All semiconductor detectors require a photo-sensitive region in which charge carriers liberated by incident photons (compton effect, photoelectric effect or pair production) will be moved or collected. In the case of Gaia, the detectors are silicon CCDs, the dominant effect (by far) is the photoelectric effect and liberated electrons are collected in potential wells. Because the Gaia CCDs are back illuminated, the photo-sensitive region is essentially the full thickness of the CCD. However, this is divided into a high field (depletion) thickness and a low-field (known as field free) thickness.
The depletion thickness of a semiconductor device that part of the material that has been depleted of majority carriers due to the presence of an electric potential. The potential may be due to an applied bias, the presence of differently doped semiconductor material (i.e. a pn junction), the presence of an interface with a metal (Schottky diode) or a Metal Oxide Semiconductor structure (MOS device), or more often than not, a combination of the above. Since the depletion region is (by definition) that part of the device that supports the potential gradient (non-zero electric field), it is the part of a device capable of moving (or collecting) electrons quickly. Photon interactions within the depletion region liberate electrons which move quickly to the nearest potential maximum, usually within the same CCD pixel in which they were liberated.
The remaining, undepleted thickness of a CCD is referred to as the field-free region. Electrons liberated by photons incident in the field-free region are also collected (eventually). However, in the absence of a strong electric field, the electrons are also able to spread laterally due to diffusion. This means that electrons liberated by photon interactions in the field free region will often be collected by pixels other than the one in which the interaction took place. In high energy, spectroscopic (photon counting) applications, many of these events would be rejected, which would constitute an overall loss of efficiency. For an integrating application such as Gaia, the efficiency is not affected as long as all electrons are collected within the relevant PSF. The sharing of electrons between pixels does however constitute a blurring of the image or a reduction in diffusion MTF.
The figure is a schematic representation of charge spreading in the depletion and field-free regions. t is the total device thickness, x_d is the depletion depth and x_ff is the field-free thickness.
Image courtesy of Alexander Short.
[High resolution version available]