When an ionizing particle passes through a semiconductor device, it transfers energy and generates electron-hole pairs along its path. The excess carriers are subsequently transported throughout the semiconductor’s volume via ambipolar diffusion until they either recombine or are collected and extracted typically by a biased contact or a reverse-biased p-n junction. To predict the transient electrical behavior of complementary metal-oxide semiconductor (CMOS) devices and circuits when exposed to ionizing radiation and assess their soft error rate (SER), it is fundamental to accurately model these diverse physical processes. In this chapter, we present a comprehensive modeling and analysis of the diffusion and collection mechanisms of radiation-induced charges through a semiconductor device. Analytical formulations of the collected charge, collection current, and collection velocity are developed. These equations are further employed to establish an analytical formulation of the soft error rate (SER), explaining its exponential dependence on the critical charge of the circuit. This formulation also links the SER to various physical and technological parameters, as well as to the characteristics of the radiation.