Photodiodes: Thermal Generation and Current Mechanisms

This lecture discusses the operation of photodiodes, focusing on thermal generation and current mechanisms. It begins by explaining the structure of a PN diode at thermal equilibrium, highlighting the depletion region and the behavior of the Fermi levels on both sides. The instructor describes how applying a forward bias reduces the barrier, allowing current to flow, while reverse bias increases the barrier, resulting in minimal current except for the dark current. The lecture emphasizes the importance of reverse bias operation for photodiodes and details the thermal generation of carriers in the depletion and diffusion regions. The generation of electron-hole pairs and their collection in the depletion region is explained, along with the impact of temperature on carrier generation. The IV curve of a diode in the dark is presented, illustrating the generation and recombination processes. The lecture concludes by distinguishing between the different currents in a PN diode, emphasizing that the current is dominated by generation in the depletion region, making it distinct from an ideal diode.