Schottky barriers to conduction

If a voltage difference is established across the junctions by a battery in the sense that the semiconductor potential is made less positive, the barrier height is lowered and electrons are more readily excited over it. The rate of passage of 

A sandwich comprising a semiconductor between two metallic electrodes presents the same effective barrier irrespective of the sense of an applied voltage. The situation is illustrated in Fig. 2.21. The resistance will be high at low voltages, because few electrons cross the barriers, but will be low once a voltage is reached which enables electrons to cross the metal-semiconductor barrier at a significant rate. The resulting characteristic is shown in Fig. 2.22 and is similar to that for two rectifiers back to back. 

In the case of wide band gap insulators the current will be low at all voltages but will initially exceed that to be expected under equilibrium conditions because of a redistribution of charges in the vicinity of the junctions. This makes it 

A complicating matter disregarded in the discussion concerns surface states . At a semiconductor surface, because of the discontinuity, the atomic arrangement is quite different from that in the interior of the crystal. In consequence the electrons on the surface atoms occupy localized energy states quite different from those in the interior. As an added complication impurity atoms carrying their own localized energy states will e adsorbed on the semiconductor surface. 

Barriers of the Schottky type control the behaviour of voltage-dependent resistors (VDRs), PTC resistors and barrier-layer capacitors. Their behaviour is by no means as well understood as that occurring in semiconductors such as silicon but, where appropriate in the text, simplified models will be presented to indicate the principles involved. 

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