The effects of pyrolytic Al2O3deposition temperature on electrical properties of an inversion‐mode InP (MISFET) metal‐insulator‐semiconductor field‐effect transistor were investigated. An Al2O3gate insulator was deposited using an aluminum isopropoxide organic source on a HCl vapor etched InP surface. An increasing current drift was seen when the insulator was deposited at a temperature below 330 °C. This became exaggerated with decreasing temperature. The observed drift is explained in terms of a time‐dependent threshold voltage associated with the polarization of organic molecules or radicals introduced into the insulator by an incomplete decomposition of the source gas during deposition of the dielectric layers at rather low temperatures. The effective electron mobility of the InP MISFET did not show any dependence on the deposition temperature below 350 °C. At higher temperatures, the effective mobility appreciably decreased.