The "Atlas user's manual " discusses traps comprehensively, which can be downloaded easily on the internet, and chapter 21.63 gives definitions and statements of traps in the bulk semiconductors, maybe you can find solutions and some examples there.

You can set interface traps in HEMT device using INTTRAP statement. You can set dicrete traps within the semiconductor band gap. In HEMTs devices people prefer to use interface traps at 2DEG rather than bulk traps.

As an example of volume traps with gaussian distribution in GaN buffer:

doping trap region = "GaN" acceptor e.level=0.8 concentration=1e18 gaussian peak=1

characteristic=0.455 degen.fac=1 sign=1e-15 sigp=1e-15

Example of interface traps between nitride and AlGaN barrier:

inttrap intname = "nitride/AlGaN" acceptor e.level=0.8 density=2e12 degen.fac=1 sign=1e-15 sigp=1e-15

thank you so much !!