The photoelectric effect is responsible for generating signal charge when incident photons interact with the photosensitive volume of the CCD. The challenge for CCD manufacturers is to let incoming photons interact. Quantum efficiency is the performance parameter that tells us how well they have accomplished this task
For frontside-illuminated CCDs, incident photons must pass through the poly gates before they can interact with the epitaxial layer. As shown below, gate absorption represents a significant QE loss mechanism, especially for photons with short absorption lengths. Reflection loss also degrades QE performance over a major portion of the working spectrum. Transmission loss is important for very long and very short wavelength photons that penetrate the epitaxial layer without interaction.
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