Schottky barrier
A Schottky barrier, named after Walter H. Schottky, is a potential barrier formed at a metal–semiconductor junction which has rectifying characteristics, suitable for use as a diode. The largest differences between a Schottky barrier and a p–n junction are its typically lower junction voltage, and decreased (almost nonexistent) depletion width in the metal.
Not all metal–semiconductor junctions form Schottky barriers. A metal–semiconductor junction that does not rectify current is called an ohmic contact. Rectifying properties depend on the metal's work function, the band gap of the intrinsic semiconductor, the type and concentration of dopants in the semiconductor, and other factors. Design of semiconductor devices requires familiarity with the Schottky effect to ensure Schottky barriers are not created accidentally where an ohmic connection is desired.
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Advantages [edit]
Schottky barriers, with their lower junction voltage, have applications in devices that are desired to better approximate an ideal diode. They are also used in conjunction with normal diodes and transistors, where their lower junction voltage is used for circuit protection (among other things).
Because one of the materials in a Schottky diode is a metal, lower-resistance devices are often possible. In addition, the fact that only one type of dopant is needed may greatly simplify fabrication. And because of their majority-carrier conduction mechanism, Schottky diodes can achieve greater switching speeds than p–n junction diodes, making them appropriate to rectify high-frequency signals.
Devices [edit]
A metal–semiconductor junction that forms a Schottky barrier as a device by itself is known as a Schottky diode.
A bipolar junction transistor with a Schottky barrier between the base and the collector is known as a Schottky transistor. Because the junction voltage of the Schottky barrier is small, the transistor is prevented from saturating too deeply, which improves the speed when used as a switch. This is the basis for the Schottky and Advanced Schottky TTL families, as well as their low power variants.
A MESFET, or metal–semiconductor FET, is a device similar in operation to the JFET, which utilizes a reverse biased Schottky barrier to provide the depletion region. A variant of this device is the high-electron-mobility transistor (HEMT), which also utilizes a heterojunction to provide a device with extremely high conductance.
Schottky barriers are commonly used also in semiconductor electrical characterization techniques. In fact, in the semiconductor, a depletion region is created by the metal electrons, which "push" away semiconductor electrons (simplification, see depletion region article). In the depletion region, dopants remain ionized and give rise to a "space charge" which, in turn, give rise to a capacitance of the junction. The metal–semiconductor interface and the opposite boundary of the depleted area act like two capacitor plates, with the depletion region acting as a dielectric. By applying a voltage to the junction it is possible to vary the depletion width: if we reverse bias the junction, the dopants electrons will be emitted and pushed away; if we forward bias the junction, the electrons will be captured. By analyzing the emission and capture of electrons by dopants (or, more frequently, by crystallographic defects or dislocations, or other electron traps) is possible to characterize the semiconductor material. The most popular electrical characterization techniques that use this type of junction are DLTS and CV profiling.
A Schottky barrier carbon nanotube FET uses the non-ideal contact between a metal and a carbon nanotube to form a Schottky barrier that can be used to make extremely small Schottky diodes, transistors, and similar electronic devices with unique mechanical and electronic properties.
