Gas sensors are frequently used for detecting toxic gases and vapors for environmental control, industrial monitoring, and household safety. Semiconductor conductivity can be modified by doping or fine-tuned by applying an electric or magnetic field in an ultra-wide range (10-7 S/cm to 102 S/cm). The conduction of semiconductor is significantly raised or reduced upon the exposure to external conditions, such as temperature variation, light, heat, mechanical stress, or chemicals. Thus, semiconductors are excellent materials for sensors and the device structures are critical for sensing performance. The commonly used semiconductors materials include Si, Ge, III-V, and metal oxide semiconductors. Recently carbon-based materials gain signification attention due to their unique electrical, optical and mechanical properties. There are two major semiconductor gas sensors: resistor-based and FET-based sensors. In this review, the semiconductor materials, sensor device structure as well as gas sensing mechanisms will be systematically categorized, described and explored, with the focus on metal oxides, GaN, SiC, 2D-TMD and carbon-based gas sensors. The recent progress in new semiconductor gas sensors will be thoroughly reviewed and summarized, with a hope to show the trend in semiconductor gas sensor technology.

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As down scaling of transistors continues, there is a growing interest in developing steep-slope transistors with reduced subthreshold slope (SS) below the Boltzmann limit. In this work, we successfully fabricated steep-slope MoS2 transistors by incorporating a graphene layer, inserted in the gate stack. For our comprehensive study, we have applied density functional theory (DFT) to simulate and calculate the change of SS effected by different 2D quantum materials, including graphene, germanene and 2D topological insulators, inserted within the gate dielectric. This theoretical study showed that graphene/MoS2 devices had steep SS (27.2 mV/dec), validating our experimental approach (49.2 mV/dec). Furthermore, the simulations demonstrated very steep SS (8.6 mV/dec) in WTe2/MoS2 devices. We conclude that appropriate combination of various 2D quantum materials for the gate-channel stacks, leads to steep SS and is an effective method to extend the scaling of transistors with exceptional performance.

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