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The accurate determination of the channel temperature in field-effect transistors (FETs) and monolithic microwave integrated circuits is critical for reliability. An original accurate closed-form expression is presented for the thermal resistance of multifinger FET structures. The model is based on the solution of Laplace's equations in prolate spheroidal coordinates and elliptical cylinder coordinates. The model's validity is verified by comparing the results with finite-element simulations, and experimental observations from liquid-crystal measurements and spatially resolved photoluminescence measurements. Very close agreement is observed in all cases.