Calculations show that phase nonlinearity in 1 &mgr;m gate length power GaAs metal‐semiconductor field‐effect transistors (MESFET’s) can be accounted for by the variation of gate‐channel capacitance with gate bias voltage. Buried‐layer GaAs MESFET’s having constant gate‐channel capacitance have been fabricated and their high‐frequency linearity measured. The devices display dramatic reductions of phase nonlinearity to 0.15°/W output power at 6 GHz for powers below saturation in 8 mm gate‐width devices confirming that nonlinear capacitance causes nonlinearity observed in conventional GaAs MESFET’s. Channel transit time, estimated at 3–6 ps, is not significant as a cause of nonlinearity and varies less than 100 fs with signal level.