An integrated circuit includes a plurality of semiconductor devices. Each of the semiconductor devices includes an internal voltage generation unit configured to generate a plurality of internal voltages, a voltage select output unit configured to output a default voltage of a plurality of internal voltages to a preset pad in response to an initial value of a select code, and selectively output the other voltages of the plurality of internal voltages to the pad in response to variations of the select code, and a stack operation control unit configured to control the voltage select output unit to output the default voltage to the pad in response to a stack signal and a predetermined value of the select code, instead of the initial value of the select code, and whether or not to activate the stack signal is determined according to whether or not the plurality of semiconductor devices are stacked.