Current Localization and Redistribution as the Basis of Discontinuous Current Controlled Negative Differential Resistance in NbO_x

Devices exploiting negative differential resistance (NDR) are of particular interest for analog computing applications, including oscillator-based neural networks. These devices typically exploit the continuous S-shaped current–voltage characteristic produced by materials with a strong temperature-dependent electrical conductivity, but recent studies have also highlighted the existence of a second, discontinuous (snap-back) characteristic that has the potential to provide additional functionality. The development of devices based on this characteristic is currently limited by uncertainty over the underlying physical mechanism, which remains the subject of active debate. In situ thermoreflectance imaging and a simple model are used to finally resolve this issue. Specifically, it is shown that the snap-back response is a direct consequence of current localization and redistribution within the oxide film, and that material and device dependencies are consistent with model predictions. These results conclusively demonstrate that the snap-back characteristic is a generic response of materials with a strong temperature-dependent conductivity and therefore has the same physical origin as the S-type characteristic. This is a significant outcome that resolves a long-standing controversy and provides a solid foundation for engineering functional devices with specific NDR characteristics.