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A measurement system allowing one to put in evidence the trap effects on the power performance of Al/sub 0.1/Ga/sub 0.9/N/GaN high electron mobility transistors (HEMTs) made on sapphire substrate is presented in this paper. This setup permits simultaneous measurements of the output power supplied by the device under test (DUT) and the I/sub D/(V/sub DS/) characteristic in large signal regime at 4 GHz for different load impedances. It shows the traps influence on the maximum drain-current at 4 GHz for different load impedances under large signal operating conditions. The measurements carried out on a device (2/spl times/50/spl times/1 /spl mu/m/sup 2/) have shown a linear decrease of the maximum drain-current when the load impedance increases. These observations make it possible to determine the origin of the power performances difference obtained at microwave frequencies opposite to the static regime.