Buffer Trap Related Knee Walkout and the Effects of Self-Heating in AlGaN/GaN HEMTs

Abstract

Mixed-mode simulations of a class A amplifier is used to study the DC/RF dispersion commonly observed in AlGaN/GaN based HEMTs. We show that the observed knee walkout at frequencies greater than the emission rates of buffer traps (time constants t a e > 1 week) is related to the steady state trap density and spatial location due to the DC operational bias. An increase in the drain bias point and an initial distortion of the RF signal, that is expected to disappear as the device global temperature reduces, is observed when a self-heating model is included. Finally, we propose that a reduction in the DC/RF dispersion is possible with a suitable location and concentration of an acceptor doping in the buffer. Direct current (DC) to radio frequency (RF) dispersion highlighted in experimental studies 1,2 of Gallium Nitride (GaN) based High Electron Mobility Transistors (HEMTs) in the amplifier operation has been attributed to trapping in the device structure. The effect of this dispersion is a reduction in the available RF output power, and effectively results in the reduced performance of GaN HEMTs in RF and power applications in spite of its very promising material properties. The reduction in RF output power can be observed in a so-called knee walkout. The knee walkout describes a situation where the minimum drain voltage under RF drive is smaller than that obtained from DC as opposed to the current collapse (CC) which refers to the reduction in the RF maximum drain current compared to that observed under DC. 3Although experiments have revealed a dependence of the knee walkout on the DC operating point, it remains not understood to what extent degradation processes due to either the surface or the buffer traps affect the device's RF performance. Pulsed measurements of the device have been employed to study the collapse of the current and the dispersion at the knee of the output characteristics, 2 mirroring thus the expected behavior of the device in amplifier operation. This approach is useful in gaining important insights to possible trapping behavior but loses its accuracy in simulations due to the inaccurate determination of the pulsed knee voltage. The simultaneous application of voltages on the drain and the gate contacts in pulsed simulations is in reality achieved with one bias ensuing the other. Here, we have employed a mixed mode simulation of a GaN HEMT in class A operation, which ensures that the instantaneous applied biases depend only on the amplifier operation. The behavior of the device is thus accurately modeled using a physically based device simulator while the remaining part of the circuit is modeled using conventional circuit simulation techniques 4 in a real spirit of TCAD. In addition to the much more realistic simulation approach resulting in increased accuracy, the internal device conditions can be examined at any given point.The investigations are carried out using the drift-diffusion transport model. The study is based on a careful calibration of device I-V characteristics to experimental ...

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