Aluminum oxide (Al2O3) thin films are synthesized by reactive d.c. magnetron sputter deposition on silicon substrates. The impact of varying plasma power Pp (i.e. 400 to 1000 W) and of thin film temperatures T up to 540 °C on the electrical performance are evaluated, as these dielectric layers with a thickness of 450 nm are targeted as potential candidates for high temperature sensor applications. From 150 °C to 500 °C, the current–voltage measurements show a leakage current behavior according to the Poole–Frenkel electron emission with an activation energy of 1.16 eV. At T > 500 °C, the conductivity increases above average, in respect to the extrapolated Poole–Frenkel behavior at T < 500 °C, most probably due to the migration of charged ions, such as Ar+, incorporated into the film during deposition. Basically, samples synthesized at higher plasma levels show an enhanced electrical insulation behavior. This result is supported by measurements applying optical ellipsometry as well as by the determination of the wet chemical etching behavior in phosphoric-based acid at different bath temperatures. At higher plasma power, the refractive index shows a slight tendency to increase, staying, however, below the value of single-crystalline Al2O3. In contrast, the etch rate decreases by a factor of 1.5 at samples deposited at 1000 W when lowering the temperature of the etchant from 90 °C to 60 °C. These results indicate an enhanced film density at higher Pp values as the microstructure of the Al2O3 films is X-ray amorphous independent of plasma power and post-deposition annealing temperatures up to 650 °C.