Silicon films on single crystal magnesium-aluminium spinel slices and similarly on sapphire are strained under compressive stress. This is due to the difference in thermal expansion and to the high growth temperature, causing a curvature of the slices. The coefficients of thermal expansion have been measured by a dilatometer. From the curvature measured by a stylus tracing method a residual stress of about 8 × 10 3 kp/cm 2 has been determined. The expected change in resistivity, effective Hall mobility, and band gap has been calculated from the known influence of stress on the electrical properties of bulk semiconductors, and has been compared with the experiments. The measured mobility as a function of carrier concentration has a maximum between 10 16 and 10 17 carriers per cm 3. The highest mobilities measured were 600 cm 2/V-sec for n-type and 350 cm 2/V-sec for p-type silicon films. The observed decrease of carrier mobility with decreasing carrier concentration (for n, p ≤ 10 16 cm −3) and the relatively low minority carrier lifetime are explained by the assumption of space-charges localized at crystal defect accumulations. In accordance with this assumption an anomalous dependence of mobility on temperature has also been found. The existence of localized space charge regions was further confirmed by the change of photo-voltage on scanning the film with a finely focused laser beam. First characteristics of MOST's on relatively thick silicon films on spinel do not show any remarkable difference from that of transistors on bulk silicon, both having been produced by applying the usual MOS technology.