Finely focused beams of silicon ions have been used to produce novel three-dimensional doping profiles in GaAs metal-semiconductor field effect transistor channels. The structure studied is a GaAs field effect transistor in which the channel consists of conducting stripes in a semi-insulating substrate, with the doping profile graded to give a step change in the doping density under the gate. The striped channel has the effect of increasing the device transconductance, while the longitudinal dopant grading increases the gate breakdown voltage. Striped and stepped-channel devices are compared with devices with uniform channels and with striped nonstepped channels; improved performance is demonstrated.