Gate-Tunable Negative Differential Conductance in Hybrid Semiconductor–Superconductor Devices

Negative differential conductance (NDC) serves as a crucial characteristic that reveals various underlying physics and transport process in hybrid superconducting devices. We report the observation of gate-tunable NDC outside the superconducting energy gap on two types of hybrid semiconductor–superconductor devices, i.e., normal metal–superconducting nanowire–normal metal and normal metal–superconducting nanowire–superconductor devices. Specifically, we study the dependence of the NDCs on back-gate voltage and magnetic field. When the back-gate voltage decreases, these NDCs weaken and evolve into positive differential conductance dips; and meanwhile they move away from the superconducting gap towards high bias voltage, and disappear eventually. In addition, with the increase of magnetic field, the NDCs/dips follow the evolution of the superconducting gap, and disappear when the gap closes. We interpret these observations and reach a good agreement by combining the Blonder–Tinkham–Klapwijk (BTK) model and the critical supercurrent effect in the nanowire, which we call the BTK-supercurrent model. Our results provide an in-depth understanding of the tunneling transport in hybrid semiconductor–superconductor devices.