transistors from saturating and therefore operate at much higher speeds. The high-speed characteristics make Schottky diodes useful in very high frequency circuits as detectors and mixers. Here they can operate at frequencies up to 24GHz or more.
References
“Schottky Diode.” In Wikipedia, January 19, 2025. https://en.wikipedia.org/w/index.php?title=Schottky_diode&oldid=1270334933.
“Walter H. Schottky.” In Wikipedia, January 23, 2025. https://en.wikipedia.org/w/index.php?title=Walter_H._Schottky&oldid=1271300418.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
