Semiconductor device having zq calibration circuit and calibration method thereof

PURPOSE: A semiconductor device and a calibration operating method thereof are provided to reduce calibration operation time by performing only the pull-down calibration operation. CONSTITUTION: A calibration circuit(110) performs a pull-down calibration operation in response to a pull-down calibration enable signal. A command controller(150) generates a pull-down calibration enable signal in response to a DLL reset signal. A first pull-up resistor(116) controls the impedance of a first pull-up resistor. A first comparator(114) compares the voltage of a first node with a reference voltage and outputs a pull-down calibration code. The pull-down resistor controls the impedance of the pull-down resistor.