With a view to using microwaves to excite the single-spin resonance of an electron trapped in a defect at the Si∕SiO2 interface of a metal-oxide-semiconductor field-effect transistor (MOSFET), we report on the experimental evidence for a stationary current in such devices operated under microwave radiation. The stationary current is examined as a function of the microwave power and of the operating voltage of the MOSFET. The transistor behavior is reproduced by a model exploiting the nonlinearity of the MOSFET channel resistance as a component of the circuit coupled with the electromagnetic field. We conclude that, in operating a MOSFET under microwaves, one has to pay attention to the generation of spurious stationary currents that may alter the likelihood to observe spin-dependent phenomena in the random telegraph signal observed in a MOSFET.