# Ammonia Sensing via Pseudo Molecular Doping in UV-Activated Ambipolar Silicon Nanowire Transistors

## TL;DR

This paper introduces a new method for detecting ammonia gas using silicon nanowire transistors that respond to both positive and negative charges.

## Contribution

The study introduces ammonia-induced pseudo molecular doping in ambipolar silicon nanowire transistors for room-temperature gas sensing.

## Key findings

- Ammonia adsorption alters both p- and n-type conduction channels in silicon nanowire transistors.
- The sensor detects ammonia with high sensitivity (200 ppb) and selectivity under UV light.
- Ambipolar transistors enable dual-channel sensing, improving response time and performance.

## Abstract

The potential of adsorbed gaseous molecules to create
shallow electronic
states for thermally excited charge carrier transport and to engineer
silicon transistor properties has been largely overlooked compared
to traditional substitutional impurities. This paper successfully
modifies the electrical properties of ambipolar silicon junctionless
nanowire transistors (Si-JNTs) using the reducing properties of ammonia
(NH3) for selective detection. Physisorption of NH3 induces a dual response in both p- and n-type conduction channels of ambipolar Si-JNTs, significantly
altering current and key parameters, including the “on”
current (I
on), threshold voltage (V
th), and mobility (μ). NH3 interaction
increases conduction in the n-channel and decreases
it in the p-channel, acting as an electron donor
and hole trap, as supported by Density Functional Theory (DFT) calculations.
This provides a pathway for charge transfer and ″pseudo″
molecular doping in ambipolar Si-JNTs. This NH3-mediated
molecular doping and conduction modulation in Si transistor enabled,
for the first time, the electrical detection of gaseous NH3 at room temperature across a wide concentration range (200 ppb to
50 ppm), achieving high sensitivity (200 ppb) and precise selectivity
under ultraviolet (UV) light. UV illumination dynamically modulates
current and reveals distinct sensing features in the p- and n-channels of the dual-responsive Si-JNTs.
The ambipolar Si-JNT sensor exhibits a fast response time of 1.91
min for 0.8 ppm of NH3 in the hole conduction channel and
a high sensitivity of 80% for 0.8 ppm of NH3 in the electron
conduction channel. This dual-channel approach optimizes sensor performance
by leveraging the most responsive parameters from each channel. Furthermore,
the ambipolarity of Si-JNTs broadens the parameter space for developing
a multivariate calibration model, enhancing the selectivity of Si-JNT
sensors for NH3 detection.