A high-performance MoS ₂ synaptic device with floating gate engineering for neuromorphic computing

Abstract

As one of the most important members of the two dimensional chalcogenide family, molybdenum disulphide (MoS2) has played a fundamental role in the advancement of low dimensional electronic, optoelectronic and piezoelectric designs. Here, we demonstrate a new approach to solid state synaptic transistors using two dimensional MoS2 floating gate memories. By using an extended floating gate architecture which allows the device to be operated at near-ideal subthreshold swing of 77 mV/decade over four decades of drain current, we have realised a charge tunneling based synaptic memory with performance comparable to the state of the art in neuromorphic designs. The device successfully demonstrates various features of a biological synapse, including pulsed potentiation and relaxation of channel conductance, as well as spike time dependent plasticity (STDP). Our device returns excellent energy efficiency figures and provides a robust platform based on ultrathin two dimensional nanosheets for future neuromorphic applications.Understanding the complexities in the functioning of the human brain has been one of the foremost challenges in the field of neuroscience. Among the several proposed models, only a few can explain the operation of a human brain and that too for a very limited set of functionalities [1][2][3] . From an electronic point of view, the computational architecture of a brain is vastly different from that of a traditional von Neumann architecture based system [4,5] . This has led to the emergence of neuromorphic computation schemes [6][7][8][9][10] . Current computation follows an architecture where processing and storage of data is handled by separate entities whereas in neuromorphic computation, processing and storage of data is handled by a single element which acts as the electrical analogue of a synapse. Mimicing the functionality and density of synapses in the brain would lead to a massive reduction in energy consumption and immensely enhance computational capabilities like parallel processing. Given the high density of synapses required, traditional silicon based devices which are plagued by power dissipation and short channel effects are rendered unsuitable for scalable neuromorphic applications [11,12] . This makes ultrathin two dimensional materials a perfect candidate for the active element of a synaptic transistor given their immunity to short channel effects and excellent gate coupling at nanometer length scales [12,13] .Biologically, a synapse functions by changing its conductivity based on the sequence of synaptic pulses it receives. This is accomplished by varying the concentration of neurotransmitters or chemical stimulants which control the conductivity of the junction between two neurons [14] . An ideal synaptic transistor must possess the ‡ e-mail:tathagata@iisc.ac.in, arindam@iisc.ac.in twin qualities of being a non-volatile memory while inculcating a learning based mechanism to deduce its conductance from the history of applied inputs [15][16][17][18][19][20][21][22][23][24][25][26][27][28...

Recent Progress in Synaptic Devices Based on 2D Materials

“…The background colors show their stabilities in ambient and vacuum. The 2D insulators supply an ultra‐flat dielectric layer in the electrochemical metallization/conductive bridge‐based resistance switching devices, as well as the barrier of tunneling effect‐based synaptic devices …”

“…Paul et al successfully improved the efficiency of gating, leading to a quasi‐ideal subthreshold swing (77 mV per decade) and reduction of the required drain bias and switching pulse, by using an extend graphene floating gate, as shown in Figure a,b . An h‐BN layer, working as tunneling barrier, separates the floating gate and channel, which could control the charge transfer and tune the conductance of the channel between them for mimicking the synaptic plasticity (Figure c–e).…”

“…g) Demonstration of asymmetric STDP in FG synaptic devices. Reproduced with permission . Copyright 2019, IOP Publishing Group.…”

“…Based on the negligible volume of 2D materials, synaptic operation by tens of femtojoule per spike has been realized widely, which is comparable with the energy consumption per spike in biological synapses . However, the disadvantages of 2D materials have also been revealed: limited synthesis methods for wafer‐scale geometry and the lack of compatibility to CMOS fabrication processes .…”

Spin-valley system in a gated MoS2-monolayer quantum dot

“…The area of application of monolayer materials for construction of electronic nanodevices is currently under strong development [1,[7][8][9][10][11][12]. Methods for building devices based on gated TMDC monolayers or nanotubes become increasingly advanced [13][14][15][16][17], opening the possibility of utilizing the spin and valley index of electrons controlled therein.…”