  We propose a circuit-level model combining the Marcus-Hush-Chidsey electron
current equation and the Yakopcic equation for the state variable for
describing resistive switching memory devices of the structure metal-ionic
conductor-metal. We extend the dynamics of the state variable originally
described by a first-order time derivative by introducing a fractional
derivative with an arbitrary order between zero and one. We show that the
extended model fits with great fidelity the current-voltage characteristic data
obtained on a Si electrochemical metallization memory device with Ag-Cu alloy.
