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Table 4 The state variable equations presented in an improved form

From: A Collective Study on Modeling and Simulation of Resistive Random Access Memory

Model Original state variable dynamics (f2) Modified using concepts from Wang and Roychowdhury100
Linear ion drift [3] f2 = μ v  × R on  × f1(vpn, s) DC hysteresis not present.
Clipping technique is used to set bounds for s, so that 0 ≤ s ≤ 1
Non-linear ion drift [46, 68] f2 = a × vpnm DC hysteresis not present.
Clipping technique is used to set bounds for s, so that 0 ≤ s ≤ 1
Simmons tunneling barrier [70,71,72] \( {f}_2=\left\{\begin{array}{c}{c}_{\mathrm{off}}\times \sinh \left(\frac{i}{i_{\mathrm{off}}}\right)\times \exp \left(-\exp \left(\frac{s-{a}_{\mathrm{off}}}{w_c}-\frac{i}{b}\right)-\frac{s}{w_c}\right),\mathrm{if}\ i\ge 0\\ {}{c}_{\mathrm{on}}\times \sinh \left(\frac{i}{i_{\mathrm{on}}}\right)\times \exp \left(-\exp \left(\frac{a_{\mathrm{on}}-s}{w_c}+\frac{i}{b}\right)-\frac{s}{w_c}\right),\mathrm{otherwise}\end{array}\right. \)
where i = f 1 (vpn, s)
No DC hysteresis present. Consists of fast growing functions.
sinh is changed to safesinh(), exp to safeexp().
Smoothing is performed and bounds for s, so that 0 ≤ s ≤ 1
TEAM/VTEAM [75,76,77] \( {f}_2=\left\{\begin{array}{c}{k}_{\mathrm{off}}\times {\left(\frac{vpn}{v_{\mathrm{off}}}-1\right)}^{a_{\mathrm{off}}},\mathrm{if}\ vpn>{v}_{\mathrm{off}}\\ {}{k}_{\mathrm{on}}\times {\left(\frac{vpn}{v_{on}}-1\right)}^{a_{\mathrm{on}}},\mathrm{if}\ vpn<{v}_{\mathrm{on}}\\ {}0,\kern10.25em \mathrm{otherwise}\end{array}\right. \) The equation is redesigned as:
\( {f}_2=\left\{\begin{array}{c}{k}_{\mathrm{off}}\times {\left(\frac{vpn-{v}^{\ast }}{v_{\mathrm{off}}}\right)}^{a_{\mathrm{off}}}, if\ vpn>{v}_{\mathrm{off}}\\ {}{k}_{\mathrm{on}}\times {\left(\frac{vpn-{v}^{\ast }}{v_{\mathrm{on}}}\right)}^{a_{\mathrm{on}}},\kern2.75em \mathrm{otherwise}\end{array}\right. \)
where
v = (1 − s) × voff + s × von,
Such that when s = 1 and s = 0, it is equivalent to the VTEAM equation in the vpn > voff and vpn < von regions, respectively.
The functions are also smoothened by:
\( {f}_{2p}={k}_{\mathrm{off}}.{\left( vpn-{v}^{\ast }/{v}_{\mathrm{off}}\right)}^{\alpha_{\mathrm{off}}} \),
\( {f}_{2n}={k}_{\mathrm{on}}.{\left( vpn-{v}^{\ast }/{v}_{\mathrm{on}}\right)}^{\alpha_{\mathrm{on}}} \),
f2 = smoothswitch(f2n, f2p, vpn − v, smoothing)
The bounds for s are set using clipping techniques.
Yakpocic [73, 74] f2 = g(vpn) × f(s),
where
\( g(vpn)=\left\{\begin{array}{c}{A}_p\times \left(\exp (vpn)-\exp \left({V}_p\right)\right),\kern0.75em if\ vpn>{V}_p\\ {}-{A}_n\times \left(\exp \left(- vpn\right)-\exp \left({V}_n\right)\right),\kern0.75em if\ vpn<-{V}_n\\ {}0,\kern13.00em \mathrm{otherwise},\end{array}\right. \)
and
\( f(s)=\left\{\begin{array}{c}\exp \left(-{\alpha}_p\times \left(s-{x}_p\right)\right),\kern0.75em if\ s\ge {x}_p\\ {}\exp \left({\alpha}_n\times \left(s-1+{x}_n\right)\right),\kern0.75em if\ s\le 1-{x}_n\\ {}1,\kern2.5em \mathrm{otherwise}\ \end{array}\right. \)
The equations are designed to get proper DC hysteresis:
\( g(vpn)=\left\{\begin{array}{c}{A}_p\times \left(\exp (vpn)-\exp \left({v}^{\ast}\right)\right),\kern0.75em if\ vpn>{v}^{\ast}\\ {}-{A}_n\times \left(\exp \left(- vpn\right)-\exp \left({V}_n\right)\right),\kern0.75em otherwise\end{array}\right. \)
where
v =  − V n  × s + V p  × (1 − s)
Also exponential function is changed to safeexp(). The whole function is made smooth. Clipping is used to set bounds for s.
ASU/Stanford [78,79,80,81] \( {f}_2=-{v}_0\times \exp \left(-\frac{q\times {E}_a}{k\times T}\right)\times \sinh \left(\frac{vpn\times \gamma \times {a}_0\times q}{k\times T\times {t}_{ox}}\right) \)
where
γ = γ0 − β0 × Gap3
The d/dt Gap is converted to d/dt s:
\( {f}_2=\left( maxGap- minGap\right)\times {v}_0\times \exp \left(-\frac{q\times {E}_a}{k\times T}\right)\times \sinh \left(\frac{vpn\times \gamma \times {a}_0\times q}{k\times T\times {t}_{ox}}\right) \)
Also exp is changed to safeexp() and sinh tosafesinh (). Clipping is used to set bounds for s.