Fig. 1a. The similar operation is available in a solid electrolyte. Such an operation
was first demonstrated using Cu 2 S as a solid electrolyte, in which migration of Cu
2+
cations is controlled by a gate bias [4]. Namely, application of a positive gate bias
moves Cu
2+ cations towards a channel region, where Cu
2+ cations are reduced and
deposited on a source and a drain, as shown in Fig. 1b. Thickening of the source and
the drain results in their electrical connection by a Cu filament bridge between them,
similar to the phenomena controlled in electrolytes (liquid). Application of a negative gate bias re-oxidizes Cu atoms deposited on a source and a drain. Dissolving of
re-oxidized Cu atoms, i.e., Cu
2+ , into a solid electrolyte (Cu 2 S) annihilates a Cu
filament, resulting in disconnection of the source and the drain. In this type of the
three-terminal atomic switch, initialization that forms a Cu filament by supplying
Cu
2+ cations from a drain electrode made of Cu, using the operating mechanism of a
gapless-type atomic switch, is very effective for reducing the filament size (length) in
the switching controlled by the gate electrode. The initialization ensures the electrical isolation of a gate electrode from a source and a drain, resulting in the high
reliability as well as the short switching time in the operation.
After the first development, the device structure that does not require the initialization was also developed by using Cu for the gate electrode and Pt for both the
drain and source electrodes [5]. In the three-terminal atomic switches, a gate
electrode should be far enough in order to avoid a Cu filament formation between
a gate electrode and source/drain electrodes, i.e., short-circuit. At the same time, a
gate electrode has a function of applying an electric field in a layer of a solid
electrolyte to move Cu
2+ cations. Because of this reason, Cu 2 S that has both
electronic conductivity and ionic conductivity was employed in the development.
An on/off ratio of higher than 10
4 and a cyclic endurance of 10
2 times have been
achieved using the new structure. The device structure and the operating results are
shown in Fig. 2.
Although the use of Cu 2 S resulted in the gate current in the order of μA, it is
acceptable when used as reconfigurable switches in programmable devices because
Fig. 1 Schematics of three-terminal switches where formation/annihilation of a metal filament is
achieved by a gate bias controlled electrochemical deposition/dissolution in (a) an electrolyte [3]
and (b) a solid electrolyte [4]
Development of Three-Terminal Atomic Switches and Related Topics
129
was first demonstrated using Cu 2 S as a solid electrolyte, in which migration of Cu
2+
cations is controlled by a gate bias [4]. Namely, application of a positive gate bias
moves Cu
2+ cations towards a channel region, where Cu
2+ cations are reduced and
deposited on a source and a drain, as shown in Fig. 1b. Thickening of the source and
the drain results in their electrical connection by a Cu filament bridge between them,
similar to the phenomena controlled in electrolytes (liquid). Application of a negative gate bias re-oxidizes Cu atoms deposited on a source and a drain. Dissolving of
re-oxidized Cu atoms, i.e., Cu
2+ , into a solid electrolyte (Cu 2 S) annihilates a Cu
filament, resulting in disconnection of the source and the drain. In this type of the
three-terminal atomic switch, initialization that forms a Cu filament by supplying
Cu
2+ cations from a drain electrode made of Cu, using the operating mechanism of a
gapless-type atomic switch, is very effective for reducing the filament size (length) in
the switching controlled by the gate electrode. The initialization ensures the electrical isolation of a gate electrode from a source and a drain, resulting in the high
reliability as well as the short switching time in the operation.
After the first development, the device structure that does not require the initialization was also developed by using Cu for the gate electrode and Pt for both the
drain and source electrodes [5]. In the three-terminal atomic switches, a gate
electrode should be far enough in order to avoid a Cu filament formation between
a gate electrode and source/drain electrodes, i.e., short-circuit. At the same time, a
gate electrode has a function of applying an electric field in a layer of a solid
electrolyte to move Cu
2+ cations. Because of this reason, Cu 2 S that has both
electronic conductivity and ionic conductivity was employed in the development.
An on/off ratio of higher than 10
4 and a cyclic endurance of 10
2 times have been
achieved using the new structure. The device structure and the operating results are
shown in Fig. 2.
Although the use of Cu 2 S resulted in the gate current in the order of μA, it is
acceptable when used as reconfigurable switches in programmable devices because
Fig. 1 Schematics of three-terminal switches where formation/annihilation of a metal filament is
achieved by a gate bias controlled electrochemical deposition/dissolution in (a) an electrolyte [3]
and (b) a solid electrolyte [4]
Development of Three-Terminal Atomic Switches and Related Topics
129