We demonstrate how the charge state of a trap at the Si/SiO2 interface in a MOSFET can be controlled by microwave irradiation. The device is immersed in a static magnetic field at 300 mK and operates at small bias. Under such experimental conditions the electron spins are almost fully polarized. The electron occupancy of the trap is reversibily raised from one to two electrons by turning on a microwave field of less than 10 μW. Such contactless method controls the charge bound to defects close to the channel and it enables the fast initialization of the charge and spin state of the trapped electrons. This is particularly important in those spin resonance quantum computation schemes where a channel current senses the charge state, to improve the switching clock and to eliminate the related noise.