A phosphorus-nitride (P3N5) CVD was newly developed for preparation of a high-quality gate insulator on an InP substrate. This film exhibited ohmic conduction with a breakdown field intensity as high as 1×107 V/cm. An inversion-mode n-channel InP MISFET was fabricated employing the phosphorus-nitride CVD film as a gate insulator. An effective electron mobility of 1000–1640 cm2/Vs was obtained for this device.