Schottky barrier
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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.
Not all metal-semiconductor junctions are Schottky barriers. This depends 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. Infact, in the semiconductor part of the junction, a depletion area is created by the potential due to metal electrons, which "push" away semiconductor electrons. In the depleted area dopants remain ionized and give rise to a "space charge" which, in turn, give rise to a capacitance of the junction. That means that the metal-semiconductor interface and the opposite boundary of the depleted area are acting like two capacitor plates, with the depleted area as dielectric. By applying voltage to the junction is possible to vary the depleted zone, which means make the dopant electron be emitted and pushed away (if we apply reverse voltage to the junction), or being captured (if we apply forward voltage). By analysing the emission and capture of electrons by dopants (or, more frequently, by crystallographic defects o dislocations, or other electron traps) is possible to characterize the semiconductor material. The most popular electrical characterization techniques that use this tipe of junction (but not only) are DLTS and CV Profiling.