DC Extraction of Gate Bias-Dependent Parasitic Resistances and Channel Mobility in an HEMT

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In all high electron mobility transistors (HEMTs), the low field channel mobility, $\mu _{0}$ , is a function of gate bias. In GaN HEMTs, which have surface traps, the parasitic source/drain resistances, $\text{R}_{S}/\text{R}_{D}$ , too are gate bias dependent, because electrons from the gate fill the traps nearby and deplete the 2-DEG over a length $\Delta $ on either side of the gate. We introduce a simple method to extract the gate bias dependence of $\mu _{0}$ , $\text{R}_{S}$ , $\text{R}_{D}$ , and the virtual gate length, $\Delta $ , from the measured dc resistance of gated and ungated devices fabricated on the same substrate.