A semiconductor-based electrolyte in a ceramic fuel cell (SCFC) has the potential to improve the device performance even at lower temperatures (≤520 °C) mainly due to its high ionic conductivity. Here, we present a chemically stable perovskite semiconductor Nb-doped SrTiO3-δ (STN) electrolyte for the SCFC, which reached a high power density of 678 mW/cm2 and a high open-circuit voltage (OCV) of 1.03 V at 520 °C. The STN showed a high ionic conductivity of 0.22 S/cm. Electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and band structure analysis revealed that the high ionic conductivity is due to an increase in oxygen vacancies and band gap modulation. It was also found that the bending of the energy band at the electrode/electrolyte interface helps to block the electrons and thus avoids the problem of short circuit. These results indicate that doping and energy band gap modulation can be effective approaches to develop advanced semiconductor electrolytes for SCFCs.