Polarization‐Engineered n+GaN/InGaN/AlGaN/GaN Normally‐Off MOS HEMTs

The proposal, processing and performance of n+GaN/InGaN/AlGaN/GaN normally-off metal-oxide-semiconductor (MOS) HEMTs with a recessed 2-μm long gate/8-μm source-drain distance are presented. It is shown that by using a negative polarization charge at the InGaN/AlGaN interface together with 10-nm thick Al2O3 gate insulation, a threshold voltage VT increases by 3.6 V reaching a value of ∼1.6 V. Moreover, the combination of the gate recessing through the n+GaN cap and gate insulation lead to an invariant maximal drain current of about 0.25 A mm−1, as well as decreased gate leakage current in the order of ∼10−9 A mm−1. Analytical equations explain the predictive setting of VT up to 7 V with the oxide thickness tox increase, if holes compensate the negative polarization charge. By applying tox = 30 nm a VT ∼ 3 V was obtained; p-doping of the cap/barrier layers is suggested to reach the theoretically predicted scalability.