In small signal GaAs metal semiconductor field-effect transistor (MESFET) fabrication technology where a gate recess is required, the optimum value of residual channel thickness is one of the most crucial parameters for high transconductance operation of the device. The effects of active channel thickness on device characteristics have been investigated. In a different strategy to that of previously reported work, in this study we have fixed gate length L-g at 200 nm and varied the channel thickness, a from 100 to 40 nm. This means that the shape of the depletion layer, which is a function of gate length, was largely fixed for all the devices. The variation in the device characteristics depended on the change in the active channel thickness, which modifies the field distribution inside the channel. It was found that, for optimum doping concentration, the value of transconductance increases in a parabolic fashion by decreasing the aspect ratio (L-g/a) of the device, and a highest value of transconductance is observed at L-g/a approximate to 2. This aspect ratio is 2-3 times lower than the conventional reported values.