Metal-oxide-silicon field-effect transistors (MOSFET's) were fabricated with Na+ ions in the gate oxide. Large peaks in the tunneling conductivity through the oxide were observed which are interpreted as resonant tunneling of electrons through localized Na+ impurities. It was demonstrated that the tunneling current was spatially localized. The temperature dependence of several peaks was measured and found to be consistent with a simple model for resonant tunneling through localized states. The position of the state in the direction of tunneling can be determined from the temperature dependence. © 1985 The American Physical Society.