A semiconductor device including an internal command generator and a bias generator is provided. The internal command generator generates first to fourth internal command signals sequentially enabled in synchronization with pulses of an external program signal. The first internal command signal controls a read operation for reading out data stored in memory cells, and the second and third internal command signals control a program operation for programming the memory cells. The bias generator generates a read bias signal for controlling a level of an output voltage signal, which is applied to an internal circuit, in response to the first and fourth internal command signals.