I've been trying to create a HEMT device over the past couple of weeks in Silvaco TCAD. My device works just fine in low voltage operation- that is, IdVg and IdVd curves are as expected. I'm getting a clear cutoff, linear and saturation region.
The problem however, is with high voltage analysis- specifically breakdown analysis. My AlGaN-GaN HEMT won't break down regardless of how high a potential difference I apply across it. I've done the obvious, which is checking what physical models and mathematical models I'm using. I'm using Selberherr’s Model for impact ionisation, along with field mobility, srh, fermi and pch.elec models. All of these should suffice for breakdown analysis of a device- atleast they do in ganfetex02, Silvaco's inbuilt example.
At first, I was using a complicated field plated structure with several other shenanigans attached, so I thought the covoluted structure was the problem. However, upon re-trying on a barebones AlGaN-GaN only HEMT structure, I still obtained the same lack of breakdown.
I have included this code along with seemingly infinite saturation characteristics of my device. Any help on what models I should change or any parameters to modify is greatly appreciated. Cheers.
IdVd curve obtained: https://i.stack.imgur.com/FKYUU.png
code of barebones HEMT structure:
#basic HEMT structure and analysing breakdown
go atlas
#meshing defintion
mesh width=50
x.mesh l=0 s=0.05
x.mesh l=1 s=0.05
y.mesh l=-0.05 s=0.05
y.mesh l=0.09 s=0.05
y.mesh l=0.1 s=0.01
y.mesh l=0.15 s=0.01
y.mesh l=0.20 s=0.05
y.mesh l=0.30 s=0.05
#region defintion
region num=1 mat=air y.min=-0.05 y.max=0.3
region num=2 mat=AlGan y.min=0 y.max=0.1 donors=3e17
region num=3 mat=GaN y.min=0.1 y.max=0.3 donors=1e15
#electrode definition
elec num=1 name=source y.min=-0.05 y.max=0.15 x.min=0 x.max=0.05
elec num=2 name=drain y.min=-0.05 y.max=0.15 x.min=0.95 x.max=1
elec num=3 name=gate y.min=-0.05 y.max=0 x.min=0.45 x.max=0.55
#doping definition
doping gaussian characterisitic=0.01 conc=1e19 n.type x.left=0 x.right=0.09 \
y.top=0.1 y.bottom=0.19 ratio.lateral=0.01 direction=y
doping gaussian characterisitic=0.01 conc=1e19 n.type x.left=0.91 x.right=1 \
y.top=0.1 y.bottom=0.19 ratio.lateral=0.01 direction=y
doping gaussian characterisitic=0.01 conc=3e17 n.type x.left=0 x.right=1 \
y.top=0 y.bottom=0.1 ratio.lateral=0.01 direction=y
doping gaussian characterisitic=0.01 conc=1e15 n.type x.left=0 x.right=1 \
y.top=0.1 y.bottom=0.3 ratio.lateral=0.01 direction=y
#displaying the structure
save outf=basicStructure.str
tonyplot basicStructure.str
#models definition
model print fermi fldmob srh pch.elec
impact selb
#contact definition
contact name=gate workfunction=4.8
#method specification
method newton itlim=35 trap maxtrap=10 carriers=1
output con.band val.band band.param charge e.mob h.mob flowlines qss
#idvd analysis
solve init
log outf=basicHEMT.log
solve vstep=0.1 vfinal=1 name=drain
solve vstep=1 vfinal=10 name=drain
solve vstep=2 vfinal=20 name=drain
solve vstep=5 vfinal=1200 name=drain cname=drain compl=1.0
save outf=basicStructure.str
tonyplot basicHEMT.log
quit
```
