Provided are a semiconductor device and a method of operating the same. A semiconductor device may include a comparator which compares a first voltage with a rectified voltage and provides a second voltage in accordance with the comparison. A timer circuit may operate a timer according to the second voltage and output a third voltage in correspondence with an operation time of the timer. A driver may drive a transistor with a fourth voltage generated by the driver according to the third voltage. A calibration circuit may generate a timer calibration signal based on the second voltage and the fourth voltage. The timer calibration signal may be provided to the timer circuit and used to calibrate the operation time of the timer. More efficient rectification, with reduced occurrence of reverse current, may thereby be realized.