Ferroelectric field-effect transistors (FeFETs) with n-type ZnO channel and BaTiO3 gate oxide show an unexpected reversed switching behavior in terms of the polarity of the gate voltage. The device's source–drain current is reduced after applying an external gate voltage pulse which polarizes the BaTiO3 such that the device is meant to be in the on-state. Subsequent switching to the off-state with the opposite voltage does not influence the source–drain current. Furthermore, repeated switching cycles with increasing voltages result in a shift of the onset gate voltage in the transfer characteristics of the FeFETs. The observed switching behavior is explained by electron trapping at the ZnO/BaTiO3 interface. In addition the charge accumulation and time, temperature and illumination dependent discharging effects are demonstrated. From current–voltage measurements we derive the trapped electron sheet density in dependence of the gate voltage. The interface charge density is in the order of 1 C m−2.