  This paper investigates the kinetics of buffer trapping in GaN-based
normally-off high-voltage transistors. The analysis was carried out on
transfer-length method (TLM) structures. By means of a custom setup, (i) we
investigated the trapping and de-trapping processes induced by a large vertical
bias and identified different mechanisms, responsible for the storage of
negative and positive charge in the buffer. (ii) temperature-dependent analysis
was carried out to evaluate the time constants associated to negative and
positive charge build-up. Remarkably, the results indicate that the activation
energy for negative charge trapping is ~0.3 eV, which is much lower than the
ionization energy of carbon acceptors (0.8-0.9 eV). This result is explained by
considering that trapping and de-trapping are not dominated by thermal
processes (thermal emission from acceptors), but by transport mechanisms, that
limit the transfer of charge to trap states. (iii) in the recovery experiments,
after low stress bias negative charge trapping dominates. After high stress
bias, also the effect of positive charge generation is detected, and the
related activation energy is evaluated. The results presented within this paper
clearly indicate that the trapping and de-trapping kinetics of normally-off GaN
HEMTs are the results of the interplay of transport-limited conduction
processes, that result in a low thermal activation (Ea~0.3 eV), compared to
that of CN acceptors.
