The Schottky diode (also known as hot carrier diode) is a special type of semiconductor diode with a very low forward-voltage drop and a very fast switching action.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 and low forward voltage drop.A normal silicon diode has between 0.6–1.7 volt drop, while a Schottky diode voltage drop is between approximately 0.1–0.4 volts. This lower voltage drop can provide higher switching speed and better system efficiency.

Advantages and Applications of Schottky Diodes

  • Primary advantages of Schottky diodes are very low forward voltage drop and switching speeds that approach zero time making, ideal for output switching of power supplies.
  • Schottky diodes are suitable for high frequency applications including very low power involving signal and switching diode requirements of less than 100 picoseconds. These require small Schottky devices with low capacitance.
  • The reverse recovery time of Schottky diodes are extremely fast (but soft) recovery characteristics. What little reverse recovery time they may exhibit is primarily dictated by their capacitance rather than minority carrier recombination as in conventional pn junction rectifiers. This characteristic provides very little reverse current overshoot when switching the Schottky from the forward conducting mode to the reverse blocking state.
  • The combination of very “fast-soft switching properties” of a Schottky can also eliminate the need for snubber circuits in many applications that may otherwise be required with fast or ultrafast rectifiers displaying abrupt recovery characteristics. These features make schottky rectifiers a very attractive choice for low parasitic switching losses.
  • 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 100 ns 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 essentially instantly with only slight capacitive loading, this is much less of a concern.
  • Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate up to 50 GHz.

Limitations of Schottky Diodes

  • Schottky devices are limited in some applications compared to PN junction rectifiers because their reverse leakage currents are many times higher.
  • Schottky rectifiers have lesser maximum rated junction temperatures typically in the range of 125°C to 175°C, compared to the typical 200°C for conventional PN junctions, which further influences leakage current behavior and thermal runaway.
  • Schottky devices are limited in available reverse blocking voltage ratings compared to conventional pn junction rectifiers.
  • Schottky rectifiers that exceed 100 volts in their working peak reverse voltage (VRWM), since devices moderately above this rating level will result in forward voltages equal to or greater than equivalent PN junction rectifiers.
  • The Schottky rectifier properties are primarily determined by the metal energy barrier height of material deposited on the silicon by the manufacturer. A metal with a low energy barrier height will minimize forward voltage, but will also be restricted in its high temperature operating capability and have very high reverse leakage currents. A high barrier metal height selection will minimize temperature and leakage current sensitivity but will increase the forward voltage.
  • Schottky rectifiers are comparatively sensitive to temperature in reverse leakage current, particularly as junction temperature (TJ) approaches maximum rating (TJ(MAX)) for their barrier metal design. Therefore Schottky rectifier applications with high temperature and reverse bias require precautions compared to operating other conventional PN junction design rectifiers.
  • For high current rated Schottky diodes in large stud metal packages, minimal thermal resistance involved in good mounting practices is vital. Good thermal management with more effective cooling methods (less thermal resistance) is particularly important for those Schottky rectifiers with low barrier heights that are used to reduce forward voltage power losses.

Alternatives to Schottky Diodes

  • Silicon Carbide (SiC) Schottky diode. SiC Schottky diodes have about 40 times lower reverse leakage current compared to silicon Schottky diodes and are available upto 1200 volt, 7.5 A in 2×2 mm chip.
  • Silicon carbide has a high 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 500 K, which allows passive radiation cooling in aerospace applications.
  • When less power dissipation is desired a MOSFET and a control circuit can be used instead, in an operation mode known as Active rectification. Active rectification is a technique for improving the efficiency of rectification by replacing diodes with actively-controlled transistors, usually power MOSFETs.
  • A super diode consisting of a pn-diode or Schottky diode and an operational amplifier provides an almost perfect diode characteristic due to the effect of negative feedback, although its use is restricted to frequencies the operational amplifier used can handle.