A new compact model for bipolar RRAMs based on truncated-cone conductive filaments—a Verilog-A approach. 

G. González-Cordero, J. B. Roldan, F Jiménez-Molinos, J. Suñé, S. Long and M. Liu. 
A new compact model for bipolar RRAMs based on truncated-cone conductive filaments—a Verilog-A approach. 
Semicond. Sci. Technol., 31 (11) 115013 (2016)

http://iopscience.iop.org/article/10.1088/0268-1242/31/11/115013/meta