Imec integrates thin-film pinned photodiode into short-wave-infrared imaging sensors

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Imec, the nanoelectronics and digital technologies research hub, has announced the successful integration of a pinned photodiode structure into thin-film image sensors.

www.newelectronics.co.uk/, Aug. 15, 2023 – 

With the addition of a pinned-photogate and a transfer gate, the superior absorption qualities of thin-film imagers - beyond one µm wavelength - can now be exploited, helping to unlock the potential of sensing light beyond the visible in a cost-efficient way.

Detecting wavelengths beyond visible light, for instance infrared light, offers a number of significant advantages. Applications include cameras in autonomous vehicles that will be able to 'see' through smoke or fog and cameras to unlock smartphones via face recognition. Whilst visible light can be detected via silicon-based imagers, other semiconductors are necessary for longer wavelengths, such as short-wave infrared (SWIR).

While the use of III-V materials can overcome this detection limitation, manufacturing these absorbers is expensive, limiting their use. In contrast, sensors using thin-film absorbers (such as quantum dots) have recently emerged as a promising alternative. They have superior absorption characteristics and potential for integration with conventional (CMOS) readout circuits. Nonetheless, such infrared sensors have an inferior noise performance, which leads to poorer image quality.

The pinned photodiode (PPD) structure was introduced in the 1980s for silicon-CMOS image sensors. This structure contained an additional transistor gate and a special photodetector structure, by which the charges could be completely drained before integration began (allowing reset operation without kTC noise nor the effect of the previous frame).

Consequently, because of lower noise and improved power performance, PPDs dominate the consumer market for silicon-based image sensors. Beyond silicon imaging, however, incorporating this structure was not possible up until now because of the difficulty of hybridising two different semiconductor systems.

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