The n-p-n and p-n-p high-voltage transistors showing high reliabilities have been developed by using semi-insulating polycrystalline-silicon (SIPOS) films for the surface passivation. SIPOS films are chemically vapor-deposited polycrystalline-silicon doped with oxygen or nitrogen atoms. The films employed for the surface passivation of high-voltage transistors are composed of triple layers, which are oxygen-doped SIPOS films of 0.5-µm thickness to stabilize the silicon interface, nitrogen-doped SIPOS films of 0.15-µm thickness to prevent water or sodium ions from reaching the silicon surface, and silicon dioxide films to prevent dielectric breakdown of the SIPOS films under very high-voltage operation. The n-p-n and p-n-p SIPOS transistors rated at 800 and 2500 V have been produced in planar-like structures with field-limiting rings. These transistors showed highly reliable characteristics, because the passivating SIPOS layer provides a good protection against ionic contamination and externally applied electric fields. Furthermore, 10-kV n-p-n SIPOS transistors with multiple rings have been fabricated and found that operation is stable.