An n-type channel transparent thin film field-effect transistor (FET) using a top-gate configuration on a sapphire substrate is presented. ZnO:Li film was used as a channel, and MgF₂ film as a gate insulator. Measurements showed that ZnO:Li films are ferroelectrics with spontaneous polarization $P_S=1$–5$$$\mu$C/cm² and coercive field $E_C=5$–10$$kV/cm. The dependences of drain–source current on drain–source voltage at various gate–source voltages in two antiparallel $P_S$ states were measured and the values of field-effect mobility and threshold voltage were determined for two $P_S$ states are as follows: (a) $\mu =1.5$$$cm²/Vs, $U_{\mathrm{\text{th}}}=30$$$V; (b) $\mu =1.7$$$cm²/Vs, $U_{\mathrm{\text{th}}}=23$$$V. Thus, $P_S$ switching leads to a change in FET channel parameters. Results can be used to create a bistable or, more precisely, digital FET.