Photodetectors

Photodetectors are devices that measure the intensity of light or electromagnetic radiation in general using for this purpose the interaction of the radiation with matter. The effect of this interaction can be either directly and visually perceived, as in the case of chemical photo sensors (radioactivity detection or photographic films), or converted to a measurable electrical magnitude (voltage, current, or resistivity change) in modern cameras. In the latter case, the detection steps are as follows:

  • Absorption of radiation from the material that is part of the detector. Since each material has its own absorption spectrum (frequency range of the electromagnetic radiation that it absorbs), the detector will only operate in the frequency range defined by it.
  • Conversion of the energy absorbed by the radiation to another form through effects triggered by absorption. For example, we can have:
    • a change in the current flowing through the device due to the generation of electron donors (electrons or pairs of electrons-holes) created by excitation from radiation exposure (photoelectric effect).
    • a voltage change at the edges of the device due to the creation of incident radiation of electron-hole pairs separated spatially (photovoltaic phenomenon).
    • conversion of energy to heat due to oscillation stimulation within the material that absorbs radiation and increases the temperature of the material. The temperature can be measured by a temperature-dependent resistor and in contact with the radiation-absorbing material.
  • Reading of the conversion signal with an appropriate (usually electronic) system.

Photodetector Applications

Traditionally, photodetectors were developed and advanced mainly because of the needs of scientific research (astronomy, nuclear physics and physical particle physics) and national security (military applications and border security). Nowadays however, in the case of the visible spectrum, photodetectors have become a fully developed consumer market (which is understandable, if one thinks that each cellphone usually has two built-in cameras).

Such an evolution is also intended for non-visible spectrum photodetectors (infrared and ultraviolet), without yet having become reality. Indeed, although the photodetectors outside visible already have plenty of applications (used for example for data transmission, in night or extreme weather conditions vision applications, for food and medicine quality control, in fire detection systems, in security cameras, car sensors, medical and environmental sensors), few of them are actually consumer product applications. The main reason for this lack is the high cost
of modern non-visible spectrum photodetectors, making commercial exploitation difficult for them.

Photodetector Materials and Technologies

Visible Spectrum

The dominant photodetector technology for visible radiation is that of silicon photodiodes (Si). These photodiodes may be:

  • p-n junctions: a typical semiconductor diode which is packed in such a way that the contact area of the two types of semiconductor can be illuminated. When in reverse polarization (n connected with the positive source pole and p with the negative) the junction is in depletion. In such conditions, valence band electrons are stimulated over the energy gap due to the radiation absorption and increase the inverse leakage current of the passage.
  • p-i-n junctions: the difference from the previous type is that a fine region of intrinsic semiconductor (i-intrinsic) is introduced that offers carriers of both types when illuminated.
  • Schottky junctions: a metal-semiconductor contact packed in the same way as described for the p-n type photodiode and operating in the same manner based on the semiconductor stripping area.

In any case, the construction is based on typical processes and materials of semiconductor integrated circuits. This results in a reduction in final construction costs and paved the way for commercial exploitation of visible spectrum photodetectors

Ultraviolet Spectrum

The success of silicon photodiodes combined with the fact that silicon has a sufficient absorption of up to 200nm has led to the use of silicon photodiodes for the detection of ultraviolet radiation too. However, given that UV radiation has a significantly higher energy content than visible light, the specific use of silicon photodiodes reduces their life span, thus increasing application costs. In addition, since the silicon absorbs visible, the photodetector response will have two components in the case of UV radiation coexisting with higher wavelength radiation.

Si absorbance (A) – Retrieved from https://slideplayer.com/slide/5009414/ (Slide #4)

A solution to the above mentioned problems would be the use of filters from ultraviolet absorbing materials (even lower wavelength than those that can be detected by silicon LEDs) and re-emit radiation to the visible spectrum. However, the final cost of the application is quite high, as the total application cost is the sum of the photodetector’s and the filter’s costs.

Another solution would be to abandon the silicon and build photodiodes using high energy energy semiconductors (e.g. ZnO or ZnMnO). In this case, the photodetectors absorb only the ultraviolet, so the use of filters is not necessary, but the cost of construction increases due to the use of non-standard materials.

Infrared Spectrum

Use of silicon photodetectors is absolutely sufficient in near infrared applications (wavelength up to 1100nm). Unfortunately, silicon ceases to absorb radiation at a higher than 1100nm wavelength. At such wavelengths, we use either heterojunction photodetectors with exotic materials (eg HgCdTe – see an example here) or thermal sensors (bolometers). The first solution significantly increases the photodetector manufacturing cost while the latter decreases the photodetector response rate (generating radiation-absorbing conductivity carriers is a phenomenon with a typical integration time of less than 1s, while the heat radiation conversion into the material and the restoration of a new higher temperature is a phenomenon with a typical integration time of more than 1 s).

Another problem with infrared sensors is that they are vulnerable to thermal noise, since at ambient temperature the energy content is ~ 26 meV comparable to the incident energy of infrared radiation. For this reason, infrared photodetectors are often cooled at temperatures well below 0 ° C, which also increases the final cost of the device.

References

  1. Photodetectors, Wikipedia
  2. Photonics Encyclopedia, rp-photonics website
  3. Applications of Photodetectors, LinkedIn
  4. Photodiode Detector Technology” and next webpages referring to every photodiode type. Electronics-Notes.com website.
  5. Photodiodes and How they Work”, Youtube video for the p-n and p-i-n photodiodes operations.
  6. What is a Photodiode?”, ElectronicsHub.org website.
  7. Photodiodes: Symbols and Types”, Physics-and-Radio-Electronics.com website
  8. Examples of commercial photodetectors, ThorLabs website. Disclaimer: This reference is by no means an ad of ThorLabs company, and is given only in order to obtain a sense of current photodetectors cost, especially in the case of non-visible spectrum photodetectors.