Update: Fraunhofer's Annual Report gives more information about the LDPD pixel and ToF sensor:
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| Fig. 1. LDPD ToF pixel cross-section |
"The photodiode is divided in two main parts: a pinned surface one and a part which resembles a buried CCD cell, as it can be observed in Fig. 1. The pixels and the entire sensor have been fabricated in the 2P4M automotive certified 0.35 μm CMOS technology at the Fraunhofer IMS with the addition of an extra surface-pinned n-well yielding a non-uniform lateral doping profile, as shown in Fig. 1 (upper picture). The doping
concentration gradient of the extra n-well was chosen in such a way that it induces an intrinsic lateral drift field parallel to the Si-surface in the direction of the pixel readout node (x-axis in Fig. 1) as well as from the periphery of the n-well in the direction of the n-well centre (y-axis in Fig. 1).
The potential distribution within this intrinsic lateral drift-field photodiode (LDPD) n-well resembles a hopper leading the photogenerated charge directly to the assigned readout nodes. It remains fully depleted during operation, sandwiched between the substrate and a grounded p+ pinning layer on top of it (see Fig. 1). In this manner, the almost noiseless reset and readout operations of the photodetector are enabled.
A buried collection-gate (CG) is fabricated at the one end of the n-well, which remains biased at a certain voltage VCG. It induces an additional electrostatic potential maximum in the system and enables the proper and symmetrical distribution of the signal charge among the readout nodes. Each of the four transfer-gates (TX) plays two main roles:
1) it serves to create a potential barrier in the well to prevent the collected charge to be transferred into any of the three “floating” diffusions (FD) aimed at pixel readout or the so called “draining” diffusion (DD) permanently biased at a reset potential
2) to facilitate the transport of the photocharge into a desired FD or the DD."
