U. Radhakrishna, P. Choi, S. Lim, T. Palacios, D. Antoniadis
Keywords: compact modeling, GaN HEMTs, HV applications, RF applications
Summary:The superior breakdown voltage, operating frequency, small form factor and high temperature performance of GaN high electron mobility transistors (HEMTs) are the reasons for their recent foray in high frequency and high voltage applications . The GaN power amplifier (PA) is at the heart of GaN based RF circuits while it is the GaN switch for the high voltage power convertors. Applications in both regimes require physics-based GaN-HEMT compact models which can accurately describe device behavior in both types of circuits with their own nuanced requirements. We have developed a physical compact model based on the virtual source top-of-the-barrier carrier transport that covers regimes ranging from drift-diffusion to ballistic transport and includes physical description of different regions of the device: gated region, access regions, and field plated-regions -. The model is called the MIT Virtual Source GaNFET (MVSG) model and is validated against measured DC, thermal, static CV, S-parameters, large-signal load/source pull, and noise figure measurements on various GaN-HEMT technologies. Secondary effects such as charge trapping, gain compression, gate currents etc. are included as well. The model is one of the candidates for the industry standardization effort by the compact model coalition (CMC) . In this work we demonstrate the model accuracy against both RF and HV measurement data encompassing both device and circuit-level. Device-circuit interactions are studied using the MVSG model using an RF front-end transceiver circuit which employs RF-GaN HEMTs in transmitter PA and receiver LNA, 3-stage ROs, and an RF-DC converter that employs GaN switches. The RF circuit performance metrics such as output power (Pout), power added efficiency (PAE), third-order inter modulation distortion (IMD) and noise figure (NF) are correctly predicted using the model. The model for HV circuit applications is studied using a hard-switched HV-buck-convertor circuit which uses HV-GaN HEMTs for the half-bridge switches. The switching waveforms, efficiency and slew rates are accurately captured in the model along with interesting dynamic non-quasi-static effects that arise out of the device interaction with the circuit that can be analyzed using the model in circuit simulations and is made possible because of the physical nature of the model. This work links device-physics in GaN HEMTs all the way to device optimization and circuit design with the aid of a physical compact model. References:  U. K. Mishra et.al, Proc. of IEEE, 90(6): 1022-1031, Jun. 2002.  U. Radhakrishna et.al, Phys. Status Solidi C, Vol.11, pp. 848-852, ICNS-2014.  U. Radhakrishna Master’s Thesis, Massachusetts Institute of Technology, 2013.  S.D. Mertens, Compound Semiconductor Integrated Circuit Symposium (CSICs), 2014 IEEE, pp.1-4, Oct. 2014.