A systematic procedure is described for determination of GaAs PIN diode equivalent circuit models from measured S-parameter data combined with an electromagnetic analysis of the feed structure. A new parasitic and intrinsic model topology is proposed, and found to be better suited than prior models for the particular GaAs PIN structures considered in this work. Models were developed for forward bias currents ranging from 0.01 to 100 mA, and example measured and modeled results are included to validate the approach.