8.6 GENERAL MODELS

General expressions for small-signal parameters can be developed for the simplified charge-sheet model ^[51]- ^[53] discussed in Sec. 4.3.2, as well as for the EKV model ^[54]- ^[57], ^[57a] discussed in Sec. 4.8. An example of a set of expressions valid in weak, moderate, and strong inversion saturation regions is shown in Table 8.1. Except for channel length modulation, short-channel effects are not included. All symbols not defined in the table are as used in the corresponding sections of this chapter.

Table 8.1: A saturation region long-channel nMOS model valid in all regions of inversion ^{[ ]}

Drain current:

Small-signal conductances:

Small-signal intrinsic capacitances:

Noise power spectral densities:

[ ]The current equation [242] has been adopted from Refs. 54-57a for using the source as a reference and for making possible the inclusion of an accurately evaluated V T. The small-signal and noise expressions are from Refs. 55 and 57. The expression for f( u) is from Refs. 52 and 53. [ ]Y. Tsividis, K. Suyama, and K. Vavelidis, "A simple 'Reconciliation' MOSFET model valid in all regions," Electronics Letters, March 1995.

The value of n shown is a compromise for medium accuracy in all regions, and it is adequate for simple estimates. In weak inversion, better accuracy is obtained if n is modeled as in (3.4.25). In strong inversion, if the expected voltage ranges are significant, better accuracy for I _DS