The fabrication of individual nanowire-based devices and their comprehensive electrical characterization remains a major challenge. Here, we present a symmetric Hall bar configuration for highly p-type germanium nanowires (GeNWs), fabricated by a top-down approach using electron beam lithography and inductively coupled plasma reactive ion etching. The configuration allows two equivalent measurement sets to check the homogeneity of GeNWs in terms of resistivity and the Hall coefficient. The highest Hall mobility and carrier concentration of GeNWs at 5 K were in the order of 100 ${\mathrm{cm}}^2/\left(\mathrm{Vs}\right)$ and $4\times {10}^{19}{\mathrm{cm}}^{-3}$, respectively. With a decreasing nanowire width, the resistivity increases and the carrier concentration decreases, which is attributed to carrier scattering in the region near the surface. By comparing the measured data with simulations, one can conclude the existence of a depletion region, which decreases the effective cross-section of GeNWs. Moreover, the resistivity of thin GeNWs is strongly influenced by the cross-sectional shape.

Semiconductor nanowire‐based photodetectors with high sensitivity and fast photoresponse in the near‐infrared wavelength range are crucial for applications in light‐wave communication switches, as well as environmental and atmospheric sensing. However, to advance this field, it is essential to develop innovative fabrication techniques that improve device performance. Here, the fabrication of an axial p–n junction along single germanium nanowires (Ge NWs) and their photoresponse characterization at near‐infrared wavelengths are reported. The resulting devices exhibit rectifying current–voltage characteristics with a high rectification ratio in dark conditions and operate with high sensitivity at zero bias under illumination. A high responsivity of 1.72 AW⁻¹, a low noise‐equivalent power of 5.68 × 10⁻¹¹ , and a high‐frequency response with a 3dB cut‐off frequency of 2.85 GHz are determined under 850 nm laser illumination at reverse bias. The high sensitivity of the Ge NW‐based photodetectors is ascribed to the radial built‐in electric field, which increases the carrier lifetime. In addition, the small size of the Ge NWs results in very small capacitance, leading to very fast response. These results have significant potential for advancing high‐speed and low‐power photodetectors in next‐generation optical communication systems and integrated optoelectronic devices.

Si nanowire transistors are ideal for the sensitive detection of atmospheric species due to their enhanced sensitivity to changes in the electrostatic potential at the channel surface. In this study, we present unique ambipolar Si junctionless nanowire transistors (Si-JNTs) that incorporate both n- and p-type conduction within a single device. These transistors enable scalable detection of nitrogen dioxide (NO₂), a critical atmospheric oxidative pollutant, across a broad concentration range, from high levels (25-50 ppm) to low levels (250 ppb-2 ppm). Acting as an electron acceptor, NO₂ generates holes and functions as a pseudodopant for Si-JNTs, altering the conductance and other device parameters. Consequently, ambipolar Si-JNTs exhibit a dual response at room temperature, reacting on both p- and n-conduction channels when exposed to gaseous NO₂, thereby offering a larger parameter space compared to a unipolar device. Key characteristics of the Si-JNTs, including on-current (I_on), threshold voltage (V_th) and mobility (μ), were observed to dynamically change on both the p- and n-channels when exposed to NO₂. The p-conduction channel showed superior performance across all parameters when compared to the device's n-channel. For example, within the NO₂ concentration range of 250 ppb to 2 ppm, the p-channel achieved a responsivity of 37%, significantly surpassing the n-channel's 12.5%. Additionally, the simultaneous evolution of multiple parameters in this dual response space enhances the selectivity of Si-JNTs toward NO₂ and improves their ability to distinguish between different pollutant gases, such as NO₂, ammonia, sulfur dioxide and methane.

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.