Using THz time-domain spectroscopy to measure the complex conductivity of doped silicon from low frequencies to frequencies higher than the THz plasma frequency and the carrier damping rate, we were able to show in the limit of extremely low carrier densitiesN<1013/cm3,that the Cole–Davidson(C–D)type complex conductivity accurately describes the conductivity of doped silicon. In the low N limit theC–Dparameter &bgr; converges to 0.83 forn-type and 0.70 forp-type silicon. In addition, we have observed a new absorption line at 1.9 THz from an unidentified defect insomeof our Czochralski, single-crystal, low-N silicon samples. ©1998 American Institute of Physics.