A semiconductor device may include a ZQ calibration circuit, a reference code setting circuit, a variable information generating circuit, and an internal circuit. The ZQ calibration circuit may perform a ZQ calibration operation in response to a ZQ calibration enable signal to generate a ZQ calibration code. The reference code generating circuit may output a predetermined code value as a reference code. The variable information generating circuit may compare the ZQ calibration code to the reference code to generate variable information. The internal circuit may determine operation timings based on a difference between the ZQ calibration code and the reference code.