Several ferroelectric thin films withn‐type orp‐type conductivity, including undoped and doped lead zirconate titanate, barium titanate, strontium barium niobate, and potassium niobate, as well as lead barium niobate, were made on silicon single‐crystal substrates by the sol‐gel process. Self‐biased heterojunction effects were observed in bothp‐ferroelectric thin film onn‐silicon andn‐ferroelectric thin film onp‐silicon by the measurement of current‐voltage characteristics. It has been observed that eitherp‐njunction orn‐pjunction in the ferroelectric‐semiconductor systems behave like a rectifying diode. However, the junction effect is weak in thep‐ferroelectric thin film onp‐silicon system. A physical model based on the consideration of energy‐band theory has been constructed in explaining this effect. Possible applications of this effect are discussed.