Charge trapping in 0.1 μm AlGaN/GaN RF HEMTs: Dependence on barrier properties, voltage and temperature

The goal of this paper is to describe and understand the de-trapping dynamics in AlGaN/GaN transistors for radiofrequency (RF) applications, as a function of three parameters: the aluminium content in the barrier (22.6%, 24.6%, and 26.6%), the stress voltage applied to the drain and temperature. The analysis is based on threshold voltage transient measurements, carried out under at different stress conditions. The original results of this analysis show that: (i) the analysed devices show the presence of a dominant charge-trapping process E1 that, at room temperature, has de-trapping time constants in the range of 10 ms, and activation energy around 0.6 eV. (ii) trap E1 (that is responsible for ~0.5 V threshold voltage shift) is located in the semiconductor material, and its effect increases with increasing aluminium content in the barrier. (iii) leakage measurements indicate that devices with higher Al content in the barrier have a higher gate leakage. Our hypothesis is that under off-state stress, traps in the semiconductor material are charged by electrons injected from the gate metal through defects located in the AlGaN barrier. (iv) a second trap E2, whose signal is independent on the properties of the barrier, was also detected. This trap is supposed to originate from surface defects, is responsible for a minor (~0.1 V) threshold voltage shift and is weakly thermally activated (0.25–0.4 eV). (v) finally, a detailed analysis of the de-trapping kinetics indicated that the signal associated to trap E1 increases linearly with the drain stress voltage, and the related kinetics are ideal exponentials (consistent with semiconductor traps), while the amplitude of trap E2 does not significantly depend on drain voltage, and its kinetics are described by a stretched exponential behaviour (consistent with surface traps).