On MoS₂ Thin-Film Transistor Design Consideration for a NO₂ Gas Sensor

MoS₂ thin-film transistors (TFTs) are fabricated and simulated to explore the NO₂ gas sensing mechanism depending on different device structures. In particular, the role of the Al₂O₃ passivation layer on the MoS₂ channel has been investigated. In the case of nonpassivated MoS₂ TFTs, increase of off-current is observed with NO₂ gas, which has been modeled with the modulation of the effective Schottky barrier height for holes because of the generation of in-gap states near the valence band as NO₂ gases interact with the MoS₂ channel. The nonpassivated MoS₂ TFTs are simulated based on nonequilibrium Green's function method, and the simulation results do confirm this sensing mechanism. On the other hand, MoS₂ TFTs with the Al₂O₃ passivation layer have been modeled with a pseudo-double gate structure as NO₂ gases on the capping layer can act like the secondary gate inducing the positive charge state. Our quantum transport simulation shows that the significant threshold voltage shift can be achieved with NO₂ gas, which matches the experimental observation, thereby exhibiting a completely different sensing mechanism of the passivated device from the nonpassivated counterpart. In addition, we also discuss competing device parameters for the passivated MoS₂ TFTs by varying the main and the secondary gate dielectric, suggesting co-optimization to realize high sensitivity and low power consumption simultaneously.