Quantum Efficiency: Photodiodes and Their Responsivity

This lecture discusses the concept of quantum efficiency and detectivity in photodiodes. It begins by explaining the ideal quantum efficiency, where each photon generates an electron-hole pair, and how this efficiency is affected by the wavelength of light. The instructor highlights that when the energy of the photon is less than the bandgap energy, the quantum efficiency drops to zero. The relationship between responsivity and wavelength is also examined, showing that responsivity increases linearly with wavelength until it reaches the gap wavelength, after which it decreases. The lecture further explores the noise equivalent power and its significance in determining detectivity, emphasizing that detectivity is inversely related to noise. The instructor illustrates these concepts with examples, particularly focusing on silicon and indium-gallium arsenide photodiodes, and discusses the practical implications of quantum efficiency in real-world applications. The lecture concludes with a comparison of the responsivity and detectivity spectra, highlighting their similarities and the impact of material properties on performance.