The Photoelectric Effect in Silicon Image Sensors: A Defense [Part: 3 Industrial Imaging]

June 25, 2016

In part 1 (Link), and part 2 (Link), I started the process of explaining the internal and external photoelectric effect with regard to the significant body of research on the topic. It is clear that the industrial giants of image sensors would also have at least some knowledge in this area, a) because they are large multi-nationals who would not risk incorrect information on their websites, b) they typically employ large teams of both industrial and academic world leaders in their design teams and c) the total combined historical world-wide R&D time is huge in comparison to the knowledge of an individual.

 

In this part 3 blog post, I'll briefly link to a number of big name industrial companies both in terms of their official websites and of course papers and patents from them. The aim? Well to empirically demonstrate that the " internal photoelectric effect " is critical to the operation and understanding of many optical and electrical systems, including CCD and CMOS image sensors, silicon photo-diodes (Si-PDs), avalanche photo-diodes (Si-APDs) and of course *all* single-photon avalanche diodes (Si-SPADs) technologies.

 

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Olympus Cameras [Link]:

  • "Light detectors that are based on the photoelectric effect include photomultiplier tubes, avalanche photo-diodes, charge-coupled devices, image intensifiers, and complementary metal-oxide semiconductor (CMOS) photo sensors." and

  • "The photoelectric effect is manifested in three different forms: photo-emissive, photo-conductive, and photovoltaic", and "The photo-emissive effect occurs when light strikes a prepared metal surface, such as cesium, and transfers sufficient energy to eject electrons into free space adjacent to the surface. In a photoelectric cell, the emitted electrons are attracted by a positive electrode, and when a voltage is applied, a current is subsequently created that is linearly proportional to the intensity of the light incident on the cell."

 

ST-Microelectronics Imaging [Link and Link]:

In this ST patent, the main patent refers to the use of IR information rather than the normal visible as a method of removing the degrading effects of IR from those visible images. They state:

  • "Specifically, infrared radiation also generates electrons by photoelectric effect. These electrons thus generated by the infrared radiation induce a signal which, by mixing with the signal corresponding to the visible radiation, will disrupt the measured signal and add an additional interference value to the useful signal. The quality of the images is thereby damaged." and

  • "The semiconducting photosite comprises a first semiconducting P-N junction 1 forming first load collection device making it possible to convert the incident radiation, mainly the visible radiation, into electron-hole pairs by photoelectric effect, and thus generate a signal from the electrons that is proportional to the number of incident photons received. The semiconducting photosite also comprises a second semiconducting junction 2 forming second load collection device making it possible to convert the radiation reaching this second junction 2, mainly the infrared radiation, into an electron-hole pair by photoelectric effect, and thus generate a signal from the electrons that is proportional to the number of photons received."

 

In a second ST Microelectronics patent, they explicitly state that the photoelectric effect is critical for image sensor development, stating:

  • "An imaging device, or sensor, is a photosensitive electronic component used to covert electromagnetic radiation into an analog electrical signal. Generally, this signal is then amplified, digitized by an analog/digital converter, and digitally processed so as to obtain a digital image. In this case, the imaging device makes use of the photoelectric effect, whereby incident photons liberate electrons at a semiconductor junction in each active element. To do this, a photosite (e.g. pixel) comprises at least one photosensitive region, for example, a photodiode, and a reading region coupled to the photodiode via a charge transfer transistor." and

  • "The photodiode 2 comprises a charge storage region 6 able to store the charge created via the photoelectric effect in the photodiode. The current created by this charge is removed by grounding the charge storage region 6 via an electrical connection 7, such as a metal contact. "

 

Nikon Corporation [Link]:

  • "The avalanche phenomenon is induced by introducing carriers, generated by a photoelectric effect in a light-receiving area, into a high electric field area formed in a semiconductor pn junction. The carriers introduced in such high electric field area collide with neutral semiconductor atoms, thus generating other carriers.", and

  • "The entering infrared light is subjected to the photoelectric effect, and the resulting carriers enter the high electric field area."

Canon Cameras [Link]:

  • "Photodiodes are designed to enable light to strike them on their p-type sides. When light strikes this side, electrons and holes are created within the semiconductor in a photoelectric effect. Light of a short wavelength that strikes the photodiode is absorbed by the p-type layer, and the electrons created as a result are attracted to the n-type layer. Light of a long wavelength reaches the n-type layer, and the holes created as a result in the n-type layer are attracted to the p-type layer. In short, holes gather on the p-type side, which accumulates positive charge, while electrons gather on the n-type side, which accumulates negative charge. And because the circuit is reverse-biased, the electrical charges generated are unable to flow. The brighter the light that hits the photodiode, the greater the electrical charge that will accumulate within it. "

 

Both Nikon and Canon together comprise the vast majority of both professional grade and consumer grade photographic equipment. The EOS 1D line for example is THE de-facto camera for photo-journalism and professional photography, while the Nikon top range models are equally important in terms of world wide professional camera use. Both Canon and Nikon have scientific imaging arms, mostly with respect to microscopy and biological imaging. Both are also well known in terms of consumer grade compact imaging and thus constitute a significant market share. 

 

TSMC [Link]:

Taiwan Semiconductor Manufacturing Co (TSMC) manufacture a large number of CMOS image sensors as they act as a silicon foundry for image sensor design houses such as Omnivision. They state:

  • "When the photo diode 130 is subjected to a light exposure 150, the photoelectric effect occurs and electrons and holes (not shown) are generated within the photo diodes", and

  • "As set forth above, the photo diode region 230 is provided for generating charges based on a photoelectric effect when the photo diode region 230 is subjected to a light exposure."

 

Omnivision [Link and Link]:

Omnivision produce CMOS image sensors primarily for mobile phones and have the current dual sourced (with Sony Corp) contract for Apple's iPhone range, in a 2014 patent they state:

  • "Upon photoelectric activation, the N type photodiode 305 collects electrons as photo generated charge carriers", and

  • "The present invention relates to a solid-state image sensing device such as a CMOS sensor, a CCD sensor, and the like which acquires images and positional information by using a photoelectric conversion effect, and particularly, to a technology of improvement of an SN ratio and extension of a dynamic range of a solid-state image sensing device, which includes MOS capacitance in a unit pixel and performs electric charge-to-voltage conversion.", and

  • "wherein during an exposure period, photoelectric charges generated by irradiation of light are accumulated in the photodiode."

 

Bell Labs US [Link]:

Bell Labs in the US are well known in electronics for being a historical hot-bed of superb cutting edge technology. In this press release on their website they discuss the 2009 Noble prize that went to Williard Boyle and George Smith, for the first use of CCD sensors for digital, solid-state image sensors. The Nobel prize recognises not only the scientific achievement but also the impact on the wider world. As such, one can only hold the opinions of Noble Laureates in the very highest regard. They write:

  • "Leveraging the photoelectric effect, by which light is transformed into electric signals, the CCD provided the first practical way to let a light-sensitive silicon chip store an image and then digitize it."

 

Teledyne Dalsa [Link]:

Teledyne Dalsa are big in terms of scientific instrumentation for imaging. On their website they have a dedicated white paper looking at the pro's and con's of CCD and CMOS imagers, they state that:

  • "CCD and CMOS imagers both depend on the photoelectric effect to create electrical signal from light".

 

Rambus Imaging [Link and Link]:

Rambus have been making quite a lot of noise recently regarding their lensless image sensors, in this Rambus blog post, they re-publish an IEEE Spectrum article that discusses a Paul Scherrer Institute team who are working with T-Hz imaging. Its nice to a) see an industrial company supporting smaller research institutes and b) in the context of this debate, see that the IEEE discuss the photoelectric effect. The article states:

  • "In common CCDs individual photons of visible light liberate individual electrons, a phenomenon known as the internal photoelectric effect. These electrons, have sufficient energy to cross silicon’s band gap, and end up stored in a potential well, from which they can be read out. Terahertz photons, with their longer wavelengths, carry much less energy and the dislodged electrons simply don't make it across the band gap."

 

Edmund Optics [Link]:

Edmund Optics are one of the UK's optics and photonics suppliers to both industry and academia, I've ordered a number of parts from them in the past and used their application notes a number of times as an introductory tool to a new topic. In this application note, they discuss the operation of machine vision cameras, stating:

  • "The solid state sensor is based on a photoelectric effect and, as a result, cannot distinguish between colors."

 

"Photodiode" - Wikipedia [Link]:

 

Before progressing to pure academic sources, perhaps I'll close with Wikipedia seeing as our modern use of Wikipedia often clouds the debate of technical issues, and indeed was a direct source of confusion in the rather heated argument I had about the photoelectric effect. Wikipedia states that:

"A photo-diode is a p–n junction or PIN structure. When a photon of sufficient energy strikes the diode, it creates an electron-hole pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photo-current is produced. The total current through the photo-diode is the sum of the dark current (current that is generated in the absence of light) and the photo-current, so the dark current must be minimized to maximize the sensitivity of the device.[2]" [Page was last modified on 4 March 2016, at 07:16.]

 

Despite Wikipedia's open source, traditionally non-citable, format, certain pages attract a significant number of readers and therefore a significant number of editors. With new rules requiring editors to be registered etc, certain high traffic pages have effectively become peer reviewed and effectively have become stable. While not robust enough for an academic debate, it is clear that Wikipedia would settle to the same fundamental facts as other more robust sources. Likewise Wikipedia management require that a page has clear references, citations and external links. In my opinion Wikipedia is therefore best viewed as a secondary source with links to primary sources, where those links and the writing style often being a way in to a new topic.

 

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So, we now ask ourselves the question: 

  • "Is the internal photoelectric effect important or indeed the manner in which image sensors obtain their optical to electrical conversion?"

It seems simply, from a google search of key world-players in the industrial sphere, that YES, indeed the internal photoelectric effect is key. I'm sure that all of the researching engineers and patent authors in the above sources would agree with me, especially given the content discussed in my previous blog posts, and indeed the posts to come.

 

I find myself still confused however when someone can argue that the photoelectric effect has nothing to do with CMOS imaging sensors or SPADs, and indeed how can they get angry about it in the face of this huge body of publicly available information. It is clear from my previous blog posts that different field utilize their nomenclature differently. This seems to be a prime example of this word usage issue. However one would assume that a professional would understand this issue and would modulate their opinions based upon which of the fields the issue is currently being discussed in. One would also assume that if that individual professes so be part of that wider peer group for a field, they should share the same nomenclature. The above companies clearly then represent the industrial manifestation of my field, the fact that they agree among themselves can only mean that there is indeed a shared and widely common scientific paradigm and that to proceed with a debate or to publish in the field would require acceptance of that shared view and shared language.

 

I hope that the list above of industrial companies has shown that I got my ideas from somewhere and that I am not digging a hole for myself, and that I am not barking up the wrong tree. In Part 4, we will however look at the academic literature and see if we can reach a consensus that is indeed formed by the directly relevant field (i.e. engineering, semiconductors and solid-state imaging sensors).

 

Stay tuned...

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Senior Electronics Design Engineer, (Digital)

Coda Octopus Products Ltd.

South Gyle, Edinburgh, Scotland, UK

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Dr Edward M.D. Fisher Ph.D MEng MIEEE MIET