We report metal-free synthesis of high-density single-crystal elementary semiconductor nanowires with tunable electrical conductivities and systematic diameter control with narrow size distributions. Single-crystal silicon and germanium nanowires were synthesized by nucleation on nanocrystalline seeds and subsequent one-dimensional anisotropic growth without using external catalyst. Systematic control of the diameters with tight distribution and tunable doping concentration were realized by adjusting the growth conditions, such as growth temperature and ratio of precursor partial pressures. We also demonstrated both n-type and ambipolar field effect transistors using our undoped and phosphorus-doped metal-free silicon nanowires, respectively. This growth approach offers a method to eliminate potential metal catalyst contamination and thus could serve as an important point for further developing nanowire nanoelectronic devices for applications.