Compact models for GaN based HEMTs describing the voltage-dependent terminal currents are essential for circuit simulations.  In this work, we extend the virtual-source (VS)-based transport model ^[1] originally developed for Si MOSFETs to GaN based HEMTs along with models for non-linear access regions and device self-heating. The model is suitable for quasi-ballistic or fully ballistic short channel devices typically used for RF and mixed-signal applications. The model has been implemented in Verilog-A language.

Access region behavior is analyzed by measuring I-Vs of TLM structures that represent those transistor access regions. Velocity versus field plot obtained from the I-Vs is shown in Figure 1. The velocity undergoes quasi-saturation at a field of about 5 KV/cm, which is lower than in ^[2] . The quasi-saturation is attributed to velocity saturation and self-heating. Access regions are modeled as non-linear resistors to capture this effect. The intrinsic transistor region is modeled using the VS model including self-heating in the channel. The developed model is compared against DC measurements of a short channel RF HEMT. The device has a gate length of 105 nm, access region lengths of 0.5 µm, and device structure as reported in ^[3] . DC characteristics obtained from the model and measurements are shown in Figure 2. The model gives a good match to the measurements, as Figure 2 shows. Results show that the access regions rather than the intrinsic channel region limit the maximum current in output characteristics. Access regions also cause reduction of transconductance (g_m) with gate voltage after reaching a peak value. The compact model captures these effects well.  The g_m estimated from the model along with gate capacitances would enable estimation of f_T and make projections for future scaling of GaN based HEMTs.

1. A. Khakifirooz, O. M. Nayfeh, D. Antoniadis, “A simple semiempirical short-channel MOSFET current–voltage model continuous across all regions of operation and employing only physical parameters,” IEEE Transactions on Electron Devices, vol.56, no.8, pp.1674-1680, Aug. 2009.
2. L. Ardaravicius, A. Matulionis, J. Liberis, O. Kiprijanovic, M. Ramonas, L. F. Eastman, J. R. Shealy, A. Vertiatchikh, “Electron drift velocity in AlGaN/GaN channel at high electric fields,” Applied Physics Letters , vol.83, no.19, pp.4038-4040, Nov. 2003.
3. D. S.  Lee, X.  Gao, S. Guo, T. Palacios, “InAlN/GaN HEMTs with AlGaN back barriers,” Electron Device Letters, IEEE, vol.32, no.5, pp.617-619, May 2011.