Two-dimensional analysis of the I-V characteristics of normally-off bipolar-mode FET devices

On the basis of the physical picture obtained by a detailed two-dimensional numerical simulation, an analytical model is developed for the I-V characteristics of a normally-off bipolar-mode FET structure. The numerical results show that the device has basically two modes of operation. When the minority-carrier injection from the gate is negligible, the device operation is governed by the modulation of the potential barrier in the channel due to the gate and drain voltages. This gives rise to an exponential shape of the I-V characteristics in the subthreshold regime, while for larger drain currents a transition to a triodelike shape takes place. The second mode of operation is due to the injection of minority carriers from the gate. This bipolar mode of operation is modeled according to a regional approximation method. The limits of the proposed quasi one-dimensional analysis are discussed by comparisons with the two-dimensional numerical results. The comprehensive analytical model obtained gives results in very good agreement with the numerical simulations and can be used to assess the influence of the various geometrical and physical parameters on the performances of the device.