# In this schematic, where is the depleted layer once voltage is applied to the the sensor?

In the following schematic from https://cds.cern.ch/record/2717863/files/CERN-THESIS-2019-340.pdf

the sensor is the top part, the readout electronic is the bottom.

Once we have applied an electric voltage, where is the depleted layer ? (Is it in region of n-silicon and p+ ? Is it in region of (n+ of middle, n-silicon) and p+ ?)

• I suggest that you read: en.wikipedia.org/wiki/P%E2%80%93n_junction and then you should conclude that 1) the depletion region is the region surrounding the PN junction. If that's p, p+, n or n+ does not matter. What matters is p touching n. 2) the depletion region forms by itself, no external voltage needs to be applied. Commented May 8, 2021 at 19:58
• @Bimpelrekkie : You are right, the depletion region does not need external voltage (but it gets extended if we apply a reverse voltage). Actually, I have already followed a one year course on semiconductor physics, and read one book on semiconductors physics. But this didn't answer to my question about the "n+" and "n", "p+", "p". So you mean that the deplete region, without application of revert tension, covers a part of the "p+" and a part of the "n-silicon". I guess that without additional revert tension, there is no reason that it cover the "n" region. Do you agree ? Commented May 8, 2021 at 20:14
• If we apply a revert tension of enough magnitude, will the depleted region also cover the n+ region ? Commented May 8, 2021 at 20:15
• I guess that without additional revert tension, there is no reason that it cover the "n" region. Do you agree ? No, I disagree, as I said, the depletion zone exists at the p-n junction (here: around the "p+" and "n-Silicon" areas) and will be partly in the p-type region and partly in the n-type region. The depletion zone might not reach the n+ region at the bottom of the sensor part. But that actually depends on doping concentrations, the actual sizes and applied voltages. So the depeletion region can extend down to the n+ region but it is unlikely due to the large n region. Commented May 8, 2021 at 20:31
• @BimpelrekkieI : I made a typo : I was meaning that without revert voltage, there is no reason that is covert the n+ region. ok, thank you for your explanation. Would you know where apply the external voltage ? I guess that the negative voltage should go to the aluminium of the top. But what about the negative voltage : should it be applied to the so-called "passivation" part ? Why ? Commented May 8, 2021 at 20:46

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 "n on n sensor") 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, "the depletion zone is free of unbound charges due to recombination of the electron excess with hole excess between the n and p part", 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 "passivation" 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.

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 "n-silicon" transforms to a part "p-effective" and a part "n-effective" ?

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 "bump".

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 "bumps").

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.