We report the reduction of low frequency gain and surface recombination in InAlAs/InGaAs metal-semiconductor-metal (MSM) photodetectors, by surface passivation with plasma-enhanced chemical vapor deposition (PECVD) of silicon nitride. A corresponding improvement in device speed, measured in the frequency-domain, is demonstrated. Large (100 µm) 2 devices exhibit neartransit-time-limited bandwidth (>10 GHz) following passivation, and device characteristics have been stable over a period of several months. We propose a physical model for electron trapping at the free surface, to explain the low frequency gain in nonpassivated devices and its subsequent elimination.