TABLE OF CONTENTS So far, we have considered impurity semiconductors with very low impurity contents, typically less than one part per million. We have characterized the impurity type and density by working out where the Fermi level is, and have found that in all cases it is well within the energy gap. This has meant, among other things, that the sums are much simpler, for we are able to approximate to the Fermi function. However, when the impurity level becomes very high (typically about 10 24m -3 or about 0.01%) the Fermi level moves right up into the conduction band (or down into the valence band for a p-type impurity). The semiconductor is then said to be 'degenerate'. The tunnel diode is the only device we shall mention in this chapter in which degenerate semiconductors are used on both the p- and n-sides of the junction. We shall, however, return to degenerate junctions in Section 12.7, where laser diodes are discussed.
The energy diagram at thermal equilibrium is given in Fig. 9.24(a), where, for simplicity, we take the difference between the Fermi level and the band edge as the same on both sides. It is interesting to see that the 'built-in' potential is larger than the energy gap; thus the number of electrons crossing over the potential barrier at thermal equilibrium must be small. Hence, I 0 in the rectifier equation is small, and the rectifying characteristic is rather elongated, as shown in Fig. 9.25(a).
