Please check and try attached .in file in DeckBuild environment in SILVACO ATLAS TCAD. This code consists of 2 HEMT structures which you could find in related literature. The first structure is a conventional AlGaN/GaN HEMT structure. The second one is recessed Gate HEMT. Also, you could probably find and study the HEMT device characteristics after running the codes for instance drain current, impact generation rate, Cgs, Cgd, current density, Unilateral power gain (dB), electric field, etc. Moreover, there is an opportunity to modify the codes according to your case study.
I hope the simulation code could come in handy for your case study.
I would like to know the code for reference purpose. I've seen the AlGaAs /GaAs HEMT example in SILVACO's example section , but I need the AlGaN/GaN based HEMT code for breakdown voltage.