A 5 GHz-band direct digital radio frequency (RF) modulator using current-mode digital-to-analog converter (DAC) with idle current is proposed and fabricated in 90-nm complementary metal oxide semiconductor (CMOS) process. Since the proposed modulator directly converts digital base band (BB) parallel input signal into RF signal, small die size and low dc operation can be achieved. To reduce the higher order distortion components at the RF output, idle current is provided to BB current output from the current-mode DAC. By providing idle current of 10% of full scale DAC output current, 2nd and 3rd order distortion components near 5 GHz-band carrier are reduced and the improvement of effective number of bits (ENOB) is achieved. Measured ENOB of 6.36 bit (spurious-free dynamic range (SNDR) of 40.0 dB) with RF modulated output power of -28.4 dBm is obtained at 5 GHz. The core size of the proposed modulator integrated circuit (IC) is 200μm × 200μm. The dc power consumption of core circuit excluding inverter section for digital BB signal converter is 2.5 mW (2.1 mA/1.2 V).