3246 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 10, OCTOBER 2011 Universal Reset Characteristics of Unipolar and Bipolar Metal-Oxide RRAM Daniele Ielmini, Senior Member, IEEE, Federico Nardi, Student Member, IEEE, and Carlo Cagli Abstract—Set and reset characteristics are studied for unipolar and bipolar metal-oxide resistive-switching memory devices. We show a universal dependence of set-state resistance and reset current on the compliance current used during set, with negligible impact of metal-oxide composition and switching condition. An analytical Joule-heating model for universal reset is presented, predicting a weak dependence of reset temperature and voltage on diffusion and migration parameters in both unipolar- and bipolar-switching modes. Data for the reset voltage are shown for a wide range of unipolar and bipolar metal oxides, in support of our calculations. Index Terms—Memory modeling, nonvolatile memory, resistive-switching memory (RRAM). I. I NTRODUCTION R ESISTIVE-switching memory (RRAM) devices based on reversible resistance change in metal oxides are consid- ered a potential alternative to existing Flash technology for future nonvolatile memory downscaling [1]. In RRAM, the resistance is decreased in the set transition, consisting of the formation of a conductive filament (CF) by dielectric break- down. The CF is subsequently disrupted in the reset transition, restoring a high-resistance state [2]. Two switching modes are generally observed: In unipolar switching, resistance change takes place irrespective of the pulse polarity [2], [3], while the polarity must be necessarily reversed for set and reset in bipolar switching [1]. Resistance switching studies have led to physical models for reset as a function of CF size, shape, and resistance [2], [4]–[6]. The reset operation is generally interpreted as a temperature-driven dissolution of the CF. However, for a deeper insight into the conduction and switching mechanisms on the nanometer scale, a comparative study of set/reset transitions for different active metal oxides is needed. This work addresses set and reset characteristics for unipolar and bipolar RRAM with different metal oxides, providing evidence for a universal dependence of set-state resistance and reset current on current compliance I C , which is the current during the set operation. Universal reset is explained by diffu- Manuscript received March 28, 2011; revised May 18, 2011 and June 11, 2011; accepted June 21, 2011. Date of publication July 29, 2011; date of current version September 21, 2011. This work was supported in part by Intel under Project 55887 and in part by Fondazione Cariplo under Grant 2010-0500. The review of this paper was arranged by Editor R. Huang. The authors are with the Dipartimento di Elettronica e Informazione and the Italian Universities Nanoelectronics Team (IU.NET), Politecnico di Milano, 20133 Milano, Italy (e-mail: ielmini@elet.polimi.it; nardi@elet.polimi.it; cagli@elet.polimi.it). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2011.2161088 Fig. 1. Measured (a) set-state resistance R and (b) Ireset as a function of I C . Data are shown for several unipolar and bipolar RRAM devices with different active materials, including NiO [3], [7]–[10], Cu 2 O [11], HfOx [12], and ZrOx/HfOx [13]. The inset shows the schematic I –V curves for (solid) unipolar set and (dashed) reset transitions, defining parameters I C , Vreset , Ireset , and R. sion/migration processes responsible for CF rupture. It is shown that, due to the exponential dependence of CF dissolution rate on the local temperature, the reset voltage is generally in the range between 0.2 and 1 V, thus resulting in an almost universal relationship between compliance and reset current. Our analysis provides a strong physical basis for the modeling of conduction and switching in filamentary RRAM devices. II. SET/RESET CHARACTERISTICS Fig. 1(a) and (b) shows the measured set-state resistance R and reset current I reset , respectively, as a function of the current compliance I C during set for various unipolar and bipolar RRAM devices [3], [7]–[13]. The inset shows the schematic I –V curves for unipolar set and reset transitions: The device is set under a compliance current I C , resulting in a relatively low resistance R in the set state. In the following reset operation, the voltage is swept with no current limitation, resulting in a reset transition from low to high resistance at a reset voltage V reset and a reset current I reset . In case the reset transition is not as abrupt as the one schematically shown in the inset, V reset was taken as the voltage at which the resistance started to increase along the I –V curve. The figure includes data for different metal-oxide materials, such as unipolar-switching NiO [3], [7]–[10], unipolar-switching Cu 2 O[11], bipolar-switching 0018-9383/$26.00 © 2011 IEEE