To investigate the influence of the thickness of the deposited epi-layers
simulations were carried out for devices with modified layer thickness.
Fig. 5.12 shows the on-resistance of a device in which the
thickness of each epi layer has been increased by
0.3 
m and one in
which the thickness has been reduced by
0.3 m in comparison to the
standard and the optimized device. The on-resistance of the device with the
thicker epi layers is considerably higher than for the optimized device, it is
even 10% higher than the resistance of the standard device. For the device
with thinner epi-layers the on-resistance is approximately 10% lower than for
the optimized device.
The maximum electric field increases slower for the device with thicker epi-layers and has smaller values than the optimized device in the whole simulated range of drain-source voltages. For the device with thinner epi-layers the maximum electric field is approximately 15% higher for a wide range of drain-source voltages as shown in Fig. 5.13. At drain-source voltage higher than 130 V the maximum electric field exceeds 300 kV cm- 1 and the impact ionization rate increases leading to breakdown.
These simulations show that the thickness of the deposited epi-layers is critical for the device performance. Especially for epi-layers that are thicker than in the optimized device the on-resistance degrades rapidly. The increase of the maximum electric field for an epi-layer thickness thinner than in the optimized device is not critical.
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