Why GaN cap layer of AlGaN/GaN HEMT creates problem ATLAS simulation?

Dear Friend,

I have just added a 2nm GaN cap over AlGaN barrier in example 3 (ganfetex03.in : AlGaN/GaN HEMT Id-Vgs and Id-Vds Characterization) in atlas tcad.

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1. Without GaN cap , the example 3 gives Id as 0.03 (A/mm) at VGS=0 V for mesh width of 100

2. I added GaN cap with polarization model (as you said). But the IDS become too low as 3*10^(-10)

3. I used polar.scale=0.8 for AlGaN barrier and GaN channel. And used polar.scale=0.4 for GaN cap

4.There is no doping in example 3. So i just let it as it is

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I am still strugle to get proper IDS-VGS. I am kindly requesting to you mention where I am making mistake

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For clarity i have given the lines below

#AlGaN barrier

region num=1 x.min=0 x.max=9 y.min=0.5 y.max=0.525 mat=AlGaN\ x.comp=0.3 polarization calc.strain polar.scale=0.8

#SiN passivation

region num=2 x.min=0 x.max=9 y.min=0.0 y.max=0.5 mat=nitride insulator

#GaN channel

region num=3 x.min=0 x.max=9 y.min=0.525 y.max=2\

mat=GaN polarization calc.strain polar.scale=0.8 substrate

#GaN cap

region num=4 x.min=0 x.max=9 y.min=0.498 y.max=0.5\

mat=GaN polarization calc.strain polar.scale=0.4