Self-Switching Diode Effect in Electrostatically Gated Silicon Nanowire Geometric Diodes

High frequency rectifying diodes are essential for microwave and terahertz detection systems, with important applications in security, healthcare, and high-speed communication. Geometric diodes are an emerging class of electrical diodes that have unique advantages as high frequency rectifiers, without a dependence on minority carriers and with simpler device fabrication using lower cost materials. They operate on the principle of ratcheting by converting a fluctuating, unbiased force into unidirectional electron motion. Silicon nanowire geometric diodes are morphologically asymmetric silicon nanostructures that function as electron ratchets at room temperature and have been shown to rectify frequencies up to 40 GHz, with a theoretical cut-off frequency above 1 THz. While these devices are considered quasi-ballistic rectifiers, their electrostatic effects, particularly at the nanowire surface, have a significant impact on diode performance and are largely unexplored. Here, we synthesize silicon nanowire geometric diodes using dopant-encoded VLS growth and dopant-selective etching and fabricate wrap-around gated single-nanowire devices to precisely control nanowire surface charge. Electrical characterization of single-nanowire devices demonstrates tunability of diode polarity, current magnitude from 1 nA to 10 µA, and DC asymmetry from 1 to 400 with gate voltage. Through finite-element modeling, we clarify the asymmetric bias-induced barrier lowering mechanism that enables electron ratcheting in these gated nanostructures and is indicative of self-switching diode devices. Notably, we observe this same mechanism and similar current and DC asymmetry results in simulations of ungated silicon nanowire geometric diode devices with sufficiently large trap densities at the nanowire surface. When we consider the surface trap states, the resulting surface charge acts like a wrap-around gate with a fixed voltage. These results demonstrate the creation of a novel silicon nanowire-based self-switching diode and the possibility for simplifying fabrication through surface modification to tune nanowire surface trap density.