InAlN/GaN HEMTs have been proposed to provide higher polarization charges without the drawback of high strain [62]. Several groups have demonstrated devices based on InAlN/GaN [390,391] with maximum current capabilities surpassing those of AlGaN/GaN structures.
Optimization of the structures has been carried out by using analytical models
[62]. In order to fully develop the potential of the device, proper
modeling of the materials is required. Based on the material
parameters discussed in Chapter 4, a simulation study is
conducted. The HEMT structures described in [391]
and [392] are used to benchmark the DC and AC simulation
results against measured data. A schematic layer structure of the
investigated InAl
N/GaN device [392] is shown
in
Fig. 5.40. All layers are non-intentionally doped.
A band gap bowing factor of 3 eV is assumed for InAlN as discussed in
Section 4.3.1. This yields a band gap of 4.58 eV for
InAl
N at 300 K (Fig. 5.41). The values
for the band offsets are
E
=0.66 eV and
=0.59 eV, corresponding to a 53%/47% setup.
The
calculated dielectric permittivity of InAl
N is
=9.86, which is in a good agreement with the value
listed in
[392]. The barrier height of the gate Schottky contact is
1 eV. The value of the sheet charge density at the InAlN/GaN interface
induced by the polarization effects is found to be
3.3
10
cm
from the DC characteristics
(simulation results for different values are given
in Fig. 5.42).
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A commensurate negative surface charge (as
the device is not passivated, a low value of 10 cm
is
assumed) at the top of the InAlN surface is also considered in the
simulation. Simulation results for the transfer characteristics
assessing different charge values are shown
in Fig. 5.43.
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Self-heating effects are accounted for by
using a thermal resistance of R
=3 K/W at the substrate
thermal contact. Fig. 5.44 compares transfer
characteristics without self-heating effects and with different values
of R
. This value lumps the thermal resistance of the
nucleation layer and the sapphire substrate, and possible
three-dimensional thermal effects.
![]() |
The simulation exhibits good agreement with the experimental data
under consideration of Ohmic contact resistances
R
=R
=1.3
mm. The simulated output
characteristics show a good agreement with the experimental data
(Fig. 5.45).
By AC analysis of the device a cut-off frequency
7 GHz
is obtained. This low value can be explained with the conservative
design of the device and the low carrier mobility in the channel
(
=230 cm
/Vs). Downscaled devices are analyzed (
)
and the effect of higher quality GaN material on the AC performance
is studied. In our simulation of a device with
reported in
[391] (carrier mobility
=530 cm
/Vs)
=36 GHz is
reached.