Large-scale molecular dynamics (MD) simulations, along with bond-order interatomic potentials, have been employed to study defect production, clustering and their evolution within high energy displacement cascades in semiconductors. Based on the MD results, the damage density within a cascade core is evaluated, and used to describe a new energy partition function. In addition, we have further developed a model to determine the non-ionizing energy loss (NIEL) for semiconductors, which can be used to predict the displacement damage degradation induced by space radiation on electronic components. The atomic-level based NIEL model has been applied to GaAs and GaN. At low energies, the most surviving defects are single interstitials and vacancies, and only 20% of the interstitial population is contained in clusters in GaAs, but a direct-impact amorphization in GaAs occurs with a high degree of probability during the cascade lifetime for Ga PKAs (primary knock-on atoms) with energies higher than 2 keV. However, a large number of atoms will be displaced during the collisional phase with a compacted cascade volume in GaN, and consequently, a great number of displaced atoms recombine significantly with vacancies at the same time, i.e., a pseudo-metallic behavior (PMB). This leads to the result that the majority of surviving defects are just single interstitials or vacancies for all recoil energies considered with only a small number of defects forming clusters. The total number of defects simulated in GaN can be very well predicted by the simplied Norgett, Robison and Torrens (NRT) formula due to the PMB, in contrast to GaAs where the defect number becomes much larger than the NRT value. The calculated NIEL in GaN is often found smaller than that predicted by a model based on the simple Kinchin-Pease formula. The comparisons of defect creation, density and effective NIEL in GaN to those of GaAs suggest that GaN may be much more resistant to displacement damage than GaAs, and therefore, very suitable for use in high-power space-energy systems and space-probe applications.