Download Premlal_B_Pillai_UoS_EMN_abstract

Memory and Learning behaviour of ZnO based transparent synaptic thin film transistors
Premlal B. Pillai, M. M. De Souza
Department of EEE, University of Sheffield, S1 3JD, Sheffield, UK
Email:[email protected], [email protected]
The development in mimicking memory or learning behavior of biological systems by nanoscale
ionic/electronic devices has spurred a great deal of interest in the scientific community in realising
neuromorphic systems1-2. Software level emulation of neuromorphic properties uses traditional CMOS
switches that have five orders of magnitude higher power consumption and are 100 times slower than a
brain3. For hardware based emulation, oxide based thin film transistors were proposed as energy efficient
synaptic devices using nanogranular Silicon dioxide based proton conductor films as the gate insulator4. The
main drawback of such synaptic devices are the requirements of a certain level of humidity to function as a
synaptic FET4. Recently, resistive switching property has been successfully applied to implement energy
efficient synaptic devices3. In this work, using resistive switching properties of tantalum oxide insulating
film, synaptic devices using biocompatible Zinc oxide are demonstrated. The performance of the synaptic
devices presented in this study are not dependent on environmental factors and offers new possibilities for
realising synaptic memory devices that feature lower processing time and cost.
The devices are composed of radiofrequency sputtered semiconducting layers of ZnO and insulating
Tantalum Oxide (Ta2O5) on Indium Tin Oxide coated glass substrates. The capacitance and electrical
characteristics were measured by using Agilent E4980A LCR meter and Keithley (4200 SCS) respectively.
The devices exhibit superior memory windows utilizing the mobile oxygen vacancies present in the
insulator, greater than 2V with an operation voltage of ± 4V, (fig 1a). Synaptic devices utilise a voltage
pulse on the gate electrode as a presynaptic spike to trigger an excitatory post synaptic current/conductance
(EPSC) on the channel, similar to the dendritic synapses in biological systems. The EPSC is measured as a
time dependent channel conductance after the application of a voltage pulse on the gate electrode. EPSC
signals for a range of gate pulse widths (6-242 ms), magnitude (3 V) and frequency (1-83 Hz) are analyzed
(figs 1b &1c) and compared with other device technologies.
Figure 1. a) Dual sweep IDS-VGS characteristics of the ZnO TFTs showing tunable memory window with
increasing +VGS from 3 to 6V b) Excitatory post synaptic current (EPSC) measured from a ZnO TFT
(W/L=300/10 µm) using 10 pre-synaptic gate pulses with width 26 mS and frequency 19 Hz c) showing the
influence pre-synaptic gate pulse width and frequency on the EPSC signal. The pulse width and frequency of
the ten applied gate voltage spikes are labelled in the figure.
In conclusion, Non-volatile memory and Synaptic behavior of low temperature processed ZnO/Ta2O5
thin film transistors are analyzed for the first time on the basis of memory retention properties and spike
timing dependent synaptic responses. The devices exhibit saturated EPSC signal > 300 nA at Vpulse = 0.3V,
significantly better than value of 10-30 nA reported for IZO and IGZO synaptic devices6,7. Single spike
power consumption analyses showed a power consumption <35 pJ smaller than reported in refs [4,6]. The
ZnO based synaptic devices proposed here are a viable, low cost alternative to current CMOS based three
terminal synaptic devices.
1. C. Sanchez et.al., Nat. Mater. 4, 277 (2005),
2. D. Kuzum, et. al., Nanotechnology, 24, 382001 (2013),
3. S. Yu et.al., IEEE Trans. Electron Devices 58, 2729 (2011).
4. C. J. Wan et. al., Nanoscale 5, 10194 (2013),
5. G Indiveri, et.al., IEEE Trans. Neural Networks 17, 211 (2006)
6. L. Q. Zhu et. al., Nat. Commun. 5,3158 (2014)
7. Z. Q. Wang et.al., Adv. Funct. Mater. 22, 2759 (2012).
Presentation Method (Invited/Regular Oral/Poster): Invited Oral