A MODFET dc model with improved pinchoff and saturation characteristics

We present an analytical dc model for the MODFET that offers several improvements over existing models. An enhanced version of the model makes use of a new approximation of the two-dimensional electron gas (2DEG) concentration versus gate-to-channel voltage, which models both the subthreshold region and the gradual saturation of carriers due to the onset of AlGaAs charge modulation. Even in this more accurate model there are no complicated numerical calculations involved; at most what is required is finding a single root of a function of one variable. We propose an electron velocity-field curve that combines the observed field-dependent mobility in the 2DEG, and the sharp velocity saturation in GaAs. We use a two-region Grebene-Ghandhi model with floating boundary for the channel. The quasi-linear region on the source side is treated by the gradual channel approximation and extends toward the drain up to a point where the field reaches its threshold for velocity saturation. Between this point and the drain-side end of the channel, the potential is determined by the two-dimensional Poisson equation in the AlGaAs region. The resuiting characteristics and their slopes are continuous. The model predicts a maximum transconductance and a finite intrinsic output conductance in the saturated region, two features experimentally observed but not predicted by previous models. In the limit of very short gate lengths the model approaches the saturated velocity model, while in the limit of very long gate lengths it approaches the classical gradual channel model.