On the staircase like structures in gate C-V characteristics of InSb/In0.8Al0.2Sb MOS-HEMT devices

Gate C-V characteristics of InSb/In_0.8Al_0.2Sb MOS-HEMT devices show staircase like behavior for both positive and negative gate voltages. We investigate the origin of this staircase nature. It is found that at low magnitudes of the gate voltages, the carriers are confined within the channel and the top InAlSb barrier behaves as a dielectric layer. Thus the gate oxide capacitance is determined by the series combination of the capacitance of the gate dielectric layer and the InAlSb barrier layer, as expected. However, when the gate voltage magnitude increases above a critical value, carriers are not confined within the channel region only. The InAlSb barrier region is also populated with carriers. As a result, InAlSb layer does not contribute to gate capacitance. The gate capacitance increases sharply and exhibits the staircase like structure.