Limitations of Field Plate Effect Due to the Silicon Substrate in AlGaN/GaN/AlGaN DHFETs

We investigated the limitations of the field plate (FP) effect on breakdown voltage
$V_{\rm BD}$
that is due to the silicon substrate in AlGaN/GaN/AlGaN double heterostructures field-effect transistors. In our previous work, we showed that in devices with large gate–drain distance
$(L_{\rm GD} > \hbox{8}\ \mu\hbox{m})$
, the breakdown voltage does not linearly increase with
$L_{\rm GD}$
because of a double leakage path between the silicon substrate and the metal contacts, which makes the device break at the silicon interface. In this paper, we showed that the effect of the FP for such large
$L_{\rm GD}$
is not significant because the breakdown is still dominated by the silicon substrate. The increase in
$V_{\rm BD}$
due to the FP is significant only for devices with small gate–drain distances
$(L_{\rm GD} < \hbox{8}\ \mu\hbox{m})$
. Indeed we show that for such small
$L_{\rm GD}$
the increase in the breakdown voltage is more than double, whereas for larger
$L_{\rm GD}$
, this is only about 10%. Simulations of AlGaN/GaN/AlGaN devices for small
$L_{\rm GD}$
are carried out with different FP lengths and passivation thickness in order to study the electric field distribution.