Coexistence of Digital and Analog Resistive Switching With Low Operation Voltage in Oxygen-Gradient HfOx Memristors

A highly reliable and multifunctional memristor demonstrating both digital and analog characteristics was fabricated utilizing HfO
_x
films with oxygen concentration gradient. Remarkably uniform and low operation voltage (magnitude less than 0.9V) at high speed (down to 20 ns) was realized in the digital switching mode. This performance is achieved by confining the switching region to the weak apex of the cone-shape conductive channel, which stems from the gradient distribution of resistivity and electric field in HfO
_x
. In addition, multi-levels resistance states were defined by the negative reset voltage/compliance current in the analog switching mode. The gradually changing conductance like synapse can be tuned by pulse series with low operation voltage (less than 0.7V). The coexistence of digital and analog modes open up opportunities for the synapse-like in-memory computing.