Hydrogen Sensors Based on TFTs with Catalytic Source/Drain Electrodes: IGZO Versus Pentacene

A novel H₂ sensor based on an InGaZnO thin-film transistor (IGZO TFT) with Pd source/drain electrodes as sensing medium is fabricated. Without the need of heating, rapid and concentration-dependent response (in the form of drain-current increase) of the sensor is observed under various cycles of H₂ exposure/evacuation with H₂ concentration ranging from 200 to 15 000 ppm. The drain-current increase is due to a decrease in barrier height at the interface between the Pd source and IGZO channel induced by the formation of a hydrogen dipole layer at the interface. By comparing this sensor with its counterpart based on pentacene OTFT having an opposite response, it can be claimed that the same hydrogen dipole layer has opposite effect on the two TFTs: it creates an electric field that can assist the flow of electrons from the source electrode to the IGZO channel, but oppose the flow of holes from the source electrode to the pentacene channel. As a result, the IGZO TFT can work in an OFF-to-ON mode during sensing, and so the standby power of sensing circuit could be reduced. Although the OTFT sensor shows higher sensitivity and faster response due to the higher flexibility of the pentacene layer, the proposed sensor has a better stability at high temperature due to the larger bandgap of IGZO and so is suitable for hydrogen-sensing applications in a wider range of operating temperature.