Light sensors optimised for the IoT era

Scientists from Incheon National University have developed a compact and robust optical sensor that can convert light to digital signals, making it ideal for deployment in the so-called ‘trillion-sensor era’ of the Internet of Things (IoT). Their research has been published in the journal Small.

An important part of the IoT economy is the presence of light/photo sensors, which are tiny semiconductor-based electronic components that detect light and convert it to electrical signals. Light sensors can be found everywhere around us, from household electronic gadgets and healthcare equipment to optical communication systems and automobiles.

Most light sensors used in cost-effective consumer products are energy efficient but are susceptible to noise (unwanted light information) in the external environment, which adversely affects their performance. To tackle this issue, products have been designed using light-to-frequency conversion circuits (LFCs), which show better signal-to-noise ratio. However, most LFCs are made of silicon-based photodetectors that can limit the range of light detection. Also, use of LFCs leads to chip area wastage, which becomes a problem when designing multifunctional electronic circuits for compact devices.

Led by Professor Sung Hun Jin, Incheon researchers demonstrated a highly efficient system of photodetectors that can overcome the limitations of conventional LFCs. They developed complementary photosensitive inverters with p-type single-walled carbon nanotubes (SWNT) and n-type amorphous indium–gallium–zinc–oxide (a-IGZO/SWNT) thin film transistors.

“Our photodetector applies a different approach with regard to the light-to-frequency conversion,” Prof Jin said. “We have used components that are light dependent and not voltage dependent, unlike conventional LFCs.”

The new design architecture allowed the team to design an LFC with high chip area efficiency and a compact form factor, making it suitable for use in flexible electronic devices. Experiments conducted using the photosensor system indicated excellent optical properties, including high tunability and responsiveness over a broad range of light. The LFC also showed possibility of large-area scalability and easy integration into state-of-the-art silicon wafer-based chips.

The LFC system could be used to build optical sensor systems that have high-level signal integrity, as well as excellent signal processing and transmitting abilities. These promising properties make it a strong contender for application in future IoT sensor scenarios.

“LFCs based on low dimensional semiconductors will become one of the core components in the trillion-sensors area,” Prof Jin said. “Our LFC scheme will find application in medical SpO₂ detection, auto-lighting in agriculture or in advanced displays for virtual and augmented reality.”

Image credit: ©stock.adobe.com/au/asb63

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