J. YU, X. XI AND S. WANG
In [4], the physical memristor device is a semiconductor thin film sandwiched between two metal
contacts. Let D, R
on
,andR
of f
denote the full length, low, and high resistances of this film,
respectively. The mechanism model of this device is formulated as
v.t/ D
R
on
w.t/
D
C R
of f
1
w.t/
D
i.t/ (3a)
dw.t/
dt
D
v
R
on
D
i.t/ (3b)
where
v
is the average ion mobility in a uniform field, w.t/ denotes the length of the high dopant
concentration region of the film, and v.t/ and i.t/ denote the voltage and current of this device,
respectively.
In view of the physical range limitation of w.t/, a class of memristor models are proposed by
multiplying window functions on the right side of (3b), which can be characterized by
v.t/ D .R
on
x C R
of f
.1 x//i.t / (4a)
dx
dt
D ˛i.t/f .x/ (4b)
where the constant coefficient ˛ D
v
R
on
=D
2
, the state variable x D w.t/=D,andf./ is a window
function of x. Many window functions have been used to model the memristor [4,13–17]. However, the
nonlinear characteristic of memristor cannot be fully described by these models. Actually, the behavior
of physical memristor devices such as in [18] is highly nonlinear and not consistent with these models.
Reviewing the mechanism of the memristor device, M. Pickett et al. propose a more accurate model,
called Simmons tunnel barrier (STB) model [19]. In this model, the state variable x is the STB width
between the metal electrode and the conducting channel generated by electroforming the TiO
2
device.
The derivative of state variable Px.t/ has an exponential dependence on the current i.t/, and it is too
complex to obtain the explicit solution of state variable x.t/. The state variable x.t/ depends on both
charge q.t/ and current i.t/, and so is the memristance M: Therefore, the model is a memristive
system.
To reduce the complexity of memristive model, a simplified model, called ThrEshold Adaptive
Memristor (TEAM) model, is presented in [20]. The derivative Px.t/ of the TEAM model depends on
both the state variable x.t/ and the current i.t/, and the state variable x.t/ is uncoupled from the
current i.t/. In the model, the functions of state variable x.t/ perform as the window functions to con-
strain the state variable x.t/ to the certain bounds. Compared with the STB model, the TEAM model
can significantly improve the simulation runtime with sufficient precision. However, it is still difficult
to obtain the explicit solution of the state variable x.t/ or the memristance M: Thus, the numerical
methods have to be applied to solve the differential equations. There may exist some serious simulation
errors [16, 21], and the computational efficiency is limited. Other memristive models such as [22–24]
also suffer from the same problem.
In this paper, a flexible memristive model is proposed to solve the problem. A piecewise window
function is applied to characterize the physical range limitation of the state variable x.t/, and a piece-
wise linear approximation model is applied to characterize the relationship between memristance M
and charge q.t/ and current i.t/. The solution of this memristive model can be almost analytically
obtained. The memristive model also shows great flexibility.
2. MEMRISTIVE MODEL WITH SIMPLEX BASIS FUNCTION
Under the assumption of the separability of the current i.t/ and the state variable x.t/, the derivative
Px.t/ of a general memristive model can be formulated as
dx.t/
dt
D f
1
.x.t // f
2
.i.t// (5)
Copyright © 2016 John Wiley & Sons, Ltd
DOI: 10.1002/jnm
2 of 9
Int. J. Numer. Model. 2017; 30: e2183