Al back-gated graphene field-effect transistors for capacitive sensing applications based on quantum capacitance effect

We report a new device design of a graphene field-effect transistor (G-FET) for capacitive sensing application. A channel area exposed to ambient conditions in G-FET is known to be a promising candidate for molecular level sensing applications because graphene can attract certain molecules with its freely hanging sigma bonds. In addition, molecules that adhere to graphene act as impurities that affect the electron transport within graphene. Two of the most common ways to evaluate such a change are measuring the changes in resistance and in quantum capacitance. Previous research studies have been largely focused on using resistive measurement due to restrictions from device design even though capacitive measurement can be cost-effective. To overcome the obstacles, we developed G-FET with high capacitance and a large exposed channel area by incorporating Al back-gate electrodes with naturally oxidized AlOx surface as an insulating layer. The measured capacitance was well-modulated in vacuum by the gate voltage due to the quantum capacitance effect. Also, the capacitance curve was shifted up to the right in the 100 ppm NO2 environment. The capacitance at zero gate bias was increased by 56.6% from the vacuum to the 100 ppm NO2 environment. These results indicate that the proposed device can be used for capacitive sensing applications.