Appendix 1: Memristive Devices: Materials and Complex Physics
83
Resistive Switching Devices and Memristor
In this section, the historical links between the research on memristor and that on
resistive switching devices are briefly discussed.
The first instances of resistive switching were identified by Hickmott in
1962 [17], where he used several different materials for the insulator layer, as
SiO, Al 2 O 3 , Ta 2 O 5 , ZrO 2 , and TiO 2 , showing that all of them exhibit reversible
resistive switching. It is argued in [17] that although the electric fields present across
the thin-films of oxide materials are large enough to induce dielectric breakdown
(the process of sudden irreversible increase in conductance of an insulator under a
large electric field), the observed negative resistance (decreasing conductance with
constant or increasing voltage) shows that this is not actually the case. The interest
in resistive switching increased following Hickmott’s observations [52, 53], and led
to the practical idea to exploit these effects as nonvolatile memory devices [19].
However, research into resistive switching devices drastically and quickly declined
due mainly to the difficulties in understanding the underlying physical mechanisms.
Almost in the same period, and apparently in an unrelated fashion, Chua
published his seminal 1971 paper [2] on the theoretic foundation of the memristor.
Quite curiously, the paper [2] does not mention the results of Hickmott and others
on resistive switching, although the qualitative similarity can be clearly observed.
The quoted paper [2] postulates the existence and the necessity to introduce a fourth
fundamental circuit element, in addition to resistor, capacitor, and inductor, acting
as a nonlinear resistor capable of memorizing its resistive state. The memristor
exhibits the distinctive property of tuning its resistive state according to the history
of the applied input (voltage or current). In addition, when the input is off, the final
resistive state is (ideally) kept forever.
In 1971 there was no known solid-state electronic device displaying a memristive
behavior. Then, L. O. Chua proposed an emulator of a passive memristor built
using active circuits, namely, a two-port network named mutator connected to a
nonlinear resistor (Example 2.19). One main practical problem of such mutatorbased emulator is that the implementation of the mutator needs a large number of
active elements as transistors and operational amplifiers.
However, the possibility to come across physical devices embedding memristor
features is reported in [2] by the following prophetic statement: “Although no
physical memristor has yet been discovered in the form of a physical device without
internal power supply, . . . . a monotonically increasing ϕ–q curve could be invented,
if not discovered accidentally. It is perhaps not unreasonable to suppose that such
a device might already have been fabricated as a laboratory curiosity but was
improperly identified! After all, a memristor with a simple ϕ–q curve will give rise
to a rather peculiar—if not complicated hysteretic—v-i curve when erroneously
traced in the current-versus-voltage plane. Perhaps, our perennial habit of tracing
the v–i curve of any new two-terminal device has already misled some of our deviceoriented colleagues and prevented them from discovering the true essence of some
new device, which could very well be the missing memristor.”
83
Resistive Switching Devices and Memristor
In this section, the historical links between the research on memristor and that on
resistive switching devices are briefly discussed.
The first instances of resistive switching were identified by Hickmott in
1962 [17], where he used several different materials for the insulator layer, as
SiO, Al 2 O 3 , Ta 2 O 5 , ZrO 2 , and TiO 2 , showing that all of them exhibit reversible
resistive switching. It is argued in [17] that although the electric fields present across
the thin-films of oxide materials are large enough to induce dielectric breakdown
(the process of sudden irreversible increase in conductance of an insulator under a
large electric field), the observed negative resistance (decreasing conductance with
constant or increasing voltage) shows that this is not actually the case. The interest
in resistive switching increased following Hickmott’s observations [52, 53], and led
to the practical idea to exploit these effects as nonvolatile memory devices [19].
However, research into resistive switching devices drastically and quickly declined
due mainly to the difficulties in understanding the underlying physical mechanisms.
Almost in the same period, and apparently in an unrelated fashion, Chua
published his seminal 1971 paper [2] on the theoretic foundation of the memristor.
Quite curiously, the paper [2] does not mention the results of Hickmott and others
on resistive switching, although the qualitative similarity can be clearly observed.
The quoted paper [2] postulates the existence and the necessity to introduce a fourth
fundamental circuit element, in addition to resistor, capacitor, and inductor, acting
as a nonlinear resistor capable of memorizing its resistive state. The memristor
exhibits the distinctive property of tuning its resistive state according to the history
of the applied input (voltage or current). In addition, when the input is off, the final
resistive state is (ideally) kept forever.
In 1971 there was no known solid-state electronic device displaying a memristive
behavior. Then, L. O. Chua proposed an emulator of a passive memristor built
using active circuits, namely, a two-port network named mutator connected to a
nonlinear resistor (Example 2.19). One main practical problem of such mutatorbased emulator is that the implementation of the mutator needs a large number of
active elements as transistors and operational amplifiers.
However, the possibility to come across physical devices embedding memristor
features is reported in [2] by the following prophetic statement: “Although no
physical memristor has yet been discovered in the form of a physical device without
internal power supply, . . . . a monotonically increasing ϕ–q curve could be invented,
if not discovered accidentally. It is perhaps not unreasonable to suppose that such
a device might already have been fabricated as a laboratory curiosity but was
improperly identified! After all, a memristor with a simple ϕ–q curve will give rise
to a rather peculiar—if not complicated hysteretic—v-i curve when erroneously
traced in the current-versus-voltage plane. Perhaps, our perennial habit of tracing
the v–i curve of any new two-terminal device has already misled some of our deviceoriented colleagues and prevented them from discovering the true essence of some
new device, which could very well be the missing memristor.”