My suggestion is to check your band diagrams(.str files) at equilibrium and some other voltages. Are they as expected?Also is the polarization charge showing at all expected interfaces?You can also check how your 2DEG is changing with voltage. This is standard debugging method.

Also, you have to mention the GaN layer as substrate in your input file, only then piezoelectric charges will be calculated for AlGaN properly.

Also, you can change workfunction values according to below example:

https://silvaco.com/examples/tcad/section9/example3/index.html

contact name=gate work=5 contact name=source work=3.93 contact name=drain work=3.93

The source and drain workfuction should be 3.93 to have a good contact with 2DEG in AlGaN

Thanks.

Prachi Pohekar Ma'am thankyou so much. I will do the same