  Effects of residual C impurities and Ga vacancies on the dynamic
instabilities of AlN/AlGaN/GaN metal insulator semiconductor high electron
mobility transistors are investigated. Secondary ion mass spectroscopy,
positron annihilation spectroscopy, steady state and time-resolved
photoluminescence (PL) measurements have been performed in conjunction with
electrical characterization and current transient analyses. The correlation
between yellow luminescence (YL), C- and Ga vacancy concentration is
investigated. Time-resolved PL indicating the C$_{\mathrm{N}}$O$_{\mathrm{N}}$
complex as the main source of the YL, while Ga vacancies or related complexes
with C seem not to play a major role. The device dynamic performance is found
to be significantly dependent on the C concentration close to the channel of
the transistor. Additionally, the magnitude of the YL is found to be in
agreement with the threshold voltage shift and with the on-resistance
degradation. Trap analysis of the GaN buffer shows an apparent activation
energy of $\sim$0.8eV for all samples, pointing to a common dominating trapping
process and that the growth parameters affect solely the density of trap
centres. It is inferred that the trapping process is likely to be directly
related to C based defects.
