Room temperature NO2 sensing in bottom gate WO3 thin film transistors: Ag versus Pd source/drain contacts and the inverse correlation between transistor and sensing performance

Abstract Metal oxide Thin Film Transistors (TFTs) are promising candidates for next-generation gas sensors as they combine gas detection with built-in signal amplification. However, it is still not confirmed whether better transistor performance will always lead to better gas sensing performance. In this work, two Bottom Gate Tungsten Trioxide (WO 3 ) TFTs with different source/drain contact metals, silver (Ag) and palladium (Pd), referred as BG TFT-1 and BG TFT-2 respectively are fabricated by RF sputtering, material (SEM, XRD) and electrical characterizations-(I D -V D ), transfer (I D -V G ), capacitance–frequency (C–f), and capacitance–gate voltage (C–V G ) measurements done. TFT parameters- transconductance (g m ), I ON /I OFF ratio, threshold voltage (V TH ), sub-threshold swing (SS), and mobility (µ FE ) were determined to assess charge transport, gate modulation, dielectric behaviour and interface quality. Later, they were systematically investigated for NO 2 sensing for concentrations ranging from 0.1 to 10 ppm at room temperature. The WO 3 thin film exhibited a monoclinic crystal structure with a preferred (002) orientation and an average crystallite size of 9.7 nm. Both TFTs showed stable transistor operation and reversible sensing behaviour toward NO 2 . Although BG TFT-2 exhibited superior electrical performance, including higher field effect mobility (µ FE = 8.6 × 10 −3 cm 2 V −1 s −1 ), higher transconductance and a larger I ON /I OFF ratio, BG TFT-1 consistently produced a stronger sensing response. At 10 ppm NO 2 , BG TFT-1 achieved a maximum response of 99.2% with a response time of 28 s, whereas BG TFT-2 showed a response of 96.0% with a response time of 32 s. The enhanced sensing performance of BG TFT-1 is attributed to its higher threshold voltage (V TH ) and lower initial carrier concentration, which allow NO 2 adsorption to produce a larger change in channel current. These findings demonstrate that electrical performance alone does not determine gas sensing performance. Instead, the properties of the metal–semiconductor interface and the channel surface may play a more important role in determining sensor response. This study highlights WO 3 TFTs with Bottom Gate configuration as promising low-power, room temperature NO 2 sensors that are compatible with future integrated sensing technologies.