Schottky barrier

A Schottky barrier is 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.

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Not all metal-semiconductor junctions are 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, and the type and concentration of dopants in the semiconductor. Design of semiconductor devices requires familiarity with the Schottky effect to ensure Schottky barriers are not created accidentally where an ohmic connection is desired.

Advantages

Schottky barriers, with their lower junction voltage, find application in areas where a device better approximating an ideal diode is desired. 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.

Overall, however, Schottky devices find only limited application compared to other semiconductor technologies.

Devices

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 particularly interesting variant of this device is the HEMT, or High Electron Mobility Transistor, 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 nonideal contact between a metal and a carbon nanotube (CNT) to form a Schottky barrier that can be used to make Schottky diodes or transistors, or so on. The scaling of semiconductor devices to ever-smaller sizes is rapidly approaching fundamental limits. Carbon nanotubes may become a practical alternative to customary devices due to their small size and unique mechanical and electronic properties.

See also

Ohmic contact

Schottky diode

Diode