New Pixel Sensors Bring Their Own Compute

Atomically thin devices that combine sensing and computation also save power

2 min read
close up image of a chip

This optical image shows the 900-pixel 2D active pixel sensor created by the researchers.

Akhil Dodda, Darsith Jayachandran, and Saptarshi Das

By giving compute powers to atomically thin versions of the CMOS sensors now found in most digital cameras, a prototype sensor array can capture images using thousands to millions of times less power, a new study finds.

CMOS sensors are a kind of active pixel sensor, which combine a light detector with one or more transistors. Although scientists have made steady progress toward more energy-efficient light detectors, the signal-conversion and data-transmission capabilities of active pixel sensors are currently extremely energy-inefficient, says study colead author Akhil Dodda, an electronics engineer who was at Penn State University at University Park, in Pennsylvania, at the time of the research.

In the new study, the researchers sought to add in-sensor processing to active pixel sensors to reduce their energy and size. They experimented with the 2D material molybdenum disulfide, which is made of a sheet of molybdenum atoms sandwiched between two layers of sulfur atoms. Using this light-sensitive semiconducting material, they aimed to combine image-capturing sensors and image-processing components in a single device.

The scientists developed a 2D active pixel sensor array in which each pixel possessed a single programmable phototransistor. These light sensors can each perform their own charge-to-voltage conversion without needing any extra transistors. “We were often surprised by the research outcomes from this nanometer-thick material,” says study colead author Darsith Jayachandran, an electronics engineer at Penn State University at University Park.

The prototype array contained 900 pixels in 9 square millimeters, with each pixel about 100 micrometers large. In comparison, state-of-the-art CMOS sensors from Omnivision and Samsung have reached about 0.56 µm in size. However, commercial CMOS sensors also require additional circuitry to detect low light levels, increasing their overall area, which the new array does not, Dodda says.

In addition, the new device required only a few dozen picowatts—trillionths of a watt—per pixel, Jayachandran says. In comparison, a conventional CMOS sensor might demand roughly thousands to 10 million times more power per pixel, he says.

When compared with commercial germanium sensors, the new devices proved roughly a million times more responsive to light and could detect signals about one-thousandth as strong. They also possessed high dynamic range, and performed in a uniform manner across different wavelengths of light and across the entire array.

The new sensors also possessed the ability to quickly reset in order to help eliminate noise in the images the array captured. The researchers also achieved higher manufacturing yields with this new sensor technology.

“We believe our low-power 2D active pixel sensor platform will be a significant boost when it comes to developing smart cameras in the years to come,” Dodda says. “With this technology, we can enhance the quality of images further with minimal battery consumption.”

A major stumbling block this research faces has resulted in the successful transfer of the 2D phototransistors onto the other electronics that make up the device. Improving the transfer process would help enable “our grand vision of developing a smart, low-power camera,” Jayachandran says.

The scientists detailed their findings 17 November in the journal Nature Materials.

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Europe Expands Virtual Borders To Thwart Migrants

Our investigation reveals that Europe is turning to remote sensing to detect seafaring migrants so African countries can pull them back

14 min read
A photo of a number of people sitting in a inflatable boat on the water with a patrol ship in the background.

Migrants in a dinghy accompanied by a Frontex vessel at the village of Skala Sikaminias, on the Greek island of Lesbos, after crossing the Aegean sea from Turkey, on 28 February 2020.

ASSOCIATED PRESS

It was after midnight in the Maltese search-and-rescue zone of the Mediterranean when a rubber boat originating from Libya carrying dozens of migrants encountered a hulking cargo ship from Madeira and a European military aircraft. The ship’s captain stopped the engines, and the aircraft flashed its lights at the rubber boat. But neither the ship nor the aircraft came to the rescue. Instead, Maltese authorities told the ship’s captain to wait for vessels from Malta to pick up the migrants. By the time those boats arrived, three migrants had drowned trying to swim to the idle ship.

The private, Malta-based vessels picked up the survivors, steamed about 237 kilometers south, and handed over the migrants to authorities in Libya, which was and is in the midst of a civil war, rather than return to Malta, 160 km away. Five more migrants died on the southward journey. By delivering the migrants there, the masters of the Maltese vessels, and perhaps the European rescue authorities involved, may have violated the international law of the sea, which requires ship masters to return people they rescue to a safe port. Instead, migrants returned to Libya over the last decade have reported enslavement, physical abuse, extortion, and murders while they try to cross the Mediterranean.

If it were legal to deliver rescued migrants to Libya, it would be as cheap as sending rescue boats a few extra kilometers south instead of east. But over the last few years, Europe’s maritime military patrols have conducted fewer and fewer sea rescue operations, while adding crewed and uncrewed aerial patrols and investing in remote-sensing technology to create expanded virtual borders to stop migrants before they get near a physical border.

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