398
P. A. Dananjaya et al.
and weak (pulse duration of ≤1 μs) programming conditions were used to toggle
between relatively more deterministic and probabilistic switching operation within
the same device. In the strong programming condition, it was observed that high
resistance state (HRS) of the device had larger distribution compared to the low
resistance state (LRS), which could be attributed to uncontrollable metal filaments
dissolution during reset process. With each device carries certain degree of stochasticity, in array level, this characteristic is amplified with the presence of device-todevice variation. The 1T1R and 1R synapse crossbar array was implemented in the
core circuit, together with input/output CMOS neuron and pseudo-random number
generator (PRNG) circuit under leaky integrate and fire (LIF) neuron model. Specific
programming schemes for 1T1R and 1R were implemented to handle asynchronous
analog streams of data for unsupervised pattern extraction and recognition. Excellent performance parameters were achieved for auditory pattern sensitivity (>2.5)
and video detection rate (95%), while maintaining extremely low power dissipation
of 0.55 and 74.2 μW for audio and video demonstrator respectively [60].
b. Finite Cations Source
Another type of device has also been engineered to improve analog properties of the
cation-based devices, in which a fixed amount of metal species within the switching
layer is used rather than an active electrode as a source of metal ions. This approach
prevents the formation of a localized conductive path during the switching operation, unlike the conventional cation-based devices. This technique was first implemented by sandwiching Ag-doped amorphous Si in between two inert electrodes
[61]. The structure was fabricated using co-sputtering technique of Ag and Si to
form a gradient mixture of Ag:Si across the switching layer. This results in the
presence of rich and poor Ag region that can be modulated under external electric field. The structure successfully achieved gradual conductance change in both
potentiation and depression process under identical programming pulse scheme. The
device was also integrated with CMOS-based neuron circuits to demonstrate spike
timing dependent plasticity (STDP) learning rules. The same approach was successfully adopted in Ag:TiO x [62]. The device was able to demonstrate the learning and
memory functionalities including STDP, PPF, and STP to LTP transition, with an
improved timescale of hundreds of nanoseconds as compared to microseconds pulse
used in Ag:Si devices.
A slightly different approach was implemented in Ag-doped WO x [63] and TaO x
[64, 65] with uniform Ag content across the switching layer. The Ag-doped WO x
demonstrated the tunability of the device characteristics with different Ag content.
Low Ag concentration within WO x leads to volatile switching behavior that was
able to mimic the forgetting effect of human memory. While, the devices with
relatively higher Ag contents enable analog non-volatile switching properties. The
Ag:TaO x device was fabricated via self-doping during the sputtering process. The
TaO x layer was deposited on top of Ag electrode, resulting in the intermixing layer
at the interface. The presence of the Ag:TaO x layer at the interface of Ag and TaO x
layer caused a double switching behavior under different external electric field. The
P. A. Dananjaya et al.
and weak (pulse duration of ≤1 μs) programming conditions were used to toggle
between relatively more deterministic and probabilistic switching operation within
the same device. In the strong programming condition, it was observed that high
resistance state (HRS) of the device had larger distribution compared to the low
resistance state (LRS), which could be attributed to uncontrollable metal filaments
dissolution during reset process. With each device carries certain degree of stochasticity, in array level, this characteristic is amplified with the presence of device-todevice variation. The 1T1R and 1R synapse crossbar array was implemented in the
core circuit, together with input/output CMOS neuron and pseudo-random number
generator (PRNG) circuit under leaky integrate and fire (LIF) neuron model. Specific
programming schemes for 1T1R and 1R were implemented to handle asynchronous
analog streams of data for unsupervised pattern extraction and recognition. Excellent performance parameters were achieved for auditory pattern sensitivity (>2.5)
and video detection rate (95%), while maintaining extremely low power dissipation
of 0.55 and 74.2 μW for audio and video demonstrator respectively [60].
b. Finite Cations Source
Another type of device has also been engineered to improve analog properties of the
cation-based devices, in which a fixed amount of metal species within the switching
layer is used rather than an active electrode as a source of metal ions. This approach
prevents the formation of a localized conductive path during the switching operation, unlike the conventional cation-based devices. This technique was first implemented by sandwiching Ag-doped amorphous Si in between two inert electrodes
[61]. The structure was fabricated using co-sputtering technique of Ag and Si to
form a gradient mixture of Ag:Si across the switching layer. This results in the
presence of rich and poor Ag region that can be modulated under external electric field. The structure successfully achieved gradual conductance change in both
potentiation and depression process under identical programming pulse scheme. The
device was also integrated with CMOS-based neuron circuits to demonstrate spike
timing dependent plasticity (STDP) learning rules. The same approach was successfully adopted in Ag:TiO x [62]. The device was able to demonstrate the learning and
memory functionalities including STDP, PPF, and STP to LTP transition, with an
improved timescale of hundreds of nanoseconds as compared to microseconds pulse
used in Ag:Si devices.
A slightly different approach was implemented in Ag-doped WO x [63] and TaO x
[64, 65] with uniform Ag content across the switching layer. The Ag-doped WO x
demonstrated the tunability of the device characteristics with different Ag content.
Low Ag concentration within WO x leads to volatile switching behavior that was
able to mimic the forgetting effect of human memory. While, the devices with
relatively higher Ag contents enable analog non-volatile switching properties. The
Ag:TaO x device was fabricated via self-doping during the sputtering process. The
TaO x layer was deposited on top of Ag electrode, resulting in the intermixing layer
at the interface. The presence of the Ag:TaO x layer at the interface of Ag and TaO x
layer caused a double switching behavior under different external electric field. The