OVERVIEW

TWO-TERMINAL STRUCTURE. Throughout this appendix, uniform substrates are assumed. In Sec. 2.5, the lower limit of the moderate-inversion region was taken to coincide with the upper limit of weak inversion. The latter is traditionally defined at ? _s = 2 ? _F. At this point, the surface electron concentration becomes equal to the bulk doping concentration N _A, as seen from (2.4.11b). Also, at this point C ? _i = C ? _b, as seen from (2.6.14) and (2.6.15). These two facts hold independently of the value of C ? _ox. However, neither of these facts says anything about the validity of common "weak-inversion approximations" [such as Q ? _I( V _GB) being an exponential] at this point. This is because the validity of such approximations cannot be discussed carefully without evoking the value of C ? _ox. For detailed work, then, it makes practical sense to take C ? _ox into account, and redefine the onset of moderate inversion at some point beyond which common weak inversion approximations become unacceptable. In weak inversion, and at points where C ? _i is negligible (Fig. 2.16), C ? _b varies little; thus, d ? _s/ dV _GB in (2.6.18) will be approximately constant. That is necessary for an exponential dependence of Q ? _I on V _GB, as can be seen from the development leading from (2.5.36) to (2.5.42). Significant departure from such...