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Schottky barrier[′shät·kē ‚bar·ē·ər]
a potential barrier that is formed in the contact region of a semiconductor, that is, in the region that adjoins a metal. The barrier is named after W. Schottky, the German scientist who investigated it in 1939. To obtain a Schottky barrier, the work functions of the metal and the semiconductor must differ, as was first pointed out in 1939 by the Soviet scientist B. I. Davydov.
If an n-type semiconductor is brought into contact with a metal whose work function Ф is higher than that of the semiconductor, the metal becomes negatively charged and the semiconductor becomes positively charged, because electrons pass from the semiconductor to the metal more easily than in the opposite direction. If a p-type semiconductor is brought into contact with a metal whose work function Ф is lower than that of the semiconductor, the metal becomes positively charged and the semiconductor becomes negatively charged. When equilibrium between the metal and the semiconductor is established, a contact potential Uc appears; Uc = (Фm – Фs)/e, where e is the charge of the electron. Owing to the high conductivity of the metal, the electric field does not penetrate the metal, and the potential difference Uc is produced in the surface layer of the semiconductor. The direction of the electric field in the semiconductor’s surface layer is such that the energy of majority carriers in the surface layer is higher than that in the bulk of the semiconductor. This means that the energy bands of the contact region are bent upward in an n-type semiconductor and are bent downward in a p-type semiconductor (see Figure 1). As a result, if Фm > ФS for an n-type semiconductor or if Фm < ФS for a p-type semiconductor, a potential barrier is produced in the semiconductor near the contact with the metal.
The height of a Schottky barrier Ф0 = Фm – ФS. In real metal-semiconductor structures, this relationship does not obtain because local electronic states are usually present at the surface of the semiconductor or in the thin dielectric layer that is often formed between the metal and the semiconductor. The electrons in the local surface states shield the effect of the metal, so that the intrinsic field in the semiconductor is determined by the surface states and the height of the Schottky barrier does not depend on Фm. As a rule, Schottky barriers obtained by depositing an Au film on an n-type semiconductor have the greatest heights. The intensity of an electric image (seeSCHOTTKY EFFECT) also affects the height of a Shottky barrier.
A Schottky barrier has rectifying properties. When an external electric field is applied, the current through such a barrier is produced almost entirely by majority carriers. The magnitude of the current is determined by the rate at which carriers from the bulk of the semiconductor arrive at the surface or—in the case of a semiconductor with a higher carrier mobility—on the thermionic current in the metal.
Schottky-barrier metal-semiconductor contacts are widely used in microwave detectors and mixers (seeSCHOTTKY BARRIER DIODE), as well as in, for example, transistors and photodiodes.
REFERENCESStrikha, V. I., E. V. Buzaneva, and I. A. Radzievskii. Poluprovodnikovye pribory s bar’erom Shottki. Moscow, 1974.
Strikha, V. I. Teoreticheskie osnovy raboty kontakta metall-poluprovidnik. Kiev, 1974.
Milnes, A., and D. Feucht. Geteroperekhody i perekhody metallpoluprovodnik. Moscow, 1975. (Translated from English.)
T. M. LIFSHITS