Numerical Optimization of On-Resistance and Transconductance in Depletion-Mode and Enhancement-Mode GaN HEMTs

In this paper, we use a numerical mobility modeling framework for GaN-based high electron mobility transistors (HEMTs) to examine the dependence of important design parameters such as on-resistance and transconductance on the device structure. The model is calibrated by matching previously published experimental mobility data for a depletion-mode AlGaN/AlN/GaN/AlGaN double-heterostructure (DH) HEMT to within +/- 10%. The structural parameters of the device are optimized and a minimum on-resistance of 541 Omega/square is achieved with a mole fraction of 0:36 and thickness of 14 nm for the AlGaN barrier. A p-GaN cap is added to the initial structure to achieve enhancement-mode operation, and the impact of cap and barrier thickness on threshold voltage and on-resistance is discussed. The transconductance of enhancement-mode devices at an overvoltage of 8 V is improved from 1.58 to 2.59 mS/square by simultaneously reducing the thickness of the p-GaN and AlGaN layers, thus increasing the gate capacitance while maintaining the same threshold voltage and on-resistance. The optimization procedure allows to obtain similar design parameters for the enhancement-mode HEMT as for the initial depletion-mode structure, proving that the addition of the p-GaN cap is an efficient way to increase the threshold voltage without compromising device performance.