This study systematically investigates the properties of (i) conductive Si-doped α-(AlxGa1−x)2O3 thin films grown via a third generation mist chemical vapor deposition system and (ii) Schottky diodes (SDs) and metal semiconductor field-effect transistors (MESFETs) fabricated on them. The use of chloro(3-cyanopropyl)dimethylsilane as the Si dopant resulted in conductive thin films in a wide range of Al contents x from 0 to 0.3. The resistivity fluctuated in the range from 0.14 Ω cm to 0.35 Ω cm at x ≤ 0.05 and abruptly increased as x exceeded 0.05. Nevertheless, a resistivity of 1.2 kΩ cm was measurable in an (Al0.3Ga0.7)2O3 film, whose bandgap of 6.22 eV was comparable to that of AlN. All the silver oxide SDs fabricated on these films functioned properly with the rectification ratios up to 105, while the MESFETs showed clear field-effect even in the α-(Al0.3Ga0.7)2O3 film. Interestingly, a linear dependence of barrier heights on ideality factors was obtained in diodes formed between the source and gate electrodes of these transistors despite the differences in the bandgaps. This plot yielded the average or “homogeneous” barrier height of ∼2.0 eV, suggesting that the Fermi level on the AgOy/Si:α-(AlxGa1−x)2O3 interface was pinned at ∼Ec −2.0 eV regardless of the Al content x.