Semiconductor device having sense amplifiers supplied with an over-drive voltage in a normal mode and supplied with a step-down voltage in a refresh mode

A semiconductor device which has a sense amplifier and is supplied with an external power supply voltage includes a drive signal line connected to the sense amplifier, a step up circuit generating a first voltage from the external power supply voltage, the first voltage being higher than the external power supply voltage, and a step down circuit lowering the external power supply voltage into a second voltage. For enabling the sense amplifier to perform sensing operation in a normal mode involving external access, the first voltage is applied to the drive signal line in an initial stage of the sensing operation, and thereafter the second voltage is applied to the drive signal line. In a refresh mode not involving external access, the step up circuit is shut down, and the second voltage is applied to the drive signal line from the initial stage of the sensing operation.