Photodiodes, the core component of image sensors, are indispensable in countless applications. Silicon is a favored material for crafting these components, but its sensitivity in the NIR range had been lacking. As a result, developers have had to use other materials which can be expensive and harmful to the environment. A project at the Fraunhofer Institute for Photonic Microsystems (IPMS) is now developing sensitive silicon-based photodiodes for the first time. Funded by Germany’s Federal Ministry of Education and Research, the three-year MesSi project will utilize €566,000 in funding to provide a more cost-effective route for the production of image sensors. Researchers at Fraunhofer IPMS are developing silicon photodiodes sensitive in the NIR range for more cost-effective manufacturing of image sensors in applications including autonomous driving, security, medical and chemical imaging, and spectroscopy, among others. Courtesy of Fraunhofer IPMS. Currently, photodiodes for the visible range are almost always made with silicon. Because of the material’s ubiquity in the semiconductor industry, it can be easily and cost-effectively produced at scale. In the NIR range, materials like indium gallium arsenide (InGaAs) have had to fill in due to silicon’s low-sensitivity to this portion of the light spectrum. The problem is, InGaAs is not compatible with silicon semiconductor technology. Because it requires its own dedicated production infrastructure, it’s expensive to produce. Further, heavy metals like arsenic are needed in its manufacturing, which stands at odds with current efforts to reduce the environmental impact of microelectronics production. Responding to the demand for lower production costs and more manageable environmental impacts, researchers at Fraunhofer IPMS are working to make silicon photodiodes more sensitive to the NIR. “The innovation is based on the implementation of a new structure in our photodiode,” said project coordinator Michael Müller from Fraunhofer IPMS. “Instead of the usual planar device topography, we use novel pyramidal and ring structures that function like a light collecting basin.” Using a very thin metal layer in the Schottky barrier, the researchers are able to increase the photodiode’s internal quantum efficiency — meaning the number of charge carriers that light generates in the semiconductor. Müller and his team believe that these two innovations will significantly increase sensitivity and enable silicon’s use for NIR applications. If successful, the project is expected to enable a range of new applications in the NIR range, particularly in price-sensitive, high-volume markets like autonomous driving, which requires new lidar sensors and fog cameras for environmental monitoring. Other areas include security technology, chemical and medical imaging, spectroscopy, and hyperspectral imaging.
