Comments on "Ill conditioning in self-heating FET models" [and reply]

A recent letter by S.A. Maas (see ibid., vol.12, no.3, p.88-9, March 2002) reported ill conditioning in nonlinear circuit simulators caused by the introduction of self-heating effects into FET models. This is true for circumstances outlined in that work but is a consequence of using an incomplete thermal model. This letter points out that an account for both thermal potential and mobility variation with temperature will eliminate the problem.