Importance of low-field drift velocity characteristics for HEMT modeling

The drift velocity versus field relationship, which is precisely calculated by taking into account electronic states and dominant two-dimensional scatterings, is introduced in numerical modeling of high electron mobility transistors (HEMT's). In comparison with conventionally used analytical drift velocity expressions such as two-piece linear model and silicon-like simple model, cutoff frequency characteristics can be explained without using excessively large saturation velocity values lacking in physical meaning. The calculated electric field near the source under the gate is a few kilovolts per centimeter. It is concluded that low-field drift velocity characteristics are very important in estimating the device performance, and must be based upon accurate physics.