We report the photoluminescence (PL) and structural properties of a new class of efficient visible-light-emitting semiconductors: low defect density a-Si:Hx alloys. For films prepared by the (thermal) homogeneous chemical vapor deposition (HOMOCVD) method, new broadband PL develops for x>0.3, reaching a peak emission energy of 2.05 eV for a hydrogen content x=0.66 (40 at. % H). We attribute the wide gaps to the influence of Si-H bonding on the density of states near the valence band edge. We ascribe the new PL process to band-to-band recombination from within the alloy band tails. This emission persists at room temperature with an integrated intensity comparable to conventional light-emitting diode (LED) materials. Qualitatively similar results are obtained for low-temperature-deposited rf plasma films prepared from Si2H6, but not from SiH4. We show that a low Si dangling bond concentration is the key factor, for all the different film types, to achieving efficient luminescence.