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Download as PDFA Virtual-source-based Transport Model for GaN based HEMTs including Non-linear Access Region Behavior and Self-heating
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 (gm) with gate voltage after reaching a peak value. The compact model captures these effects well. The gm estimated from the model along with gate capacitances would enable estimation of fT and make projections for future scaling of GaN based HEMTs.
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- Figure 1: Velocity vs. field plot of TLM structures of lengths 5, 10, 15, and 25 µm. Velocity shows quasi-saturation at low electric field.
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- Figure 2: Comparison of HEMT compact model against measured output I-V, transfer I-V, output conductance, and transconductance characteristics. The model produces a good match with measured data.
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