Editing Semiconductor detector

{{Short description|Device using a semiconductor (silicon or germanium) to measure radiations}}
{{Hatnote|This article is about ionizing radiation detectors. For semiconductor detectors in radio, see [[Detector (radio)]], [[Crystal detector]], [[Diode#Semiconductor diodes|Semiconductor diodes]], and [[Rectifier]].}}
{{Use British English|date=October 2021}}
{{Use dmy dates|date=October 2021}}

In ionizing radiation detection physics, a '''semiconductor detector''' is a device that uses a semiconductor (usually [[silicon]] or [[germanium]]) to measure the effect of incident charged particles or photons.

Semiconductor detectors find broad application for [[radiation protection]], [[gamma spectroscopy|gamma]] and [[x-ray spectroscopy|X-ray spectrometry]], and as [[particle detector]]s.

== Detection mechanism ==
In semiconductor detectors, ionizing radiation is measured by the number of [[charge carrier]]s set free in the detector material which is arranged between two [[electrode]]s, by the radiation. Ionizing radiation produces free [[electron]]s and [[electron hole]]s. The number of electron-hole pairs is proportional to the energy of the radiation to the semiconductor. As a result, a number of electrons are transferred from the [[valence band]] to the [[conduction band]], and an equal number of holes are created in the valence band. Under the influence of an [[electric field]], electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer [[electrical network|circuit]], as described by the [[Shockley-Ramo theorem]]. The holes travel in the opposite direction and can also be measured. As the amount of energy required to create an electron-hole pair is known, and is independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the energy of the incident radiation to be determined.<ref>{{cite book|last=Knoll|first=G.F.|title=Radiation Detection and Measurement|edition=3rd|page=365|publisher=Wiley |date=1999|isbn=978-0-471-07338-3}}</ref>

The energy required to produce electron-hole-pairs is very low compared to the energy required to produce paired ions in a gas detector. Consequently, in semiconductor detectors the [[statistical variability|statistical variation]] of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good, and is dependent upon [[rise time]].<ref>Knoll, p119</ref> Compared with [[gaseous ionization detectors]], the [[density]] of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.{{Citation needed|date=May 2023}}

== Detector types ==
{{unreferenced section|date=November 2020}}

=== Silicon detectors ===
[[File:A Forward Silicon Vertex Detector (FVTX) sensor on a microscope.jpg|thumb|upright=1.0|right|A Forward Silicon Vertex Detector (FVTX) sensor of [[PHENIX detector]] on a microscope showing silicon strips spacing at 75 microns.<ref>{{cite journal|url=https://www.phenix.bnl.gov/WWW/publish/brooks/silicon/reviews/Nov10/talks/Sensors_FPHX_11_10.pdf|title=Sensors/FPHX Readout Chip WBS 1.4.1/1.4.2|last1=Kapustinsky |first1=Jon S.|date=17 November 2010|access-date=7 August 2017}}</ref>]]

Most silicon [[elementary particle|particle]] detectors work, in principle, by [[doping (semiconductor)|doping]] narrow (usually around 100 micrometers wide) [[microstrip detector|silicon strips]] to turn them into [[diode]]s, which are then [[p–n junction#Reverse bias|reverse biased]]. As charged particles pass through these strips, they cause small ionization currents that can be detected and measured. Arranging thousands of these detectors around a collision point in a [[particle accelerator]] can yield an accurate picture of what paths particles take. Silicon detectors have a much higher resolution in tracking charged particles than older technologies such as [[cloud chamber]]s or [[wire chamber]]s. The drawback is that silicon detectors are much more expensive than these older technologies and require sophisticated cooling to reduce leakage currents (noise source). They also suffer degradation over time from [[radiation]], however, this can be greatly reduced thanks to the [[Lazarus effect]].

=== Diamond detectors ===
[[Diamond]] detectors have many similarities with silicon detectors but are expected to offer significant advantages – in particular a high radiation hardness and very low drift currents. They are also suited to neutron detection. At present, however, they are much more expensive and more difficult to manufacture.

=== Germanium detectors ===
[[File:HPGe detector.jpg|thumb|100px|right|High-purity germanium detector (disconnected from liquid nitrogen dewar)]]

[[Germanium]] detectors are mostly used for [[gamma spectroscopy]] in [[nuclear physics]], as well as [[x-ray spectroscopy]]. While silicon detectors cannot be thicker than a few millimeters, germanium can have a sensitive layer ([[depletion region]]) thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to a few MeV. These detectors are also called high-purity germanium detectors (HPGe) or hyperpure germanium detectors. Before current purification techniques were refined, germanium [[single crystal|crystals]] could not be produced with purity sufficient to enable their use as spectroscopy detectors. Impurities in the crystals trap electrons and holes, ruining the performance of the detectors. Consequently, germanium crystals were doped with [[lithium]] ions (Ge(Li)), in order to produce an [[Intrinsic semiconductor|intrinsic]] region in which the electrons and holes would be able to reach the contacts and produce a signal.

When germanium detectors were first developed, only very small crystals were available. Low efficiency was the result, and germanium detector efficiency is still often quoted in relative terms to a "standard" 3″ x 3″ NaI(Tl) scintillation detector. Crystal growth techniques have since improved, allowing detectors to be manufactured that are as large as or larger than commonly available NaI crystals, although such detectors cost more than €100,000 (US$113,000).

{{As of|2012}}, HPGe detectors commonly use lithium diffusion to make an [[N-type semiconductor|n<sup>+</sup>]] [[ohmic contact]], and boron implantation to make a [[P-type semiconductor|p<sup>+</sup>]] contact. Coaxial detectors with a central n<sup>+</sup> contact are referred to as n-type detectors, while p-type detectors have a p<sup>+</sup> central contact. The thickness of these contacts represents a dead layer around the surface of the crystal within which energy depositions do not result in detector signals. The central contact in these detectors is opposite to the surface contact, making the dead layer in n-type detectors smaller than the dead layer in p-type detectors. Typical dead layer thicknesses are several hundred micrometers for a Li diffusion layer and a few tenths of a micrometer for a B implantation layer.

The major drawback of germanium detectors is that they must be cooled to [[nitrogen|liquid nitrogen]] temperatures to produce spectroscopic data. At higher temperatures, the electrons can easily cross the [[band gap]] in the crystal and reach the conduction band, where they are free to respond to the electric field, producing too much electrical noise to be useful as a spectrometer. Cooling to liquid nitrogen temperature (77K) reduces thermal excitations of valence electrons so that only a gamma ray interaction can give an electron the energy necessary to cross the band gap and reach the conduction band. Cooling with liquid nitrogen is inconvenient, as the detector requires hours to cool down to [[operating temperature]] before it can be used, and cannot be allowed to warm up during use. Ge(Li) crystals could never be allowed to warm up, as the lithium would drift out of the crystal, ruining the detector. HPGe detectors can be allowed to warm up to room temperature when not in use.

Commercial systems became available that use advanced refrigeration techniques (for example [[pulse tube refrigerator]]) to eliminate the need for liquid nitrogen cooling.

Germanium detectors with multi-strip electrodes, orthogonal on opposing faces, can indicate the 2-D location of the ionization trail within a large single crystal of Ge. Detectors like this have been used in COSI balloon-born astronomy missions (NASA, 2016) and will be used in an orbital observatory (NASA, 2025) [[Compton Spectrometer and Imager]] (COSI).

Because germanium detectors are highly efficient in photon detection,<ref>{{Cite journal |last1=Sangsingkeow |first1=Pat |last2=Berry |first2=Kevin D |last3=Dumas |first3=Edward J |last4=Raudorf |first4=Thomas W |last5=Underwood |first5=Teresa A |date=June 2003 |title=Advances in germanium detector technology |url=https://doi.org/10.1016/S0168-9002(03)01047-7 |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |volume=505 |issue=1–2 |pages=183–186 |doi=10.1016/s0168-9002(03)01047-7 |bibcode=2003NIMPA.505..183S |issn=0168-9002|url-access=subscription }}</ref> they can be used for a variety of additional applications. High-purity germanium detectors are used by Homeland Security to differentiate between naturally occurring radioactive material (NORM) and weaponized or otherwise harmful radioactive material.<ref>{{Cite web |date=2020-12-18 |title=Deciphering Radiation Alarms: Using High Purity Germanium Detectors for Nuclear Security |url=https://www.iaea.org/newscenter/news/deciphering-radiation-alarms-using-high-purity-germanium-detectors-for-nuclear-security |access-date=2024-05-06 |website=www.iaea.org |language=en}}</ref><ref>{{Cite web |title=High-Sensitivity Detectors {{!}} Homeland Security |url=https://www.dhs.gov/publication/high-sensitivity-detectors |access-date=2024-05-06 |website=www.dhs.gov |language=en}}</ref> They are also used in monitering the environment due to the concern of the use of nuclear power.<ref>{{Cite web |title=Document Display {{!}} NEPIS {{!}} US EPA |url=https://nepis.epa.gov/Exe/ZyNET.exe/9101GWDH.txt?ZyActionD=ZyDocument&Client=EPA&Index=1976%20Thru%201980&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5CZYFILES%5CINDEX%20DATA%5C76THRU80%5CTXT%5C00000030%5C9101GWDH.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=8 |access-date=2024-05-06 |website=nepis.epa.gov |language=en}}</ref> Finally, high-purity germanium detectors are used for medical imaging and nuclear physics research, making them a rather diverse detector as far as applications go.<ref>{{Cite journal |last1=Cooper |first1=R.J. |last2=Amman |first2=M. |last3=Luke |first3=P.N. |last4=Vetter |first4=K. |date=September 2015 |title=A prototype High Purity Germanium detector for high resolution gamma-ray spectroscopy at high count rates |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |volume=795 |pages=167–173 |doi=10.1016/j.nima.2015.05.053 |bibcode=2015NIMPA.795..167C |issn=0168-9002|doi-access=free }}</ref> 

=== Cadmium telluride and cadmium zinc telluride detectors ===
[[Cadmium telluride]] (CdTe) and [[cadmium zinc telluride]] (CZT) detectors have been developed for use in [[X-ray spectroscopy]] and [[gamma spectroscopy]]. The high density of these materials means they can effectively attenuate X-rays and gamma-rays with energies of greater than 20 [[Electronvolt|keV]] that traditional [[silicon]]-based sensors are unable to detect. The wide [[band gap]] of these materials also means they have high [[Electrical resistivity and conductivity|resistivity]] and are able to operate at, or close to, room temperature (~295K) unlike [[germanium]]-based sensors. These detector materials can be used to produce sensors with different electrode structures for [[Medipix|imaging]] and high-resolution [[HEXITEC|spectroscopy]]. However, CZT detectors are generally unable to match the resolution of germanium detectors, with some of this difference being attributable to poor positive charge-carrier transport to the electrode. Efforts to mitigate this effect have included the development of novel electrodes to negate the need for both polarities of carriers to be collected.<ref>{{cite journal|last=Luke|first=P. N.|date=1994-11-01|title=Unipolar charge sensing with coplanar electrodes -- Application to semiconductor detectors |doi=10.2172/34411|osti=34411|url=https://digital.library.unt.edu/ark:/67531/metadc684991/|doi-access=free}}</ref><!-- someone added this isolated reference --><ref>J. S. Kapustinsky, Nucl. Instrum. Methods A 617 (2010) 546 – 548.</ref>

== Integrated Systems ==
Semiconductor detectors are often commercially integrated into larger systems for various radiation measurement applications.

=== Automated Sample Changing for Germanium Detectors ===
[[File:GammaGIF.gif|thumb|upright=1.0|right|alt=HPGe automated with a low-cost, open-source autosampler.|HPGe automated with a low-cost, open-source autosampler.]]

Gamma spectrometers using HPGe detectors are often used for measurement of low levels of gamma-emitting radionuclides in environmental samples, which requires a low background environment, usually achieved by enclosing the sample and detector in a lead shield known as a 'lead castle'. Automated systems have been developed <ref>{{cite web | url=https://www.nuclearsystem.com/robotic-gamma-spectrometer-with-sample-changer | title=Robotic Gamma Spectrometer with sample changer – Nuclear System }}</ref> to sequentially move a number of samples into and out of the lead castle for measurement. Due to the complexities of opening the shield and moving the samples, this automation has traditionally been expensive, but lower-cost autosamplers have recently been introduced.<ref>{{cite journal|last=Carvalho|first=Matheus|title=Auto-HPGe, an autosampler for gamma-ray spectroscopy using high-purity germanium (HPGe) detectors and heavy shields|journal=HardwareX|volume=4|pages=e00040|doi=10.1016/j.ohx.2018.e00040|year=2018|doi-access=free}}</ref>

=== Radioactive Waste Assay Machines ===
Semiconductor detectors especially HPGe are often integrated into devices for characterising packaged radioactive waste. This can be as simple as detectors being mounted on a moveable platform to be brought to an area for in-situ measurements and paired with shielding to restrict the field-of-view of the detector to the area of interest for one-shot "open detector geometry" measurements,<ref>ISO 19017:2015</ref> or for waste in drums, systems such as the Segmented Gamma Scanner (SGS) combine a semiconductor detector with integrated [[mechatronics]] to rotate the item and scan the detector across different sections.<ref>LALP-91-011, Application Note, Segmented Gamma-Ray Scanner</ref> If the detector field of view is scanned across small areas of the item in multiple axes as is done with a Tomographic Gamma Scanner (TGS), [[Tomography]] can be used to extract 3D information about the density and gamma emissions of the item.<ref>LA-UR-93-1637, Tomographic gamma scanning (TGS) to measure inhomogeneous nuclear material matrices from future fuel cycles, LANL, 1993</ref>

=== Gamma Cameras ===
Semiconductor detectors are used in some [[Gamma camera|Gamma Cameras]] and Gamma imaging systems <ref>{{cite web | url=https://h3dgamma.com/h100.php | title=H100 - H3D, Inc }}</ref>

== See also ==
* [[Lazarus effect]]
* [[Pandemonium effect]]
* [[Synthetic diamond#Applications|Synthetic diamonds]]
* [[Total absorption spectroscopy]]
* [[X-ray spectroscopy]]
* [[Microstrip detector]]
* [[Hybrid pixel detector]]
* [[Liulin type instruments]]

== References==
{{Reflist|30em}}

== External links ==

* [https://indico.cern.ch/event/124392/contributions/1339904/attachments/74582/106976/IntroSilicon.pdf Silicon Detector powerpoint] delivered for EDIT (Excellence in Detectors and Instrumentation Technologies) 2011 at CERN, M. Krammer, F. Hartmann.

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[[Category:Experimental particle physics]]
[[Category:Ionising radiation detectors]]
[[Category:Medical imaging]]
[[Category:Particle detectors]]
[[Category:X-ray instrumentation]]