This chapter focuses on the characterization of optoelectronic processes in semiconductors, with a particular emphasis on localized carriers in III-V semiconductors. The inherent short-range disordering in III-V alloys leads to carrier localization, which significantly influences device performance. To probe these localized states non-destructively, we employ advanced techniques including time-resolved photoluminescence (TRPL) and photocurrent (TRPC). We introduce the Localized-State Ensemble (LSE) model, extended for transient analysis, to quantitatively interpret the steady-state and time-resolved luminescence of localized carriers. Furthermore, we establish the analytical link between TRPC and TRPL, enabling a deeper understanding of the recombination dynamics of the minority carriers. This combined approach of novel characterization and physical modeling provides critical insights into the recombination mechanisms of localized carriers, offering significant guidance for the design of high-efficiency optoelectronic devices.