Photodiodes are light-sensitive semiconductor diodes that produce a current proportional to the number of photons the diode is exposed to. Photodiodes are an incredibly important type of sensor with numerous applications, including fire safety, photovoltaics and automation.1-3 Here, we dive into the properties and applications of InGaAs photodiodes.
Image Credit: Heintje Joseph T. Lee/Shutterstock.com
Photodiodes only have a limited operating wavelength range. The operating range typically spans ~ 320 – 1100 nm for silicon photodiodes, so most of the visible and the near infra-red region (NIR).4 Many sensing applications benefit from using longer NIR wavelengths through to the short-wave infrared (SWIR) region.
Imaging with SWIR radiation is a popular, non-destructive way of ‘seeing through’ objects to look at their contents or layers underneath surfaces. SWIR imaging has become particularly popular in art history and restoration as well as for recycling complex plastics that may be covered in labels or be a complex mixture of materials.5,6
Successful detection of SWIR or longer wavelength region needs photodiodes made of the right material – and for these wavelength region, Indium gallium arsenide (InGaAs) photodiodes are the typical choice.
InGaAs photodiodes use a mixture of three elements to efficiently absorb light in the ~ 800 – 2600 nm. Depending on the exact bandgap required for the sensor, the ratio of indium to gallium can be modified to achieve the right energy separation and maximize the efficiency of the InGaAs photodiode in the desired wavelength region.
InGaAs photodiodes are commonly used in spectroscopy and spectrophotometry measurements and sensing. Using InGaAs photodiode arrays for machine vision and automation applications is becoming increasingly common as InGaAs photodiodes work efficiently in low visible light conditions and therefore do not require external illumination as visible cameras would.7
Some devices that require sensing over an extended wavelength range may make use of dual diodes made from different materials, such as silicon which offers better performance in the UV to the visible range.
How do They Work?
The operating principle that underpins most photodiodes uses a P-N junction in the semiconductor material. The P-type carriers are mostly holes and the N-type carriers are electrons. Once a reverse bias is applied to the photodiode there is only a very slight movement of charge carriers through the circuit, resulting in a small level of current. For InGaAs photodiodes, the name comes from the semiconductor material that forms the P-N junction.
While there is a small amount of current measured on a photodiode, even in the absence of any light, what is known as the dark current, a significant amount more charge carriers are created when light shines on the diode. The absorbed photons must be sufficiently energetic to create new charge carrier species and increase the overall current measured.
There are several different photodiode types, including avalanche photodiodes and PIN photodiodes. Avalanche InGaAs photodiodes typically offer reduced noise levels and improved sensitivity over more traditional PIN InGaAs photodiodes.
For applications where absolute numbers of photons must be measured, such as for the characterization of laser sources, the calibration of the InGaAs photodiode is very important. As the diode response of the InGaAs photodiode can vary from diode to diode, it is important that the output current levels for individual devices are calibrated.
Calibration measurements are not the only important part of ensuring devices containing InGaAs photodiodes make accurate and reliable measurements. Sensors can become contaminated by their local environment, affecting the level of current readout or damaged by irradiation with intense light sources or particle collisions.
For medical, aerospace and more demanding applications, recent developments in the InGaAs photodiodes from Marktech Optoelectronics have shown that hermetic sealing can help improve sensor performance.8 Hermetic sealing keeps the sensors air and moisture tight so no other species can adsorb to the sensor's surface or affect the electronic properties, which could lead to inaccuracies in the current readout.
Multispectral imaging, or its higher resolution counterpart, hyperspectral imaging, is becoming a more mainstream imaging method.9 Multispectral imaging records signals from electromagnetic radiation across several different wavelength ranges and can be used to recover more complex datasets for improved identification of chemical species or complementary temperature and species identification information.
InGaAs photodiode arrays and their wide wavelength operating range are ideal for multispectral imaging and the high quantum efficiency makes them very sensitive. While normally achieving wider wavelength coverage than can be achieved in a device from an InGaAs photodiode alone is done by using multiple photodiodes, there are now developments to make single monolithic InGaAs photodiodes that integrate a GaAs layer to make a photodiode capable of detecting in the IR and visible regions.
For many applications, particularly in automation where feedback systems are used, the readout and response time of an InGaAs photodiode is very important. The new award-winning Marktech InGaAs photodiodes are also designed to have good response times for quick detection. Their key focus is very high levels of sensitivity for detection in challenging environments such as for point-of-care- medical devices or wearable technologies.
References and Further Reading
Tina, G. M., et al. (2013). Intelligent sun-tracking system based on multiple photodiode sensors for maximisation of photovoltaic energy production. Mathematics and Computers in Simulation, 91, pp.16–28. doi.org/10.1016/j.matcom.2012.07.020
Yakuphanoglu, F. (2007). Photovoltaic properties of hybrid organic/inorganic semiconductor photodiode. Synthetic Metals, 157(21), pp.859–862. doi.org/10.1016/j.synthmet.2007.08.012
Stoppa, D., et al. (2002). A CMOS photosensor test-chip for smoke detection applications. Proceedings - IEEE International Symposium on Circuits and Systems, 2, pp.161–164. doi.org/10.1109/iscas.2002.1010949
Young, E. T. (2013). Long-Wavelength Infrared Detectors. In T. D. Oswalt & I. S. McLean (Eds.), Planets, Stars and Stellar Systems: Volume 1: Telescopes and Instrumentation (pp.565–585). Springer Netherlands. doi.org/10.1007/978-94-007-5621-2_14
Gavrilov, D., et al. (2013). Experimental Comparative Study of the Applicability of Infrared Techniques for Non- destructive Evaluation of Paintings. Journal of the American Institute for Conservation, 52(1), pp.48–60. doi.org/10.1179/0197136012Z.0000000002
Hashagen, J. (2015). Seeing Beyond the Visible in machine vision. Optik&Photonik, 3, pp.34–37. https://onlinelibrary.wiley.com/doi/epdf/10.1002/opph.201500021
Malchow, D. S., et al. (2008, February). Development of high-speed InGaAs linear array and camera for OCT and machine vision. In Optical Components and Materials V (Vol. 6890, pp. 94-102). SPIE. doi.org/10.1117/12.771540
Kardys, G. (2023) Marktech optoelectronics, https://marktechopto.com/marktech-optoelectronics-wins-2023-best-of-sensors-awards-in-the-aerospace-and-space-category/, accessed July 2023
Coffey, V. C. (2012). Multispectral imaging moves into the mainstream. OPN Optics & Photonics News, 23(4), pp.18–24. doi.org/10.1364/OPN.23.4.000018
Geum, D. M., et al. (2019). Monolithic integration of visible GaAs and near-infrared InGaAs for multicolor photodetectors by using high-throughput epitaxial lift-off toward high-resolution imaging systems. Scientific Reports, 9(1), pp.1–12. doi.org/10.1038/s41598-019-55159-x