TCAD Simulation and Modeling of AlGaN / GaN HFETs

KUANG, WEIWEI. TCAD Simulation and Modeling of AlGaN/GaN HFETs. (Under the direction of Dr. Robert J. Trew and Dr. Griff L. Bilbro). This work focused on the TCAD simulation and modeling of AlGaN/GaN HFETs. AlGaN/GaN HFETs have demonstrated excellent RF performance, which benefits from the high sheet charge density in these hetero-structures, the high carrier mobility and saturation velocity in the channel, and the high breakdown voltage inherent in the GaN material. However, these devices experience physical phenomena that degrade their performance. In particular, AlGaN/GaN HFETs often demonstrate Cgs variations with input power drive that are opposite to classical FET behavior. The transconductance is also degraded at high current levels. The behavior affects the frequency performance and linearity of the device. It has been numerically confirmed in this work that the nonlinear source resistance due to the on-set of space charge limited current transport condition existing in AlGaN/GaN HFETs is the origin of the extrinsic gm and Cgs degradation at high drain current levels. AlGaN/GaN HFETs suffer from large gate leakage and reliability problems where dc drain current and RF output power are degraded as a function of stress time. In this work, TCAD simulations were performed to reproduce the measured bias-dependent and stress time-dependent drain current and gate current characteristics of AlGaN/GaN HFETs with excellent accuracy. A surface electron hopping model is proposed to explain the gate leakage and the reliability problem associated with the high voltage operation of AlGaN/GaN HFETs. According to the model, electrons that tunnel from the gate can accumulate at the gate edge on the drain side and/or travel along the AlGaN surface toward the drain through a trap-to-trap hopping mechanism. The extracted value for the activation energy of the surface traps is in the range G Δ = 0.25~0.35eV, which is consistent with the measured energy level associated with nitrogen vacancies and/or dangling bonds in the device. A discussion of the AlGaN/GaN HFET device design and optimization was also presented in this work. It is suggested that reduced gate-to-source spacing can be used to reduce the nonlinear source resistance and to improve RF performance and linearity of the device. Various techniques including processing methods (using passivation to reduce or immobilize the surface states), field engineering methods (using field plates) and polarization control methods (using a GaN cap layer above the AlGaN layer, or growing the device in M-plane, or using a lower AlN mole fraction in AlGaN) can be employed to mitigate the large gate leakage current and reliability problems. TCAD Simulation and Modeling of AlGaN/GaN HFETs