circuits to reduce the growing power consumption. Although there are various
nonvolatile devices such as used in memories, most of them are two-terminal devices
that cannot replace CMOS.
Nonvolatile three-terminal devices that controls ionic movement in solid electrolytes were developed and commercialized in 1970s [1]. Since the size of the devices
was in the order of 10 mm, the early-developed nonvolatile three-terminal devices
did not have chance to be used in information processing systems. In 1990s,
‘memistor’, the three-terminal device aiming to be used in neural computing systems, was developed [2]. The memistor moves H
+ ions to a channel region of WO 3 ,
which becomes conductive by being doped with H
+ ions. Namely, a source and a
drain are electrically connected. After stopping bias application, H
+ ions gradually
leave from the channel region typically in several minutes. The gradual decrease in
conductivity was expected to emulate a neural function. Therefore, the memistor is a
volatile device although it has a certain decaying time.
The nonvolatile three-terminal operation truly in a nanoscale was demonstrated in
2004 by electrochemical deposition of metal atoms on two Au electrodes dipped in
an electrolyte [3]. The electrochemical deposition of metal (Ag) atoms onto the two
Au electrodes thickened the two electrodes, making a gap between the two electrodes smaller and finally bridging them. Bias application in an opposite polarity
dissolved the deposited metal atoms into an electrolyte, resulting in a disconnection
of the two electrodes. Precise control of a gate bias achieved switching between
quantized conductance states.
2 Metal Cation-Controlled Three-Terminal Atomic
Switches
Metal cation-controlled type is developed based on the operating mechanism of the
two-terminal atomic switches. It is classified into two types by a rate limiting process
in the metal filament formation in their switching on process. In the first type, drift of
metal cations is a rate limiting process, where a conductive filament gradually grows
by incorporating metal atoms drifting from a gate electrode. In the second type,
nucleation of metal atoms is a rate limiting process, where nucleus is formed when a
concentration of metal cations at a channel region reaches a certain value. Depending
on applications, one can choose either of the two types.
2.1 Filament Growth Controlled Type
As mentioned in the former section, deposition and dissolution of metal atoms
between two electrodes in an electrolyte (liquid), such as AgNO 3 + HNO 3 [3] and
CuSO 4 [4], can be used for nonvolatile three-terminal operations, as shown in
128
T. Hasegawa et al.
nonvolatile devices such as used in memories, most of them are two-terminal devices
that cannot replace CMOS.
Nonvolatile three-terminal devices that controls ionic movement in solid electrolytes were developed and commercialized in 1970s [1]. Since the size of the devices
was in the order of 10 mm, the early-developed nonvolatile three-terminal devices
did not have chance to be used in information processing systems. In 1990s,
‘memistor’, the three-terminal device aiming to be used in neural computing systems, was developed [2]. The memistor moves H
+ ions to a channel region of WO 3 ,
which becomes conductive by being doped with H
+ ions. Namely, a source and a
drain are electrically connected. After stopping bias application, H
+ ions gradually
leave from the channel region typically in several minutes. The gradual decrease in
conductivity was expected to emulate a neural function. Therefore, the memistor is a
volatile device although it has a certain decaying time.
The nonvolatile three-terminal operation truly in a nanoscale was demonstrated in
2004 by electrochemical deposition of metal atoms on two Au electrodes dipped in
an electrolyte [3]. The electrochemical deposition of metal (Ag) atoms onto the two
Au electrodes thickened the two electrodes, making a gap between the two electrodes smaller and finally bridging them. Bias application in an opposite polarity
dissolved the deposited metal atoms into an electrolyte, resulting in a disconnection
of the two electrodes. Precise control of a gate bias achieved switching between
quantized conductance states.
2 Metal Cation-Controlled Three-Terminal Atomic
Switches
Metal cation-controlled type is developed based on the operating mechanism of the
two-terminal atomic switches. It is classified into two types by a rate limiting process
in the metal filament formation in their switching on process. In the first type, drift of
metal cations is a rate limiting process, where a conductive filament gradually grows
by incorporating metal atoms drifting from a gate electrode. In the second type,
nucleation of metal atoms is a rate limiting process, where nucleus is formed when a
concentration of metal cations at a channel region reaches a certain value. Depending
on applications, one can choose either of the two types.
2.1 Filament Growth Controlled Type
As mentioned in the former section, deposition and dissolution of metal atoms
between two electrodes in an electrolyte (liquid), such as AgNO 3 + HNO 3 [3] and
CuSO 4 [4], can be used for nonvolatile three-terminal operations, as shown in
128
T. Hasegawa et al.