Abstract
A new technique is presented which allows various components that contribute to the total parasitic source resistance in a conventional GaAs AlGaAs HEMT to be measured. These components are the contact resistance to the n + GaAs cap, the sheet resistance of the unrecessed region between the ohmic contact pad and the gate recess, the barrier resistance between the cap and the 2DEG sheet, and the sheet resistance of the 2DEG layer. A standard transmission line arrangement with addition of slots of various widths in the n + cap between adjacent ohmic pads is used. Measurement of inter-pad resistances followed by a regression analysis allows various components to be separated. The data presented shows the variation in barrier resistance with HEMT material layer structure e.g. Al mol fraction in the AlGaAs, space layer thickness, doping concentration in the AlGaAs etc. The paper also shows that for most device applications, it is not necessary to make direct alloyed contact to the 2DEG sheet and that conduction via the n + cap is a more desirable contact mechanism.
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