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The Photoelectric Effect in Silicon Image Sensors: A Defense [Part: 5 Academic Imaging]

In this series of blog posts, I've systematically looked at the literature, physical mechanisms and the nomenclature used within Physics and Engineering for the external and internal photoelectric effect. Contrary to unprofessional and overly critical critique, the internal photoelectric effect is directly relevant to solid-state image sensors and indeed is the nomenclature (used within the field of literature) to describe the process.

This blog post looks, cites and quotes from a number of academic peer reviewed journal papers and conference proceedings. The purpose is to demonstrate that I, as a researcher in the field of optical detection and instrumentation, was not digging myself a hole when I included the photoelectric effect in my written work.

In part 1 (Link) we discussed that there were only three effects that constitute optical absorption in solid-state materials, and that if we were to incorrectly disregard the photoelectric effect, we would struggle to explain the operation of SPADs, CCDs, CMOS sensors and photo-diodes in the UV, visible and IR wavelength regions with any of the remaining physical phenomena (Compton Scattering and Pair Production).

In part 2 (Link) we discussed quotes from a number of sources including optical physics texts, some dictionary definitions, some books on semiconductor electronics, academic papers and legal technology patents. The key here was to demonstrate that there is not necessarily a disagreement in the field as to the process, but that there is distinct nomenclature for the two photoelectric processes.

  1. The External Photoelectric Effect: Electrons receiving energy from the absorption of an optical photon, obtain enough energy whereby they are able to overcome the vacuum work function of the metal. If this is the case, the electrons have enough kinetic energy they are able to leave the surface of the metal. The key here is vacuum work function and indeed metallic materials.

  2. The Internal Photoelectric Effect: Electrons again obtain energy from the absorption of an optical photon, however if this energy is greater than the band gap of the semiconductor (typically less than the work function of that material), the electron is promoted from the valance band into the conduction band. It is not ejected from the material surface, but is internally able to flow forming a photo-current or is able to modify the conductivity.

In part 3 (Link) we bolstered the nomenclature of the field by discussing the phrase "the internal photoelectric effect" within corporate and industrial manufacturers of CCD and CMOS image sensor technologies, i.e. world leading and world wide companies that typically employ large teams of experts and indeed generate significant income from products that rely on the physical process. Due to the competitive market these companies would not risk incorrect or pseudo science being on their websites, particularly in web sections that act as white papers on the topic and indeed including incorrect terminology within the legal nature of industrial patents, viewable by their competitors, would be inadvisable. The debate however is if academic and industrial usage of a phrase constitutes the accepted nomenclature of the phrase, and if there is consistent agreement, then this must be the case.

In part 4 (Link), I looked at academic text books that directly proved that the internal photoelectric effect is a well used term with relation to the physical process of valance to conduction band promotion by photonic absorption and indeed is well used in the literature. The interesting book listed here is actually a very recent book (2014) that included chapters from my Ph.D supervisor. As such, the academic that told me the photoelectric effect was "irrelevant" was therefore also having a go at that supervisor by proxy, as well as the editors and publishers of the book. The book also contained a chapter by R. Turchetta from the UK's STFC, therefore by telling me I was digging myself a hole and was fundamentally wrong, this particular academic was also, by proxy, stating that R. Turchetta was also wrong. Hmm, perhaps we should organise a scientific congress to discuss this matter among all these authors and companies I've referenced within this series of posts.


So, onto today's post where will will discuss academic research papers, both journals and conference proceedings that directly demonstrate that the nomenclature ("the internal photoelectric effect"), is in direct use within the academic field, and is correctly linked to both the physical process and indeed photodiodes, avalanche photodiodes, SPADs, CCD sensors and CMOS Imaging sensors.

Source (Paper): "A View on Progress of Silicon Single Photon Avalanche Diodes and Quenching Circuits", Cova, Ghioni, Zappa, SPIE, Advanced Photon Counting Techniques, Edited by W. Becker 2006 (Link)

  • "Furthermore, they have inherently higher photon detection efficiency, since they do not rely on electron emission in vacuum from a photocathode as PMT, but instead on the internal photoelectric effect."

Source (Paper): "Progress in Silicon Single-Photon Avalanche Diodes", Ghioni, Gulinatti, Rech, Zappa, Cova, IEEE Journal of Selected Topics in Quantum Electronics, 2007 (Link)

  • "Furthermore, they have inherently higher photon detection efficiency, since they do not rely on electron emission in vacuum from a photocathode as PMT, but instead on the internal photoelectric effect."

  • This is actually a a very similar paper, with the SPIE paper being an invited conference paper and this IEEE journal paper being a much extended peer reviewer version of the same. The point however is that due to the differences in publisher, IEEE vs SPIE, the paper and its content will have been effectively peer reviewed multiple times.

Source (Paper): "Study of high speed quenching circuits in photon counting imaging LIDAR system", X. ZHENG, Y. DING, G. HUANG and R. SHU, SPIE: Optical and Optoelectronic Sensing and Imaging Technology, 2015 (Link)

  • "SPAD is a kind of photoelectric device based on internal photoelectric effect"

Source (Paper): "Quantum efficiency of silicon photodiodes in the near-infrared spectral range", C. Hicks, M. Kalatsky, et al, J. Appl Optics, Vol. 42, No. 22, p4415-4422, 2003 (Link)

  • "Numerous studies explored the physics of internal photoelectric effect in Si and the ways to improve Si photodiode characteristics in the whole spectral range from ultraviolet UV to near infrared NIR."

Source (Conference/Tutorial Slides): "Tutorial: Photodetection", Remi Barbier (University of Lyon), 6th International Conference on New Developments in Photodetection (NDIP), Lyon, France, 2011 (Link)

  • This is perhaps the best set of slides/images that exactly explains the differences between the internal and external photoelectric effects and demonstrates the issues of nomenclature within a large physics and electronics field. It is interesting that R. Barbier directly discusses the two processes and links them appropriately to vacuum technologies such as PMTs and solid-state technologies such as SPADs. [Below are images of selected slides from this conference tutorial]

Source (Conference Slides): "Photodetection: Principals, Performances and Limitations", Slide #1, Joram (CERN), Dinu (LAL, ORSAY), Gys (CERN), Korpar (Uni of Maribor), Musienko (Fermilab), Renker (TU Munich), Excellence in Detectors and Instrumentation Technologies (EDIT), 2011 (Link and Link)

  • Notice, they show the standard portrayal of the band gap with Ec and Ev relating to the conduction and valence bands respectively.

  • The figure also shows the action of mid band traps or levels.

  • They also quote the band gap of silicon at 1.12eV with an approximate cut off wavelength (1.1um).

  • They show a very nice figure for the internal quantum efficiency, i.e. the number of photoelectrons produced per photon, for different wavelengths.

Source (Paper): "Line Image Sensors for Spectroscopic Applications in the Extreme Ultraviolet" Banyay, Brose, Juschkin, IOP Measurement Science and Technology, Vol 20, No 10, 2009 (Link)

  • "The basic principle of CCDs is to create electron– hole pairs by the internal photoelectric effect inside a p–n junction and to store and shift these charge packages through a pixel matrix."

  • "The maximum number of electron–hole pairs generated by an incident photon of energy Eph due to the internal photoelectric effect can at best be (equation)."

Source (Paper): "Physics of Photon-Flux Measurements with Silicon Photodiodes", Geist, Gladden, Zalewski, Journal of the Optical Society of America (JOSA), Vol 72, Issue 8, 1982 (Link)

  • "First, we briefly review the theory of the internal photoelectric effect in a semiconductor and argue that the fundamental photodetection process in an ideal photodiode does indeed respond to photon flux and to no other property of electromagnetic radiation."

  • Indeed this paper has an entire section, section II, titled "Internal Photoelectric Effect", with the text: "The quantum-mechanical description of the photogeneration of electron-hole pairs in silicon has been studied in considerable detail."

Source (Paper): "Semiconductor Ultraviolet Detectors", A. Rogalski and M. Razeghi, Opto-Electronics Review, Vol 4, No1/2, 1996 (Link)

  • "In the semiconductor detectors, the photons are absorbed in the bulk of the semiconductor material producing electron hole pairs which are separated by an electric field. These detectors make use of the internal photoelectric effect, where the energy of the photon is large enough to raise the electrons to the conduction band of the semiconductor material."

  • "In the case of photovoltaic detectors, the electron-hole pairs are separated by the electric field of p-n junctions, schottky barrier or MIS capacitors, which leads to an external photocurrent proportional to the number of detected photons."

Source (Paper): "Design and properties of silicon avalanche photodiodes", Wegrzecka, Wegrzecki, Grynglas et al, Opto-Electronics Review, Vol. 12, No. 1, p95-104, 2004 (Link)

  • "Detection in APDs takes advantage of the internal photoelectric effect as well as the phenomenon of avalanche multiplication in a reverse-biased silicon p-n junction", and

  • "The absorbed flux causes generation of electron-hole pairs in the semiconductor. The pairs are separated in a reverse-biased junction within the photodetector structure. The effect, that consists of generation, transport and recording in the external circuit of optical generated charge carriers, is called the internal photoelectric effect and is characterized by an internal quantum efficiency, eta_i"

Source (Paper): "Quantum Efficiency of the Internal Photoelectric Effect in Silicon and Germanium", O. Christensen, Journal of Applied Physics, 47, p689, 1976 (Link)

  • "For photon energies above the absorption edge, each of the absorbed photons creates on the average, eta, electron-hole pairs, where, eta, is the quantum efficiency of the internal photoelectric effect"

Source (Teaching Slides): "Detectors III: EMCCD, IR-APD, MKIDS, STJ, TES", "Ay122a: Astronomical Measurements and Instrumentation", Dimitri Mawet, 2015-16, California Institite of Technology (Caltech) and The Jet Propulsion Lab (JPL) (Link)

Source (Book): "Quantitative Biomedical Optics: Theory, Methods, and Applications", I. Bigio and S.Fantini, Cambridge University Press, 2016 (Link). In particular page 398:

  • "The emission of electrons from a photocathode is referred to as the external photoelectric effect, whereas the excitation of electrons into the conduction band of a semiconductor is referred to as the internal photoelectric effect."

Source (Book): "Optoelectronic Devices and Systems", S. Gupta, PHI Learning Pvt. Ltd, 2nd Edition, 2014 (Link)

  • "When radiation falls on the surface of a semiconductor material, a portion of the energy is absorbed by the semiconductor provided the energy of each photon hv is greater than the forbidden energy gap, Eg, of the semiconductor. As a result of this absorption, electron-hole pairs are created. Thus the conductivity of the semiconductor material increases. This property is known as the internal photoelectric effect which was discovered in 1873 while studying the electrical conductivity of Selenium in the presence of light"

  • "Devices such as photo-diodes and photo-transistors are based on this effect."

  • "The internal photoelectric effect must not be confused with the external photoelectric effect where free electrons are emitted from the surface of the semiconductor when radiation of appropriate frequency falls on it."

Source (Book): "Semiconductor Physics: An Introduction", Karlheinz Seeger , Springer Science & Business Media, 2004 (Link)

  • Particularly page 118

Source (Society Bulletin): "Einstein’s Photon Hypothesis and Its Impact on Science and Technology", Prof. Kunio Tada (Kanazawa Institute of Technology, Japan), Association of Asia Pacific Physical Societies (AAPPS) Bulletin Vol. 15, No. 2, 2005 (Link)

  • "In order to induce this effect, the absorption of incident light by matter should cause a generation of charged carriers, such as conduction electrons and positive holes in the case of a semiconductor, or free electrons (photoelectrons) emitted from a metal surface with immobile positive ions left behind. These two phenomena are called the internal photoelectric effect and external photoelectric effect, respectively."

  • "It is rather surprising that the internal photoelectric effect found in 1873 was much earlier than the discovery of external photoelectric effect made in circa 1888. It was discovered in the photoconduction of crystalline selenium by Willoughby Smith [see ref 11]."

Source (Paper): "Dynamic image processing on the solid-state photoelectric image sensor", Andrey A. Mancvetov, Proc. SPIE 2051, International Conference on Optical Information Processing, 248 (January 21, 1994) (Link)

  • This paper explicitly links CCD image sensors to the key phrase "photoelectric", however it demonstrates the issue in the literature with the confusion between the internal and external effect as which variety of photoelectric effect is not disclosed.

Source (Paper): "Determination of the quantum efficiency of a light detector", Y. Kraftmakher, European Journal of Physics, Volume 29, Number 4, 2008 (Link)

  • "The quantum efficiency (QE) η of a light detector is the ratio of number of electrons (or electron–hole pairs) released by the photoelectric effect to the number of incident or absorbed photons [1–3]. That means the QE is a quantity defined as the percentage of photons hitting the photosensitive area that will produce electrons or electron–hole pairs. It is an accurate measurement of the electrical sensitivity of a device to light."

  • "For monochromatic light of frequency ν and power P, the number of photons per second is P/hν, where h is the Planck constant. The electric current I due to the photoelectric effect equals (equation 1)"

  • "For a silicon photodiode used here, the I/P ratio given by the manufacturer is 0.62 A W−1 at λ = 0.9 µm. The QE for this wavelength thus equals 0.85."

  • This paper, is in fact, a method of demonstrating the photoelectric effect and quantum efficiency for undergraduate students as a teaching tool

Source (Paper): "Modeling, fabrication and measurement of a novel CMOS UV/blue-extended photodiode", C. P. Chen, Y. J. Zhao et al, Journal of Central South University, October 2014, Volume 21, Issue 10, pp 3821–3827 (Link)

  • "Photoelectric characteristics of this presented photodiode were studied..."

  • This paper links photoelectric as a key phrase with CMOS photodiodes and even SPAD technologies

Source (Paper): "Theory of the quantum efficiency in silicon and germanium", E. Antoncik and N. Gaur J. Phys. C: Solid State Phys., Vol. 11, 1978 (Link)

  • Abstract: "The spectral dependence of the quantum efficiency of the internal photoelectric effect in silicon and germanium as recently measured by Christensen, is interpreted in terms of the primary absorption process and the secondary relaxation processes. In particular, the interband Auger effect and emission of phonons directly connected with the generation of additional electron-hole pairs are discussed in detail."

  • Introduction: "The quantum efficiency of the internal photoelectric effect is defined as the number of electron-hole pairs arising from the absorption of one photon. This number depends on the energy of the photons absorbed; while in the neighbourhood of the absorption edge every photon creates only one electron-hole pair, for higher energies more electron-hole pairs can be generated and the quantum efficiency is greater than unity"

Source (Paper): "Various improved techniques for reduction of dark current in silicon photoelectric detector", Guoxiang Zheng, Proc. SPIE 1982, Photoelectronic Detection and Imaging: Technology and Applications '93, 164 (April 1, 1993) (Link)

  • "Silicon photodiodes. photoelectric transistor and other silicon photoelectric devices have many advantages such as high response speed, good linearity of current vs. illuminance, high quantum efficiency over a wide spectrum range and long life"

  • As with other papers, this effectively links the photoelectric effect with photodiode detectors through the use of the nomenclature of the "photoelectric detector".

Source (Book): "An Introduction to Optoelectronic Sensors", G. Righini, A. Tajani and A. Cutolo (Eds), World Scientific, Series in Optics and Photonics - Vol.7, 2009. (Link) - In particular: (Part 1, Chapter 14, page 303, Carlo Corsi, Centro Ricerche Elettro Ottiche)

  • "Infrared (IR) detectors are devices transforming the radiant energy of the IR region (0.7um to 300um) of electro-magnetic (EM) spectrum incident on the sensor into another form of energy which can be easily measurable like, generally, in the form of electric signal. This conversion of energy normally is done by:

  • Photoelectric Effects:​

  • ​a) photoelectric external effects: by generating free electrons from the surface of the sensor that has been hit by photons with a sufficient energy.

  • b) photoelectric internal effects: the most relevant phenomena in the sensor development, by generating a couple of electron-hole inside a photoelectric material, generally a photoconductor or a photovoltaic structure.

  • Bolometric Effects: ... ... ... etc."

Source (Paper): "Some Features of Photocurrent Generation in Single and Multibarrier Photodiode Structures", A. V. Karimov and D. M. Yodgorova, ISSN 10637826, Semiconductors, 2010, Vol. 44, No. 5, pp. 647–652. © Pleiades Publishing, Ltd., 2010. (Link)

  • While this paper does not include the "internal photoelectric effect" as a distinct phrase, it does discuss photodiode technologies. Crucially however it acts to muddy the nomenclature of the field by using the phrases "photoelectric amplification" and "photoelectric gain". I presume this is photoelectron multiplication due to avalanche effects. This would apear to be the case due to discussion of the multiplication factor.

  • "The feature of the internal photoelectric amplification effect is that the internal photoelectric gain in multi barrier photodiodes, depending on the connection mode (current or voltage generator), provides both current and voltage amplification due to the high out put resistance of these devices."


To close, I have a further industrial patent that again muddies the waters with respect to nomenclature in the field:

Source (Patent): "Solid-state image sensor and camera including a plurality of pixels for detecting focus", US 9065992 B2, Canon (Link)

  • This suggests use of a "photoelectric converter" as a field specific name for a pixel or photosite. Interesting...


Given the above, and the references in the previous four posts in this series, I fail to see how I was incorrect in my interpretation, I fail to see how I was digging myself a hole and indeed I fail to see what other process would be responsible for optical to electrical conversion in solid-state semiconductor technologies.

Clearly, the "internal photoelectric effect" is not "irrelevant" for SPAD sensors and clearly, beyond any doubt the phrase is in constant, long term, world wide and interdisciplinary use within both academia and industry. I sincerely hope the academic in question is humbled by this review of the literature and the state of the nomenclature and that one should only shout and become unprofessional in an academic capacity if you can clearly demonstrate you are right.

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