S. Ghosh, S.A. Ahsan, S. Khandelwal, A. Pampori, R. Dangi, Y.S. Chauhan
Keywords: GaN HEMT, ASM-HEMT, compact model
Summary:HEMTs based on GaN have become immensely ubiquitous in the past decade and offer a strong competition to mature technologies like silicon primarily due to the superior characteristics offered by the GaN material system over existing technologies. A high bandgap allows the HEMT to perform under significantly higher voltages coupled with a high mobility two-dimensional electron gas (2-DEG) allowing high power designs to be implemented on a considerably smaller area. The presence of the 2-DEG, along with an undoped system which minimizes scattering, leads to a better frequency response and makes GaN HEMTs a candidate of choice in RF applications as well. However, to expedite the circuit development process, a model is required which should be accurate, computation/memory efficient, and robust in terms of operation and should scale credibly. The existing literature is replete with models however a huge majority of them are empirical or table-based in nature, that altogether ignore the underlying physics of the GaN device. We have recently developed a physics-based compact model, known as the Advanced SPICE Model for GaN HEMTs (ASM-HEMT), and have validated it against measured data for DC-IV, Capacitances, high-frequency performance etc. Due to the strong piezoelectric polarization effect that exists between AlGaN/GaN hetero-interface, a high-density 2DEG results that forms the transistor channel. In our model, we preserve the 2DEG nature of the channel by self-consistently solving the Schrodinger and Poisson equations to obtain an analytical expression for the surface-potential after considering two energy sub-bands in the triangular quantum well at the hetero-interface. We proceed to calculate all other important quantities such as the intrinsic charges, drain current etc. in terms of the surface potential, valid for a wide range of bias conditions. To make the model more realistic, we have incorporated effects such as Access Resistances, Drain Induced Barrier Lowering (DIBL), Mobility Degradation, Channel Length Modulation, Self-Heating, Gate Current, Noise, Field Plates etc. to realize a more realistic device. Moreover, GaN HEMTs are known to exhibit the phenomenon of trapping which significantly affects the dynamic performance of the device particular at RF. We have a working trap model incorporated into the main model and it is validated against pulsed IV measurements, harmonic balance power sweeps and load pull for a commercial GaN HEMT. The ASM model is geometrically and temperature scalable, thanks to its physical nature. Also, the core model parameters are few and can be easily related to the device physics and their extraction procedure is fairly simple. The ASM model is in the final stage for getting standardized by the Compact Model Coalition.