Metal-Semiconductor Contacts Schottky Junctions

This section is of special interest because at first sight there are certain similarities between semiconductor-metal junctions and semiconductor-liquid interfaces. This will be discussed in more detail in Chapter 7. 

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The term photovoltaic effect is further used to denote non-electrochemical photoprocesses in solid-state metal/semiconductor interfaces (Schottky barrier contacts) and semiconductor/semiconductor pin) junctions. Analogously, the term photogalvanic effect is used more generally to denote any photoexcitation of the d.c. current in a material (e.g. in solid ferroelectrics). Although confusion is not usual, electrochemical reactions initiated by light absorption in electrolyte solutions should be termed electrochemical photogalvanic effect , and reactions at photoexcited semiconductor electodes electrochemical photovoltaic effect . 

Eq. (11.1) is also valid for pure solid state devices, such as semiconductor-metal contacts (Schottky junctions) and p-n junctions, as described in Chapter 2. The physics of the individual systems occurs only in y o- The main difference appears in the cathodic forward current which is essentially determined by /o. In this respect it must be asked whether the forward current is carried only by minority carriers (minority carrier device) or by majority carriers (majority carrier device). Using semiconductor-liquid junctions, both kinds of devices are possible. A minority carrier device is simply made by using a redox couple which has a standard potential close to the valence band of an n-type semiconductor so that holes can be transferred from the redox system into the valence band in the dark under cathodic polarization. In this case, the dark current is determined by hole injection and recombination (minority carrier device) and /o is given by Eq. (7.65), i.e. 

Schottky barrier diodes - a diode consisting of a metal-semiconductor contact which has rectifying characteristics similar to a p-n junction differs from a p-n junction diode in that the diode s forward voltage is different (lower for commonly used materials), and there is no charge stored when the diode is forward biased device can therefore be turned off very rapidly by application of reverse bias, as storage time is negligible. 

The main advantage of poly-p-phenylene is that, due to its nonacetylenic composition, it has a much higher thermal stability (450°C in air and 550°C in inert atmosphere). Potential applications of poly-p-phenylene are similar to those envisaged for polyacetylene, such as Schottky barriers in photocells. (A Schottky barrier is a metal semiconductor contact that has rectifying characteristics similar to a p-n junction.)... 

Semiconductor surfaces can be modified by electrochemical deposition of metals. This is of great technological importance because metal-semiconductor contacts such as ohmic contacts and Mott-Schottky diodes are required for many semiconductor devices. For instance, an ideal Mott—Schottky junction was formed by electrochemical deposition of Cu on n-GaAs as discussed in Section 2.2. The electrochemical method is very attractive because the adjustment of the electrode potential offers a unique tool of controlling and structure of the interface. 

Schottky barrier — Energy barrier formed at metal-semiconductor junctions. When a metal is intimately brought into contact with a semiconductor, charge... 

Figure 6 Schottky junction between a metal and an n-type semiconductor (a) before contact (b) after contact, without bias (c) forward bias (d) reverse bias.

Contacts are the elementary building blocks for all electronic devices. These include interfaces between semiconductors of different doping type (homojunc-tions) or of different composition (heterojunctions), and junctions between a metal and a semiconductor, which can be either rectifying (Schottky junction) or ohmic. Because of their primary importance, tire physics of semiconductor jrmc-tions is largely dealt with in numerous textbooks [11, 12]. We shall concentrate here on basic aspects of the metal-semiconductor (MS) and, above all, metal-insulator-semiconductor (MIS) junctions, which are involved in the organic field-effect transistors. 

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