Presentation - Ammonia Sensing via Pseudo Molecular Doping in UVActivated Ambipolar Silicon Nanowire Transistors

Abstract: The potential of adsorbed gas molecules to modulate semiconductor behaviour, by forming shallow electronic states that influence charge carrier transport, remains relatively untapped compared to traditional doping with substitutional impurities. In this work, we exploit the reducing properties of ammonia (NH3) to electrically modify ambipolar silicon junctionless nanowire transistors (Si-JNTs) for selective and room-temperature gas detection. Si-JNTs offer several intrinsic advantages as gas sensors: a high surface-to-volume ratio, direct surface interaction with analytes, and compatibility with standard CMOS processes. Unlike traditional field-effect transistors (FETs), JNTs eliminate the need for a physical gate junction, simplifying fabrication and enhancing electrostatic control. Their ambipolar nature, supporting both electron (n-type) and hole (p-type) conduction, enables complementary electrical responses to a single analyte within the same device, opening new possibilities for dual-mode detection. Upon exposure to NH3, the ambipolar Si-JNTs exhibit a distinct and simultaneous response in both conduction channels. NH3 acts as an electron donor and hole trap, increasing n-channel current while decreasing p-channel conduction. This leads to notable shifts in device parameters such as threshold voltage (Vₜₕ), on-current (Iₒₙ), and carrier mobility (μ). Density Functional Theory (DFT) simulations support this observation, showing charge transfer from NH3 to the nanowire surface, resulting in shallow donor-like states. This interaction mimics doping without permanent alteration to the silicon lattice, a mechanism we describe as “pseudo” molecular doping. Our device achieves robust NH3 sensing across a wide concentration range (200 ppb to 50 ppm) at room temperature. Notably, it detects concentrations as low as 200 ppb, with enhanced selectivity under ultraviolet (UV) illumination. UV exposure further activates surface states, dynamically modulating the sensing response and accentuating channel-specific behaviour. For instance, the p-channel shows a rapid response time of 1.91 minutes at 0.8 ppm NH3, while the n-channel exhibits a strong sensitivity of 80% at the same concentration. This dual-channel architecture allows us to optimise sensitivity and selectivity by extracting the most responsive parameter from each channel. In conclusion, this study demonstrates that ambipolar Si-JNTs provide a powerful, compact, and tunable platform for gas sensing. Their intrinsic dual-conduction capability offers built-in redundancy and selectivity, while room-temperature operation ensures energy efficiency. By enabling real-time, low-power detection with complementary channel responses, ambipolar SiJNTs pave the way for miniaturised environmental sensors and wearable air quality monitors. Future work will explore functionalising the nanowire surface with organic or hybrid materials to extend detection to transient atmospheric radicals such as hydroxyl (OH·) and nitrate (NO3·). These enhancements could further enable the real-time monitoring of short-lived reactive species critical to atmospheric chemistry and climate modelling