We demonstrate the efficacy of a method for reducing the effects of charge associated with the InP substrate-InGaAs interface. By incorporating an AlInAs buffer layer we have dramatically improved the performance of InGaAs metal-semiconductor-metal-photodetectors grown by molecular beam epitaxy. The buffer layer reduces the parasitic capacitances and also reduces the carrier concentration in the absorbing InGaAs layer. This allows complete depletion at low bias, giving rise to a more uniform field strength. The relatively large, 50-μm-diam detectors, operating at 5 V, have an extremely linear response, and a 3 dB power bandwidth of ∼30 GHz. Low frequency gain was not observed for bias voltages to 20 V.