Radio frequency identification (RFID) devices are provided including a contactless internal voltage generator configured to generate a rectification voltage responsive to a radio frequency (RF) input signal and an internal voltage responsive to the generated rectification voltage and a reference voltage; a clock generator configured to sense an amount of current to a sink path of the contactless internal voltage generator and to generate a clock signal using a variable resistance value, the variable resistance value based on the amount of current sensed; and an internal circuit driven by the internal voltage and the clock signal.