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.
Keywords: NO2; chemical sensor; field-effect transistors; passivation; transition-metal dichalcogenides.