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Goddard Space
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Schematic section of a circular DEPFET device for the WFI. For optimal charge collection and efficient area-filling, the DEPFETs must have a circular geometry and are surrounded by a global drift ring structure focusing the signal electrons towards the pixel center. Click the image for a larger view
The DEPFET sensor array is a combined detector-amplifier structure. Every pixel consists of a p- channel MOSFET, integrated onto a fully depleted silicon bulk. With the help of an additional deep-n implantation, a potential minimum for electrons, the so-called internal gate, is generated and laterally constrained to the region below the transistor channel. Incident X-rays interact with the bulk material, and the resulting charge is separated; while holes drift to a p-contact, the electrons are collected in the internal gate of the adjacent pixels. There, their presence influences the conductivity of the MOSFET channel; sensing the increase of channel conductivity of the MOSFET is therefore a measure of the quantity of collected charge.
Since in DEPFET pixel sensors the signal charge is stored in the pixel cell, a pixel does not need to be read out continuously. Compared to other pixel sensor concepts, this can be used to significantly reduce the number of readout channels, avoiding bump bonding and resulting in a small scale readout chip.
In an APS, all pixels have a common bulk and back contact. As the bulk is fully depleted the sensor is irradiated through the homogeneous, non structured, thin backside entrance window. The two-dimensional pixel array has no insensitive gaps, and the DEPFET APS has a fill factor of 100%. Back to WFI page...
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