OVERVIEW

The energy band model is often used in the description of MOS structures (see, for example, Refs. 1 to 4 in Chap. 2). The main features of such a description are summarized below. This material can best be understood by referring to Chap. 2, Secs. 2.2 to 2.5, and Appendix A.

Consider a two-terminal MOS structure on a p-type substrate, as shown in Fig. D.1a. Assume for the present that the effective interface charge per unit area, Q ? _o, is zero. Let W _M and W _S be the work functions of the gate and substrate materials, respectively (Appendix A). As an example, assume W _M > W _S. If the MOS structure is short-circuited ( V _GB = 0), electrons in the substrate, where the work function is lower, will find it attractive to leave and, traveling through the external wire, enter the gate. This will leave behind positive charges in the substrate, as has been shown in Fig. 2.2b, and will cause in it a potential drop. The other energy levels will thus bend until the Fermi levels in the gate and substrate can line up (as in the case of the pn junction in Fig. A.4). If it is desired to prevent this from happening and, instead, keep the bands "flat" in the substrate region next to the oxide, no potential drop and no net charges should exist there. This can be achieved by inserting in the...