Performance and Reliability Modeling of AlGaN/GaN Hetero-junction Field Effect Transistors

Two independent problem areas in AlGaN/GaN HFETs that affect device performance and reliability are investigated. The first is an investigation into the nonlinear resistance phenomenon present in these devices under high current RF operation. High-voltage microwave AlGaN/GaN HFETs operating under high-power conditions suffer from degraded RF performance and linearity due to a nonlinear resistance effect in the gate-source region. During RF operation, the nonlinear resistance is due to the onset a of space-charge-limited current (SCLC) transport mechanism within the device channel. Under high current injection conditions, SCLC transport can set in and, consequently, the source resistance becomes a function of the injected charge resulting in a rapid increase and limiting device performance. The threshold for SCL current is dependent on the donor-like states on the AlGaN surface which are responsible for supplying electrons to the channel. To understand the effect of nonlinear source resistance we show on an un-gated HFET channel model that the critical current density of space-charge effects and thus the onset of nonlinear source resistance can be reduced or can be shifted above the normal operating current density of the device by modification of the charge on the AlGaN surface.;Secondly, nonlinearities stemming from avalanche breakdown due to high electric field magnitudes in the device channel while operating under high voltage conditions is investigated. A temperature dependent, impact ionization initiated RF breakdown model in the 2DEG channel of AlGaN/GaN HFETs is reported. When operating these devices in RF power amplifier circuits, impact ionization in the channel has a significant effect upon gain saturation, power-added efficiency and output power. An analytical physics-based model of channel breakdown is formulated based on TCAD investigations of the internal device behavior. This model is integrated with an existing physics-based HFET compact model and accurately predicts large-signal device performance. Values of thermal resistance and the breakdown temperature coefficient were extracted from simulations of an industrial HFET and are in agreement with the published measurements as well as an independent model of thermal resistance in AlGaN/GaN HFETs thus validating that the model accurately captures the dominant breakdown mechanism.