Mechanisms of Step-Stress Degradation in Carbon-Doped 0.15-μm AlGaN/GaN HEMTs for Power RF Applications

We discuss the degradation mechanisms of C-doped 0.15-
$\mu \text{m}$
gate AlGaN/GaN HEMTs tested by drain step-stress experiments. Experimental results show that these devices exhibit cumulative degradation effects during the step-stress experiments in terms of either (i) transconductance (
$g_{\mathrm{ m}}$
) decrease without any threshold-voltage (
$V_{\mathrm{ T}}$
) change under OFF-state stress, or (ii) both
$V_{\mathrm{ T}}$
increase and
$g_{\mathrm{ m}}$
decrease under ON-state stress conditions. To aid the interpretation of the experiments, two-dimensional hydrodynamic device simulations were carried out. Based on the obtained results, we attribute the
$g_{\mathrm{ m}}$
decrease accumulating under OFF-state stress to hole emission from CN acceptor traps in the gate-drain access region of the buffer, resulting in an increase in the drain access resistance. On the other hand, under ON-state stress, channel hot electrons are injected into the buffer under the gate and in the gate-drain region where they are captured by CN traps, leading to
$V_{\mathrm{ T}}$
and
$g_{\mathrm{ m}}$
degradation, respectively.