voltage, in contrast to the behavior of conventional ReRAMs and memristors [8], for
which an applied signal is required to cause a change in conductance. In the second
behavior, a persistent enhancement of the conductance can be produced by frequent
repetition of stimuli. As the intervals are decreased to 2 s, the atomic switch showed
a transition to a higher conductance state, which persisted for a long time even after
the input pulse stimuli were stopped (Fig. 2b). This permanent transition to a higher
conductance state resembles the persistent increase in the synaptic weight that results
from increasing the repetition rate of stimuli by action potentials in a biological
nervous system, corresponding to the LTP mechanism.
The STP and LTP synaptic behaviors in the atomic switch can be explained in
terms of the formation of a metal bridge and its stability in a nanogap. In the STP
mode, the precipitated Ag atoms form an incomplete bridge, and approximately one
quantized channel could not be maintained after each input pulse. That is, a large Ag
protrusion formed temporarily in the nanogap, which then slowly shrank by
dissolving back into the Ag 2 S and/or by diffusing away on the Ag 2 S surface,
gradually decreasing the conductance. On the other hand, in the LTP mode, a
complete and robust Ag atomic bridge formed, and a conductance higher than
77.5 μS was maintained after the last input pulse. The key point of this achievement
is the use of small input pulses, where each pulse does not turn on the Ag 2 S-based
gap-type atomic switch.
Another type of synaptic behavior has been obtained using a Ag 2 S-based atomic
switch made of small grain Ag 2 S [6]. Figure 3 shows the change in resistance of the
atomic switch with small Ag 2 S crystal grains. The initial resistance before inputting
the voltage pulse was set to 1 MΩ. Inputting the voltage pulses resulted in the
resistance of the switch remaining constant until the third input signal was applied.
However, applying a fourth input pulse caused the resistance to abruptly decrease
from 1 MΩ to 10 kΩ. This turn-on switching process shows that the atomic switch
stored the information during the first three pulse inputs and turned on at the fourth
input pulse (Fig. 3a). The same switching operation was observed for the turn-off
process, for which no change in the resistance of the atomic switch was observed
during the first several inputs, but the resistance increased significantly to 1 MΩ after
a certain number of signal inputs (Fig. 3b). In this experiment, a limited amount of
Ag atoms can precipitate from a Ag 2 S crystal; therefore, the small Ag 2 S grain size
must gather Ag cations from neighboring grains for precipitation to occur, as
illustrated in Fig. 3c. The energy potential barrier at the grain boundary of a Ag 2 S
crystal limits the diffusion of Ag cations crossing the grain boundary, although the
Ag cations can move freely inside the Ag 2 S grains. Thus, Ag precipitation from a
small grain requires that the grain be prepared (by increasing the number of Ag
cations) using a finite number of input pulses. This behavior is analogous to the
experience required to activate a synaptic connection in the human brain, which
contrasts with devices exhibiting multilevel and memristive operations that change
their output with every input signal.
The atomic switch not only mimics the control of synaptic weight in biology, but
may also be useful for implementing a model of memory in the human brain. In
psychology, human memory is believed to be created by the dynamic change of
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T. Tsuruoka et al.
which an applied signal is required to cause a change in conductance. In the second
behavior, a persistent enhancement of the conductance can be produced by frequent
repetition of stimuli. As the intervals are decreased to 2 s, the atomic switch showed
a transition to a higher conductance state, which persisted for a long time even after
the input pulse stimuli were stopped (Fig. 2b). This permanent transition to a higher
conductance state resembles the persistent increase in the synaptic weight that results
from increasing the repetition rate of stimuli by action potentials in a biological
nervous system, corresponding to the LTP mechanism.
The STP and LTP synaptic behaviors in the atomic switch can be explained in
terms of the formation of a metal bridge and its stability in a nanogap. In the STP
mode, the precipitated Ag atoms form an incomplete bridge, and approximately one
quantized channel could not be maintained after each input pulse. That is, a large Ag
protrusion formed temporarily in the nanogap, which then slowly shrank by
dissolving back into the Ag 2 S and/or by diffusing away on the Ag 2 S surface,
gradually decreasing the conductance. On the other hand, in the LTP mode, a
complete and robust Ag atomic bridge formed, and a conductance higher than
77.5 μS was maintained after the last input pulse. The key point of this achievement
is the use of small input pulses, where each pulse does not turn on the Ag 2 S-based
gap-type atomic switch.
Another type of synaptic behavior has been obtained using a Ag 2 S-based atomic
switch made of small grain Ag 2 S [6]. Figure 3 shows the change in resistance of the
atomic switch with small Ag 2 S crystal grains. The initial resistance before inputting
the voltage pulse was set to 1 MΩ. Inputting the voltage pulses resulted in the
resistance of the switch remaining constant until the third input signal was applied.
However, applying a fourth input pulse caused the resistance to abruptly decrease
from 1 MΩ to 10 kΩ. This turn-on switching process shows that the atomic switch
stored the information during the first three pulse inputs and turned on at the fourth
input pulse (Fig. 3a). The same switching operation was observed for the turn-off
process, for which no change in the resistance of the atomic switch was observed
during the first several inputs, but the resistance increased significantly to 1 MΩ after
a certain number of signal inputs (Fig. 3b). In this experiment, a limited amount of
Ag atoms can precipitate from a Ag 2 S crystal; therefore, the small Ag 2 S grain size
must gather Ag cations from neighboring grains for precipitation to occur, as
illustrated in Fig. 3c. The energy potential barrier at the grain boundary of a Ag 2 S
crystal limits the diffusion of Ag cations crossing the grain boundary, although the
Ag cations can move freely inside the Ag 2 S grains. Thus, Ag precipitation from a
small grain requires that the grain be prepared (by increasing the number of Ag
cations) using a finite number of input pulses. This behavior is analogous to the
experience required to activate a synaptic connection in the human brain, which
contrasts with devices exhibiting multilevel and memristive operations that change
their output with every input signal.
The atomic switch not only mimics the control of synaptic weight in biology, but
may also be useful for implementing a model of memory in the human brain. In
psychology, human memory is believed to be created by the dynamic change of
178
T. Tsuruoka et al.