Analytical Physics Based AlGaN/GaN HFET Large Signal Model and Nonlinearity Analysis with Nonlinear Source Resistance

Abstract

In this work the impact of nonlinear source resistance and RF channel breakdown on AlGaN/GaN HFETs RF and linearity performance were studied. AlGaN/GaN HFETs are well known strong candidates for high power devices due to its superior material properties, such as high electric field to achieve electron saturation velocity and high mobility. Practical amplifiers, however, do not demonstrate the good RF linearity performance predicted from fundamental semiconductor materials properties. In particular, it has been demonstrated that a nonlinear source resistance is generated in these devices due to the onset of space-charge limited (SCL) transport in the gate-source region. It has been demonstrated that the nonlinear source resistance modulation degrades the linearity of the device, even under modest RF drive conditions. Breakdown mechanism has been found to be a major factor that limits FET’s performance at large signal condition. For AlGaN/GaN HFETs, breakdown could happen both in the channel and on the surface. The lower critic value for channel breakdown determines that it dominates in the breakdown process and contributes most in device’s operation. A physics based large signal FET model was modified to include nonlinear source resistance effect and RF channel breakdown. The model was further modified to include multi-tone and wide-band signal simulation capability to study the device’s linearity performance. The goal of this work is to understand the physical mechanisms that determine device’s RF and linearity performance and provide the optimization options of improving these effects effectively and efficiently. The physical mechanisms were able to be quantified and determined based on the study using the analytical physics based model. It allows the development and optimization of power amplifier before the device is fabricated. The physical device model used in this work permits determination of the nonlinear distortion under large signal operation as a function of bias, device physical parameters such as structural dimensions, doping, etc, and circuit tuning conditions.