We report on passivation and antireflection coating of InP/InGaAs metal-semiconductor-metal photodetectors by low-temperature deposited silicon dioxide. The passivating performance of silicon dioxide films applied by nonreactive radio frequency magnetron sputtering and remote plasma enhanced chemical vapor deposition are comparatively investigated. Different wet chemical treatments Of the InP surface prior to deposition including sulfur passivation are performed and their influences on the device performance are presented. Under optimized deposition conditions and pretreatments, both processes result in a stable and reproducible surface passivation as reflected by a drastic reduction of excessive leakage currents and photocurrent gain. The improvement of the device characteristics due to the silicon dioxide coating is attributed to a substantial lowering of the density of interface states at the insulator-InP interface as compared to nonpassivated devices.