We investigate the structural, energetic, and electronic properties of hydrogen-passivated doped and undoped germanium nanowires along ͓001͔, ͓110͔, and ͓111͔ directions, with diameter d up to 3 nm, using ab initio methods. A critical diameter d c Ϸ 2 nm is found, above which all wires have faceted cross sections determined by the symmetry of their axis. The wires possess several electronic properties relevant for sensing and other nanoelectronic applications: ͑i͒ Quantum confinement has a substantial effect on the electronic band structure and, hence, the band gap ͑E g ͒, which increases with decreasing diameter. ͑ii͒ Wires oriented along ͓110͔ are found to have a direct E g , while the wires along ͓111͔ are found to have an indirect E g. Wires along ͓001͔ show a crossover from a direct to an indirect E g as diameter increases, the value of the critical diameter for the transition being 1.3 nm. ͑iii͒ The electronic band structure shows a significant response to changes in surface passivation with hydrogen. ͑iv͒ Doping of wires with n-and p-type atoms produced a response in the band structure similar to that in a doped bulk crystal.

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