Example embodiments of the present invention relate to methods of manufacturing a semiconductor device. Other example embodiments of the present invention relate to methods of manufacturing a semiconductor device having a gate electrode. In the method of manufacturing the semiconductor device, a gate electrode may be formed on a semiconductor substrate. Damage in the semiconductor substrate and a sidewall of the gate electrode may be cured, or repaired, by a radical re-oxidation process to form an oxide layer on the semiconductor substrate and the gate electrode. The radical re-oxidation process may be performed by providing a nitrogen gas onto the semiconductor substrate while increasing a temperature of the semiconductor substrate to a first temperature to passivate a surface of the gate electrode under a nitrogen gas atmosphere, providing an oxygen gas onto the semiconductor substrate while increasing the temperature from a first temperature to a second temperature to perform a first oxidation process and/or performing a second oxidation process at the second temperature.