A parallel implementation of the selfconsistent-charge density-functional based tight-binding (SCC-DFTB) method is used to examine large scale structures in III - V semiconductors. We firstly describe the parallel implementation of the method and its efficiency. We then turn to applications of the parallel code to complex GaAs systems. The geometries and energetics of different models for the 19 ? 19 reconstruction at the (1-1-1-) surface are investigated. A structure containing hexagonal rings of As at the surface consistent with STM experiments is found to be stable under Ga-rich growth conditions. We then examine voids in the bulk material which are mainly caused by the movement of dislocations. Void clusters of 12 missing atoms are found to be energetically favorable. This is in very good agreement with recent positron annihilation measurements. Additionally, we investigate the diffusion of C in p-type material and suggest a diffusion path with an activation energy of less than 1 eV which is consistent with experimental studies. Finally, focusing on GaN we provide atomistic insight into line defects in wurtzite GaN threading along the growing c-axis. We highlight the stability and electronic properties of screw and edge dislocations, discuss reasons for the formation of nanopipes and relate the yellow luminescence observed in highly defected materials to deep acceptors, V_Ga and V_Ga-(O_N)_n, trapped at threading edge dislocations.