This system is used to observe the Gm dispersion phenomena in the FET device. Transconductance of FET exhibits the frequency dispersion at frequencies lower than 1 MHz, namely Gm is decreased (negative Gm dispersion) and/or increased (positive Gm dispersion) with frequency. A function generator, a digitized oscilloscope, and a power supply were used to measure Gm, i.e. the ratio of incremental change of drain current to an incremental change of gate voltage. A small sinusoidal signal was applied to the gate of device through the matching circuit and the corresponding sinusoidal current was measured at the drain side of the FET though a load resistor using an oscilloscope. This system is effective in characterize the surface and bulk traps in FET devices.