Diamond metal-insulator-semiconductor diodes and field-effect transistors (MISFETs) have been prepared usingCaF2gate insulator and nondoped (in some cases, boron was doped) diamond homoepitaxial films. The resultant capacitance-voltage(C-V)curves and drain current-drain voltage(ID−VD)curves strongly depended on the amount of oxygen contamination of diamond surface. From analyses ofC-VandID−VDcurves, it was found that the oxygen contamination induced the surface states with two distribution peaks locating very near the valence band edge and at the energy of∼1 eVfrom the valence band edge. Although fluorination of oxygen-terminated diamond surface proceeded to a certain extent duringCaF2deposition at the elevated temperatures in vacuum, it still allowed surface state formation of about∼1014/cm2 eVnear the valence band edge due to uncompleted exchange of oxygen by fluorine atoms and easy penetration of residual oxygen in the chamber through theCaF2insulator at elevated temperatures. Reduced-oxygen process by diamond surface passivation with hydrogen (hydrogenation) and room temperature deposition ofCaF2greatly improved the surface stability, and consequently, the surface state density near the valence band edge was reduced to∼1012/cm2 eV.In this manner, the effective hole mobility of∼10 cm2/V swas obtained from the diamond MISFET, which can be well compared with the surface Hall mobility of35 cm2/V s.©1997 American Institute of Physics.