Photodiodes remain a key focus in various fields of electronics due to their remarkable properties. These properties are primarily determined by the materials used in their fabrication. Gallium arsenide (GaAs) is a promising material for photodiode manufacturing and serves as an excellent model for simulating transport phenomena in these structures. Notably, the presence of trapping centers significantly impacts their performance, as detailed in this chapter. This chapter presents both physical and mathematical models that exploit the properties of GaAs and the phenomena observed in a PIN diode under dark conditions. The study initially focuses on the behavior of the diode in the absence of illumination. However, the chapter also incorporates the physical processes associated with the photovoltaic mechanism into the mathematical approach, providing a simplified model to govern and predict phenomena under illumination. The basic operation of the PIN photodiode is explained, followed by an analysis of the physical phenomena occurring in dark conditions, including the role of trapping centers in recombination and space charge storage mechanisms. Furthermore, the chapter distinguishes between conduction simulations in a standard PIN diode and a PIN photodiode. Finally, a simple analytical model derived from simulation results is introduced to explore the electronic behavior. Detailed physical insights are provided to better understand the observed phenomena and validate the model.