At microwave frequencies, variable resistors can be implemented with cold-MESFETs. An accurate ultrawideband model for cold-MESFETs is presented. The equivalent circuit model is based on the physical structure of the device and has a simple topology that requires only two bias dependant elements. The model has been used to accurately characterise coldMESFETs from 0.05 to 20.05 GHz. Moreover, unlike that for a switched-MESFET, the model is accurate over a continuous and wide range of bias points.