  The decoupling of epitaxial factors influencing on the dynamic instabilities
of AlGaN/GaN metal-insulator semiconductor high electron mobility transistors
is investigated. Three different sets of samples have been analyzed by means of
dynamic instabilities in the threshold voltage (V$_{\mathrm{th}}$ shift).
Secondary ion mass spectroscopy, steady-state photoluminescence (PL)
measurements have been performed in conjunction with electrical
characterization. The device dynamic performance is found to be significantly
dependent on both the C concentration close to the channel as well as on the
distance between the channel and the higher doped C region. Additionally, we
note that experiments studying trapping should avoid large variations in the
sheet carrier density (N$_{\mathrm{s}}$). This change in the N$_{\mathrm{s}}$
itself has a significant impact on the V$_{\mathrm{th}}$ shift. This
experimental trends are also supported by a basic model and device simulation.
Finally, the relationship between the yellow luminescence (YL) and the band
edge (BE) ratio and the V$_{\mathrm{th}}$ shift is investigated. As long as the
basic layer structure is not changed, the YL/BE ratio obtained from
steady-state PL is demonstrated to be a valid method in predicting trap
concentrations in the GaN channel layer.
