A gallium arsenide diode structure is simulated using a particle-mesh computer model. The model incorporates two dimensions in configuration space, and three dimensions ink-space, with a full description of the material scattering cross-section, which is implemented using Monte-Carlo techniques. Detailed information about electric field and valley population profiles is presented for both dipole domain, and l.s.a. modes of operation. Comparison is made with the widely differing previous theoretical results.