A semiconductor memory device changes a pulse width of an over driving signal according to operation modes, which differ by a degree of accessing memory banks during an over driving operation. An over driver supplies an RTO line of the bit line sense amplifier with an over driving voltage in response to the over driving signal and an over driving signal generator changes a pulse width of the over driving signal according to the operation modes. An increase in the VCORE due to excess supply voltage VDD in the over driving operation is prevented.