In this schematic, where is the depleted layer once voltage is applied to the the sensor?
Under a high fluence of charged particles in high energy physics accelerator experiment, the n bulk is type inverted, forming a p substrate on the n+ side, located at the bottom beside p-spray implants. Effectively, the p-n junction here (and the depletion zone, respectively) is p-substrate (inverted from n bulk) on n+ implant. This design with p-spray implants is used to achieve pixel separation required for high detector resolution. The upper p+ backside implant is common for all pixel.
When in operation, the depletion voltage applied to a pixel sensor depletes the whole upper-half chip of the detector (an &quot;n on n sensor&quot;) and the depletion zone extends all the way from the top p+-n junction to the bottom n-n+ transition. Sure, there is no such thing as n+-n-junction, but, because of the n-bulk type-inversion, &quot;the depletion zone is free of unbound charges due to recombination of the electron excess with hole excess between the n and p part&quot;, and further on following the Section 2.2 The Pixel Detector of the document you cited in your question.
A quite unusual configuration of the depletion zone, isn't it?
As for the &quot;passivation&quot; mentioned in your comments, surface passivation is the part of the device fabrication process and refers to the chemistry of device materials. In the n-on-n sensor, the passivation layer is also important because we need a means to isolate edge of the substrate from a readout chip. The sensor is biased by a few hundred volt voltage and this significant voltage should be dropped across the passivation layer, from the p-side to the readout chip potential. The passivation layer must be of good quality to protect the device from breakdowns.
UPDATE: answers to comment questions

What prevents, from physics laws, than the depleted region to continue
in the full region of p+ and in the full region of n+ ?

Nothing prevents. In a bulk not damaged by irradiation the depletion voltage providing a 300um thick depletion zone can be as low as 50V. With the process of donor removal launched by the increasing fluence of the charged particles the depletion voltage even decreases and at ca. a few 1012cm-2 fluence the donor removal by irradiation of the bulk results in a 300um thick depletion zone even with near-zero depletion voltage. With higher fluences, the radiation induced defects result in space charge accumulation and consequently lead to the type reversal of the bulk: an original n-type bulk becomes a p-type bulk. Radiation-induced charge accumulation outweighs the effect of dopants like phosphorus. The effective doping concentration is complicated by the annealing aftermath.

do you mean that the &quot;n-silicon&quot; transforms to a part &quot;p-effective&quot;
and a part &quot;n-effective&quot; ?

No, read it as is written: the process of donor removal launched by the increasing fluence of the charged particles can lead to the type reversal of the bulk: an original, without radiation induced defects, n-type bulk becomes a p-type bulk.

Where do we apply the polarisation for the electronic readout (the
bottom device below the &quot;bump&quot;.

See @jonk's comment to your Q Where can I find a clear analysis of how the krummenacher circuit works?

What is the role of oxid ? What is the role of nitrid ?

Silicon oxide is a standard isolation material used in semiconductor devices. Silicon nitride, with its higher dielectric constant as compared to silicon oxide, is used as a second dielectric to increase the coupling of the signal to the readout electrodes (via &quot;bumps&quot;).
You can find a more detailed explanations of semiconductor particle detector principles and implementations on the CERN Document Server (https://cds.cern.ch/record/368412), the SLAC site (https://www.slac.stanford.edu/econf/C020909/trpaper.pdf), and the DESY site (https://www.desy.de/~schleper/lehre/Det_Dat/SS_2018/).
The recommended option to study particle detectors is to enroll in the school offering such courses. Also, you can try CERN's or SLAC's summer schools, they offer the remote learning.
Good luck with your studies!