A semiconductor device includes: a first buffer circuit configured to receive a chip select signal in a power-down mode in response to a first select signal, a second buffer circuit configured to receive the chip select signal in an active mode in response to the first select signal, a power supply circuit configured to supply external power to a plurality of logic elements in the active mode in response to a second select signal, and not supply the external power to the plurality of logic elements in the power-down mode, and a select control circuit configured to transition a logic level of the second select signal at a first edge of a first chip select signal in the power-down mode, and then transition a logic level of the first select signal at a following second edge of the first chip select signal to exit from the power-down mode and enter the active mode.