A simple yet comprehensive unified physical model of the 2D electron gas in delta-doped and uniformly doped high electron mobility transistors

This paper presents a new approach to model the 2DEG concentration (n/sub s/) versus gate voltage (V/sub G/) behavior and the equilibrium 2DEG concentration (n/sub s0/) in a HEMT. The approach results in a model which is "comprehensive" in the following sense. It is valid for both delta-doped and uniformly doped HEMTs. It captures all the three n/sub s/-V/sub G/ nonlinearities (subthreshold, gradual pinchoff, and gradual saturation), and the effects of all the device parameters including temperature, in relation expressing n/sub s/ as an explicit closed-form integrable function of V/sub G/ having continuous first derivatives; thus, the function readily yields a device current-voltage/capacitance-voltage (I-V/C-V) model which can be incorporated in software meant for simulation of circuits, particularly of the analog variety. The simple model is shown to predict the results of complex numerical calculations and experiments. The closed-form n/sub s0/ expression of the model is the first of its kind reported for delta-doped HEMT's, and reveals an interesting feature unexposed by earlier n/sub s0/ models: the reciprocal of n/sub s0/ in both delta-doped and uniformly doped devices decreases linearly with reduction in spacer width, and saturates at low spacer widths in some delta-doped devices. It is shown that the measured n/sub s0/ in delta-doped devices is predicted correctly if the electrons in the V-shaped well are assumed to be trapped at the donor sites rather than to be filling the conduction subbands.