Transport Mechanisms and Measurements

Several possible current transport mechanisms are illustrated in Fig. 3. The schematic represents a Schottky barrier on an undoped sample under forward bias. The three arrows for electron transport are drawn for comparison with crystalline semiconductors in which thermionic emission, tunneling via thermionic field emission, or field emission represent the usual mechanisms. 

At room temperature it is usual to consider that electron transport over the barrier dominates the forward-bias current. For high-mobility materials such as crystalline Si, it is presumed that the electron population at any energy above the conduction-band minimum is independent of distance from the surface. The transport is then limited by the emission of electrons into the metal. For low-mobility materials it is possible that the current transport is limited by electron diffusion through the depletion region. In this case, the electron population at any energy will vary as a function of distance from the interface. Thus if the thermionic emission limits the transport, then 

At lower temperatures, tunneling currents may become more important. The tunneling of electrons from the conduction band into the metal is certainly observed in crystalline Si and has been reported for amorphous Si (Alkaisi and Thompson, 1979-1980). The process is more complicated in a-Si H because the localized states near the conduction-band tail may significantly enhance the tunneling current. 

The last possibility, which corresponds to field emission in crystalline semiconductors, is a much less likely current path in a-Si H Schottky barriers under forward bias. Since the Fermi energy is near midgap, there is a relatively low density of states and the states are localized. Thus there are few states that can field emit into the metal. In reverse bias, field-emission breakdown can be observed because the emission is from the metal Fermi energy into the a-Si H conduction band. Measurements and an analysis have been 

The forward-bias current-voltage characteristic is the most used method of characterizing the barrier. Following the formalism developed for crystalline semiconductors (Rhoderick, 1978), the current density J can be written as 

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The second part of this chapter will describe some of the general aspects of the Schottky barrier and properties specific to a-Si H. The third part is devoted to the transport mechanisms and measurements, and the fourth describes the effects due to atomic structural properties of the interface. Next, the effects due to doping are addressed, and following that the origin of the Schottky barrier is discussed. By presenting the question of the origin of the barrier near the end of this chapter, we imply that it is an unsolved problem. The chapter is concluded with comments about applications and important problems to be addressed for further understanding. 

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