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P. A. Dananjaya et al.
in 1-transistor-1-RRAM (1T1R) structure during the weight update. Thus, both
gradual potentiation and depression can be achieved in anion devices. However, this
approach still requires non-identical programming pulses and resistance state verification before the programming step, thus overhead on circuitry is needed during the
learning process. Furthermore, the inherent oxygen vacancy defects variation within
the device structure makes achieving symmetrical and linear weight update with
sufficient read margin remain a huge challenge in these devices. Several structures,
i.e., AlO x , HfO x and TaO x —based structures, have been comprehensively investigated, improved, and implemented as synaptic devices to meet the requirements of
an ideal synaptic device.
Aluminum Oxide (AlO x )-based Devices
AlOx-based RRAM devices have been investigated as both digital and analog
memory devices under different systems, i.e., Ti/Al 2 O 3 /Pt [13], TiN/Al 2 O 3 /Pt [14],
Ni/Al 2 O 3 /Pt [15], CNT/AlO x /CNT [16], Ti/AlO x /TiN [17], and Al/AlO x /Pt [18]. In
general, the reported AlO x -based devices have high dynamic ratio (ranging from 10
to 1000), high scalability (down to 36 nm
2 device active area) [16], and low switching
energy (below 2 pJ) [14, 17]. The potentiation and depression characteristic of the
AlO x -based structure was experimentally tested in Ti/AlO x /TiN [17]. The linear
gradual conductance change in both directions was achieved under non-identical
pulses scheme. Different compliance currents (CCs) from 50 to 900 μA under 1.5 V,
500 μs voltage pulse were imposed during potentiation while different pulse amplitudes from −1 V to −1.6 V with 500 μs duration were used in depression mode.
The device was able to achieve an average of 1.2 and 1.7% conductance change per
programming pulse with 85 potentiation and 60 depression steps while maintaining
~10 dynamic ratio. However, due to non-identical pulses scheme required during the
operation for both SET and RESET, significant overhead must be implemented on
the peripheral circuit, which is not ideal for on-chip learning application.
AlO x has relatively high oxygen scavenging immunity, which is reported to result
in significantly smaller filament dimension [19]. While it is a desired property to
achieve high ON/OFF ratio, high speed, and excellent uniformity, it also raises a
challenge in achieving linear and symmetrical weight update. Thus, rather than being
implemented as the main switching layer, AlO x has been more widely used as an
insertion layer interfacing the main switching layer to improve the synaptic performance of the RRAM devices in terms of uniformity and linearity of the conductance
update [20–27].
Hafnium Oxide (HfO x )-based Devices
One of the first structures explored for synaptic device applications is HfO x -based
device with Ti oxygen reservoir electrode. Different weight update schemes have
been demonstrated for this system, i.e., identical and non-identical pulses. Under
identical pulses scheme, the device can only achieve gradual depression while still
having abrupt potentiation. The gradual potentiation can be achieved under nonidentical pulse condition in which the current flowing through the 1T1R device is
closely controlled by pulsing the transistor’s gate, resulting in a well-control filament
P. A. Dananjaya et al.
in 1-transistor-1-RRAM (1T1R) structure during the weight update. Thus, both
gradual potentiation and depression can be achieved in anion devices. However, this
approach still requires non-identical programming pulses and resistance state verification before the programming step, thus overhead on circuitry is needed during the
learning process. Furthermore, the inherent oxygen vacancy defects variation within
the device structure makes achieving symmetrical and linear weight update with
sufficient read margin remain a huge challenge in these devices. Several structures,
i.e., AlO x , HfO x and TaO x —based structures, have been comprehensively investigated, improved, and implemented as synaptic devices to meet the requirements of
an ideal synaptic device.
Aluminum Oxide (AlO x )-based Devices
AlOx-based RRAM devices have been investigated as both digital and analog
memory devices under different systems, i.e., Ti/Al 2 O 3 /Pt [13], TiN/Al 2 O 3 /Pt [14],
Ni/Al 2 O 3 /Pt [15], CNT/AlO x /CNT [16], Ti/AlO x /TiN [17], and Al/AlO x /Pt [18]. In
general, the reported AlO x -based devices have high dynamic ratio (ranging from 10
to 1000), high scalability (down to 36 nm
2 device active area) [16], and low switching
energy (below 2 pJ) [14, 17]. The potentiation and depression characteristic of the
AlO x -based structure was experimentally tested in Ti/AlO x /TiN [17]. The linear
gradual conductance change in both directions was achieved under non-identical
pulses scheme. Different compliance currents (CCs) from 50 to 900 μA under 1.5 V,
500 μs voltage pulse were imposed during potentiation while different pulse amplitudes from −1 V to −1.6 V with 500 μs duration were used in depression mode.
The device was able to achieve an average of 1.2 and 1.7% conductance change per
programming pulse with 85 potentiation and 60 depression steps while maintaining
~10 dynamic ratio. However, due to non-identical pulses scheme required during the
operation for both SET and RESET, significant overhead must be implemented on
the peripheral circuit, which is not ideal for on-chip learning application.
AlO x has relatively high oxygen scavenging immunity, which is reported to result
in significantly smaller filament dimension [19]. While it is a desired property to
achieve high ON/OFF ratio, high speed, and excellent uniformity, it also raises a
challenge in achieving linear and symmetrical weight update. Thus, rather than being
implemented as the main switching layer, AlO x has been more widely used as an
insertion layer interfacing the main switching layer to improve the synaptic performance of the RRAM devices in terms of uniformity and linearity of the conductance
update [20–27].
Hafnium Oxide (HfO x )-based Devices
One of the first structures explored for synaptic device applications is HfO x -based
device with Ti oxygen reservoir electrode. Different weight update schemes have
been demonstrated for this system, i.e., identical and non-identical pulses. Under
identical pulses scheme, the device can only achieve gradual depression while still
having abrupt potentiation. The gradual potentiation can be achieved under nonidentical pulse condition in which the current flowing through the 1T1R device is
closely controlled by pulsing the transistor’s gate, resulting in a well-control filament