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 Al 2 O 3 gate insulation, a threshold voltage V T 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 V T up to 7 V with the oxide thickness t ox increase, if holes compensate the negative polarization charge. By applying t ox  = 30 nm a V T  ∼ 3 V was obtained; p‐doping of the cap/barrier layers is suggested to reach the theoretically predicted scalability.