leakage current is the current of the gate Ig

I highly recommend to rearrange your question, otherwise you have no chances to have an answer (The Answer is as What the Question was).

As I see TCAD entries in topics, you probably mean something about setting HEMT parameters in simulation rather than HEMT fabrication things. It's better to include some phrases about TCAD and simulation in your question directly and not only in topics. Unfortunately I'm not a TCAD user.

For the real HEMT the leakage current (which is actually only reverse part .of gate IV-curve) is defined by electrophysical properties of near gate area, which are subsequently defined by many factors: heterostructure design, quality, metallization, surface condition near the gate, type and quality of gate passivation. You cannot modulate it once you have got a HEMT in your hand. It is as is.