Millimeter-wave/terahertz rotational spectroscopy offers ultra-wide-detection range of gas molecules for chemical and biomedical sensing. Therefore, wideband, energy-efficient, and fast-scanning CMOS spectrometers are in demand. Spectrometers using narrow-pulse sources and electromagnetic scattering [1] are broadband, but their resolutions do not meet the requirement (<;10kHz) of the absolute specificity. Alternatively, a scheme using a single tunable tone exhibits significant trade-off between bandwidth and performance. The 245GHz spectrometer in [2] presents 4mW radiated power, but only has a 14GHz bandwidth. In [3] and [4], broader bandwidths are achieved at the expense of degraded radiated power (0.1mW) and noise figure (NF=18.4 to ~23.5dB). In addition, given a typical 10kHz resolution and 1ms integration time, scanning a 100GHz bandwidth with a single tone takes as long as 3 hours. This paper reports a rapid, energy-efficient spectrometer architecture based on dual-frequency-comb scanning. A 220-to-320GHz CMOS spectrometer prototype based on this architecture is demonstrated with a total radiated power of 5.2mW and a NF of 14.6 to ~19.5dB.