We present first-principles calculations of the properties of atomic and molecular hydrogen in pure bulk GaAs. Our results indicate that H penetrates into GaAs in atomic form. Inside GaAs, atomic H tends to form H-2 molecules in tetrahedral sites, which are deep energy wells for H-2. The H-2* defect, formed by one H in a bond-center site and one H in an adjacent tetrahedral position, has higher energy than H-2 but lower-energy barriers for diffusion. Isolated H could be present as a metastable species. We compute the stable charge state of isolated H as a function of the Fermi energy. Our results suggest that H behaves as a negative-U defect. As a consequence, isolated H is expected to be present only as a charged species (positively charged in p-doped samples, negatively charged in undoped and n-doped samples). Our conclusions are compared with experimental results and with the results of calculations for H in other semiconductors. The main features of H in GaAs are quite similar to what has been found in Si.
