Impact of intrinsic stress in diamond capping layers on the electrical behavior of AlGaN/GaN HEMTs

A finite-element model coupling 2-D electron gas (2-DEG) density, piezoelectric polarization charge Q-P, and intrinsic stress induced by a nanocrystalline diamond capping layer, was developed for AlGaN/GaN high electron mobility transistors. Assuming the surface potential is unchanged by an additional stress from diamond capping, tensile stress from the diamond cap leads to an additional tensile stress in the heterostructure and, thus an increase in the 2-DEG under the gate. As a result, additional compressive stress near the gate edges would develop and lead to decreased 2-DEG in the regions between the source and drain contacts (SDCs). Increased saturation drain current will be due to the reduced total resistance between SDC. Integration of the 2-DEG density from SDC revealed a redistribution of sheet density with total sheet charge concentration remaining unchanged. The modeling results were compared with the experimental data from Raman spectroscopy and I-V characterization, and good agreements were obtained.