  We develop a novel field effect transistor (FET) device using solid ion
conductor (SIC) as a gate dielectric, and we can tune the carrier density of
FeSe by driving lithium ions in and out of the FeSe thin flakes, and
consequently control the material properties and its phase transitions. A
dome-shaped superconducting phase diagram was mapped out with increasing Li
content, with $T_c$ $\sim$ 46.6 K for the optimal doping, and an insulating
phase was reached at the extremely overdoped regime. Our study suggests that,
using solid ion conductor as a gate dielectric, the SIC-FET device can achieve
much higher carrier doping in the bulk, and suit many surface sensitive
experimental probes, and can stabilize novel structural phases that are
inaccessible in ordinary conditions.
