source-drain current (I SD ) changes over the range of about five orders of magnitude
in the repeated on/off switching by gate bias sweeping. On the other hand, a gate
current (I G ) remains in the order of 0.1 pA during the whole switching processes.
Although variations in the switching-on bias (220–300 mV) and switching off bias
(À30 to À100 mV) are observed, nonvolatile three-terminal operations are clearly
demonstrated with the high on/off ratio. From the gate leakage current (I G ), the
power consumption is estimated to be very small less than fW. The reduction of a
gap between a source and a drain also improved repeatability of the atomic switch.
Another important factor in terms of energy consumption is a sub-threshold
slope. Namely, steep sub-threshold slope enables faster switching with less energy
consumption. The sub-threshold slope of the atom transistor is of about 10 mV/
decade (Fig. 5), which is much steeper than 60 mV/decade that is the value of ideal
MOSFET. The steeper sub-threshold slope of atom transistor can also drastically
reduce power consumption in logic circuits.
3 Oxygen Ion Controlled Type
Three-terminal operation has also been demonstrated by controlling oxygen ion’s
drift to make a conductive channel between a source and a drain. The initial target of
this oxygen ion controlled type was to develop a nonvolatile three-terminal atomic
switch that turns on with a negative gate bias application. Since the atom transistor
mentioned in the former section turns on with a positive gate bias application, the
development of the three-terminal atomic switch that turns on with a negative gate
bias would have enabled a nonvolatile complementally operating system that can
fully replace CMOS systems.
For that purpose, we decided to use the phenomena that some metal oxides
become conductive with increase in the concentration of oxygen ions from the
stoichiometric condition. Since a negative gate bias application brings oxygen ions
towards a channel region, the phenomena should turn on an oxygen ion controlled
three-terminal switch.
In order to confirm the mechanism, we fabricated a sidewall gate type device, as
shown in Fig. 6. We employed TaOx as a channel material instead Ta 2 O 5 in order to
easily move oxygen ions. Another difference from the atom transistor is that a gate
electrode consists of Pt so that ions that can move in a metal oxide layer is only
oxygen ions. The operating result is shown in Fig. 7. As can be seen in Fig. 7a,
resistance between a source and a drain decreased to the order of 10 kΩ from the
order of 10 GΩ, while resistance between a gate and source/drain was kept at around
the order of GΩ, suggesting a three-terminal switching due to the increase in oxygen
ions occurred. Figure 7b shows I/V characteristics between a source and a drain
measured in the off-state and the on-state. The I/V characteristics in the on state
shows nonlinearity, suggesting that a conductive path is semiconductive. As such we
succeeded in turning on by increasing oxygen ions at a channel region. However, the
repeatability was not good because the increasing oxygen ions more than the
134
T. Hasegawa et al.
in the repeated on/off switching by gate bias sweeping. On the other hand, a gate
current (I G ) remains in the order of 0.1 pA during the whole switching processes.
Although variations in the switching-on bias (220–300 mV) and switching off bias
(À30 to À100 mV) are observed, nonvolatile three-terminal operations are clearly
demonstrated with the high on/off ratio. From the gate leakage current (I G ), the
power consumption is estimated to be very small less than fW. The reduction of a
gap between a source and a drain also improved repeatability of the atomic switch.
Another important factor in terms of energy consumption is a sub-threshold
slope. Namely, steep sub-threshold slope enables faster switching with less energy
consumption. The sub-threshold slope of the atom transistor is of about 10 mV/
decade (Fig. 5), which is much steeper than 60 mV/decade that is the value of ideal
MOSFET. The steeper sub-threshold slope of atom transistor can also drastically
reduce power consumption in logic circuits.
3 Oxygen Ion Controlled Type
Three-terminal operation has also been demonstrated by controlling oxygen ion’s
drift to make a conductive channel between a source and a drain. The initial target of
this oxygen ion controlled type was to develop a nonvolatile three-terminal atomic
switch that turns on with a negative gate bias application. Since the atom transistor
mentioned in the former section turns on with a positive gate bias application, the
development of the three-terminal atomic switch that turns on with a negative gate
bias would have enabled a nonvolatile complementally operating system that can
fully replace CMOS systems.
For that purpose, we decided to use the phenomena that some metal oxides
become conductive with increase in the concentration of oxygen ions from the
stoichiometric condition. Since a negative gate bias application brings oxygen ions
towards a channel region, the phenomena should turn on an oxygen ion controlled
three-terminal switch.
In order to confirm the mechanism, we fabricated a sidewall gate type device, as
shown in Fig. 6. We employed TaOx as a channel material instead Ta 2 O 5 in order to
easily move oxygen ions. Another difference from the atom transistor is that a gate
electrode consists of Pt so that ions that can move in a metal oxide layer is only
oxygen ions. The operating result is shown in Fig. 7. As can be seen in Fig. 7a,
resistance between a source and a drain decreased to the order of 10 kΩ from the
order of 10 GΩ, while resistance between a gate and source/drain was kept at around
the order of GΩ, suggesting a three-terminal switching due to the increase in oxygen
ions occurred. Figure 7b shows I/V characteristics between a source and a drain
measured in the off-state and the on-state. The I/V characteristics in the on state
shows nonlinearity, suggesting that a conductive path is semiconductive. As such we
succeeded in turning on by increasing oxygen ions at a channel region. However, the
repeatability was not good because the increasing oxygen ions more than the
134
T. Hasegawa et al.