Silicon nanowires have been employed effectively in innovative electronic devices including sensors, solar cells and logic circuitry [1]. Field-effect-transistors (FET) based on silicon nanowires are extensively used for sensing applications, benefitting from their compact nanoscale structures that allow excellent regulation of electrostatic potential across the nanowire channel and enhanced sensitivity owing to a high surface-to-volume ratio [2]. One such nanowire FET concept is the junctionless nanowire transistor (JNT) [3]. A JNT is a highly doped nanowire channel without p-n junctions, where the gate electrode regulates the flow of charge carriers. Usually, JNTs act as either a p- or an n-type device depending on the type of doping. In this work, we optimize the doping concentration of the silicon JNT devices to perform as ambipolar FETs by controlling through the back gate. The use of both top and back gates enables enhanced polarity control in the JNT devices, leading to unipolar behaviour much like electrostatic doping [4]. Such functionality is similar to Schottky barrier-based FETs known as reconfigurable field effect transistors (RFET) [5]. Furthermore, these properties of dual polarity in JNT are advantageous for a wide range of sensing applications. Previously, silicon JNTs have shown excellent sensitivity to record-low concentrations of the protein streptavidin in the liquid phase [6]. However, they have not yet been operated as gas sensors. Here we also explore the sensing properties of ambipolar JNT in NO 2 atmosphere.