The world’s highest sensitivity infrared sensor.

Mitsubishi Electric Corp developed what it claims is an infrared sensor having the world’s highest sensitivity.

The sensor realizes a sensitivity that is more than 10 times higher than that of quantum-type infrared sensors, which had the highest sensitivity. This time, Mitsubishi Electric used graphene (carbon material) and utilized its high electron mobility.

Graphene channels controlled for detecting infrared light

The newly-developed sensor detects middle-infrared light. A device mounted with graphene is formed in the channel part of an FET (field effect transistor), and a light-amplifying part that produces electrons and positive holes at the time of receiving infrared light is placed under the graphene.

Moreover, a negative electric potential (back gate voltage) is applied to the light-amplifying part from the side opposite to the graphene. At this point, when the light-amplifying part receives infrared light, electrons generated by the back gate voltage gather on the graphene side in response. Those electrons are used to change the channel characteristics realized by the graphene.

The current-voltage characteristics applied to both side of the graphene are changed by the electrons generated by infrared light. In the right graph of Fig 1, the “VDP” current increases due to electrons generated by infrared light. The current (⊿Iph) increases because of the high electron mobility of graphene and the steep angle of the graph, increasing sensitivity. When an index of sensitivity was measured, it turned out that sensitivity had increased by 1,000%.

Mitsubishi Electric is also developing an infrared sensor capable of detecting far-infrared light.
It uses a dielectric substance for the light-amplifying device of the middle-infrared sensor. Polarization caused at the time of receiving far-infrared light is used to change the channel characteristics of graphene.

Existing quantum-type far-infrared sensors reduce thermal noise by using liquid nitrogen for cooling. On the other hand, with the new method, it might become possible to raise temperature to a room temperature, etc by increasing cooling sensitivity, Mitsubishi Electric said.

The prototyped graphene is mounted on a silicon (Si) water by transcription. First, a graphene film is formed on a copper (Cu) plate. Then, a plastic film is mounted on the graphene. The Cu is removed by using a chemical. The graphene is placed on the Si wafer by using the plastic film as a supporting material. After that, the plastic film is removed by using a chemical.

If it becomes possible to form a graphene film by using a semiconductor process in the future, the cost of the newly developed infrared sensor will be drastically reduced.

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