The Advanced Technology R&D Center of Mitsubishi Electric Corp is developing an image sensor that can sense a wide frequency band of light from visible light to terahertz waves with one device.

It can be a multi-spectrum image sensor with a lower cost and higher performance, compared with existing multi-spectrum image sensors. Currently, multiple kinds of image sensors are combined in accordance with wavelength to realize a multi-spectrum image sensor, and high-cost materials and liquid nitrogen-based cooling are necessary to detect lights other than visible light.

Mitsubishi Electric exhibited a prototyped chip at MEMS Sensing & Network System 2018, which took place from Oct 17 to 19, 2018, at Makuhari Messe, in Chiba City, Chiba Prefecture, Japan, along with Ceatec Japan 2018. It was developed in a joint research project with the research laboratory of Kazuhiko Matsumoto of the Institute of Scientific and Industrial Research, Osaka University.

The prototyped chip

This time, the Advanced Technology R&D Center used a sheet-like graphene material that is made by arranging carbon atoms on a single layer. The state of electrons of graphene is different from those of other materials and semiconductors. And the new sensor uses this characteristic.

Attention paid to no-bandgap property

In general, electrons exist as valence electrons, which are bound by the perimeters (outermost shells) of atomic nuclei, and free electrons, which can freely move in a substance (among multiple atoms). Between low-energy valence electrons and high-energy free electrons, there is a bandgap where they cannot exist as electrons. When energy corresponding to the gap is given, valence electrons become free electrons.

The optical sensors of existing image sensors turn valence electrons into free electrons by using the energy of irradiated light. The electric current generated by free electrons are electronically read out to detect light. The energy for turning valence electrons into free electrons depends on material and structure. So, when light with a wavelength corresponding to the energy is taken in, light is detected.

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