What are Schottky Barrier Diodes?

Answer 1

Good question, and one for us "older timers". First, let's talk about a "standard" silicon diode. These are fairly simple semiconductors with a single junction of P/N doped silicon. Sand if you will, with a little help... The Schottky diode is named after the inventor, Walter Schottky and is well known for having an extremely low forward voltage drop. Silicon junctions typically have a Vf (forward voltage) of about 0.63 volts at the point they begin to conduct current. This can easily rise up to 0.7 or even 1.0 Volts for large, high current diodes but this is mainly due to the Vf of the junction, plus resistive losses due to the high currents. But here, we'll focus on small signal diodes, those used for switching and low currents, in the hundredths of amps or 10mA. The Schottky junction is designed differently and uses a "Schottky barrier", or metal-to-semiconductor junction rather than the typical semi-to-semi conductor PN junction. The end result is a much lower Vf of around 0.15 up to 0.5V. This compares with the ubiquitous germanium diode with a Vf of 0.2V and because of it's low Vf, was very useful in the "diode radio" where a much lower RF power was needed to reach the conduction voltage and thus rectify an AM signal, and in combination with a high impedance headphone, allowed hearing radio stations, using no external power. Many people built their own and required winding antenna coils of hundreds of turns of tiny wire. However, Si-Si (silicon to silicon) junctions other than having a much higher Vf, also take much longer to block a reverse current after having been conducting a forward current. This is also called the recovery period and ranges in the 100's of microseconds which is fast, but slow when compared with modern switching power supplies (for example), which use frequencies into the hundreds of kHz and thus need to switch faster than a silicon diode can recover and block the current. Thus results in a large amount of power dissipation in the diode and makes the power supply less efficient. The Schottky diode, has the two important qualities needed in such uses: first, it's Vf is low reducing the power dissipation when conducting. Second, they're much faster at recovering and thus dissipate much less power during the recovery phase, when the diode is conducting "backwards". The physics involved are beyond the scope of this question, but to summarize: Schottky diodes differ from silicon diodes by using a different type of junction; a metal to Si or "barrier junction" and, have a lower Vf and faster recovery speed. They also cost a lot more than an ordinary diode, like a 1N4001 diode/rectifier. The term "rectifier" simply means a diode generally has enough current carrying ability to be used in a power supply to convert AC into DC. Finally, Schottky diodes, while having attractive capabilities, also have some significant limitations making them less usable in other applications. A main limitation is the Vr or reverse voltage rating, or the voltage that the diode can block without being forced to conduct, typically less than 50V and also "leaking" more current when blocking than an Si diode. Using an example of a 1N4001, it has a Vr rating of 50V, which is the lowest "grade". But also available are the same diodes, with better performance. The 1N4002 has a Vr of 100V, the 1N4003 200V, 1N4004 400V, going up to the 1N4007 with 1000V Vr. These voltages would totally destroy the Schottky diode's ability to block reverse current. But then too, the 1N400x diodes are much slower and cannot be used in high speed circuits like the Schottky diode can. Actually diodes conducts as soon as a current flow is possible and follow an exponential curve whereby eventually any more current flow will not appreciably increase the voltage drop. This empirical voltage drop can be assumed to be ~.7v or~.6v since the current flow will make a difference in voltage drop. This forward voltage drop can be assumed to be 6v to .7v or higher depends on current flow.