Role of the AlGaN Cap Layer on the Trapping Behaviour of N-Polar GaN MISHEMTs

We investigate the static and dynamic (trapping) performance of N-polar Gallium Nitride MIS-HEMT devices as a function of the aluminum concentration in the top cap layer (22%, 34% and 46%). The analysis is based on combined dc characterization, double pulse measurements, and threshold voltage transient investigation. The de results demonstrate that the use of high aluminum concentrations in the cap layer results in a lower gate leakage current (around 9 µA/mm for % Al=46, compared to 70 µA/ mm for %Al=22, measured at V
Gs
=-7 V and V
DS
=15 V). In addition, pulsed and transient investigation showed that the use of high Al concentration in the cap layer can substantially suppress the current collapse (slump ratio = 15 % for %Al=46, compared to 26 % for %AI=22). Trapping is ascribed to the presence of a defect state located at E
C
-0.5 eV, which is responsible for a threshold voltage shift. The results point out the key role of the AlGaN cap layer on the performance of AlGaN-based HEMTs, and give indication on how to optimize the performance of the devices.