Positive and negative charge trapping GaN HEMTs: interplay between thermal emission and transport-limited processes

TL;DR

This study explores the charge trapping mechanisms in GaN HEMTs, revealing that transport-limited processes, rather than thermal emission, dominate trapping and de-trapping kinetics, with implications for device reliability.

Contribution

It provides a detailed analysis of charge trapping dynamics in GaN HEMTs, highlighting the role of transport mechanisms over thermal processes and quantifying activation energies.

Findings

Negative charge trapping activation energy ~0.3 eV

Transport processes limit charge transfer to traps

Positive charge generation occurs after high stress bias

Abstract

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.