Quantum efficiency is also dependent upon gate voltage, with lower voltages producing small depletion regions and visa versa. As photons above 700 nanometers penetrate deep into the silicon substrate close to the buried drain, they have a greater chance of liberating electrons that will diffuse into the drain and be instantly removed. These devices are also usually equipped with vertical antibloom drains that produce a reduced response to longer wavelength photons. Interline-transfer CCDs have photodiodes that deviate from standard polysilicon gate structure, a factor that reduces interference effects and produces a more ideal and uniform spectral response.

Polysilicon transmittance starts to decrease below 600 nanometers and the material becomes essentially opaque to photons at 400 nanometers, but absorption depends upon gate thickness and interference effects of light passing through thin films on the CCD surface. These structures, used to transfer the charge from the imaging area and to protect the CCD from humidity and electrostatic discharge, absorb the shorter wavelengths (450 nanometers and lower), reducing the blue sensitivity of the device. The spectral sensitivity of the CCD differs from that of a simple silicon photodiode detector because the CCD surface has channels used for charge transfer that are shielded by polysilicon gate electrodes, thin films of silicon dioxide, and a silicon nitride passivation layer. Standard CCDs, which are illuminated in the front of the device through the gate electrodes and oxide coatings, are more sensitive to green and red wavelengths in the region between 550 and 900 nanometers. This property is especially important for low-light imaging applications such as fluorescence microscopy where emission photon wavelengths are often in the 375-550 nanometer range and have a relatively high absorption coefficient in silicon.

The quantum efficiency of a charge-coupled device (CCD) is a property of the photovoltaic response defined as the number of electron-hole pairs created and successfully read out by the device for each incoming photon. Concepts in Digital Imaging Technology Quantum Efficiency

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