Abstract: A variable-frequency I-V measurement system for testing the IC device, such as a GaAs MESFET, has probes for measuring the bias voltage, drain voltage, drain current, and gate signal voltage, a gate signal source, and a computer controller for controlling the gate signal source to provide, in sequence, gate signals varied in waveform, voltage, and frequency, in order to measure the I-V characteristics of the device in response to variable frequency gate signals. Values for maximum drain current (Imax) and percentage drop in Imax at different frquencies are derived and used to predict the RF performance characteristics of the device and to select good wafers early in the fabrication process, as well as for non-destructive testing of frequency dispersion effects on the device.