Rectifiers and Transistors

Abstract

Although in solid-state rectifiers one usually employs one semiconducting contact, semiconduction itself is not essential for the rectification process. This may be illustrated by considering two metals of different work function separated by a thin vacuum gap. As we have seen in Sec. 9-10, the Fermi levels of the two metals must coincide in thermal equilibrium, leading to the situation depicted in Fig. 14-1a; the metal of low work function acquires a positive surface charge, the other acquires a negative surface charge. The total potential drop across the gap isequal to (ϕ₁ − ϕ₂)/e. It is convenient to consider this situation a dynamic equilibrium in which the electronic current from 1 to 2 is equal to that from 2 to 1. Let us denote this current density by I₀. Suppose now metal 2 is made negative with respect to 1 by applying an external voltage smaller than the voltage drop (ϕ₁ − ϕ₂)/e. The energy levels of 2 are then raised relative to those in 1, and the situation corresponding to Fig. 14-lb results. The current I_1→2 is still equal to I ₀ because the barrier viewed from the position of metal 1 has not changed. On the other hand, the potential energy hill as viewed from metal 2 is lowered by an amount eV, which makes the probability for an electron to cross the hill larger by a factor e^ev/kT Hence the net electron current is