Improvement of Synaptic Properties in Oxygen‐Based Synaptic Transistors Due to the Accelerated Ion Migration in Sub‐Stoichiometric Channels

Abstract

arrays, are being actively studied as synaptic devices for use in neuromorphic computing. However, the need for a new type of synaptic device has emerged, owing to write-disturbance issues [6] and the difficulty of obtaining sufficient synaptic properties. [7,8] Among various synaptic memory devices, ion-based synaptic transistors are being studied as promising next-generation solutions based on nearideal synaptic behaviors, low-power operations, reasonable retention, and excellent endurance characteristics.Ion-based synaptic transistors mainly use protons and Li ions as doping elements, which can be easily moved by external biases, owing to their small mass. [9] The electrical conductivity changes linearly according to the concentration of ions injected into the channel [10] and by using it as a synaptic weight ideal training characteristics can be achieved. In the case of a protonbased synaptic transistors, low-voltage operation is possible owing to the small atomic size (ion radius 0.04 Å), and excellent weight-update linearity and endurance characteristics are obtained. [11][12][13][14][15] However, to provide excellent proton-hopping characteristics, polymer materials, such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, are used as channels and electrolytes. Hence, it is difficult to utilize conventional complementary metal-oxide-semiconductor (CMOS) technology. [14,15] The structure and operation mechanism of a Li-based synaptic transistor is very similar to that of a Li-ion battery. Li ions are injected or extracted into a channel that acts as a cathode, and its electrical conductivity (i.e., synaptic weight) is determined by the concentration of Li ions in the channel. [10] Li-ion-based synaptic transistors have excellent synaptic properties owing to their small atomic size (ion radius 0.9Å), [16][17][18][19][20] but there are disadvantages, such as Li dendrite generation and open-circuit potential issues. [16,21] Accordingly, on the basis of various prior studies in the RRAM and solid oxide fuel cell fields, research on oxygen ion-based synaptic transistors (OISTs) that can utilize conventional CMOS technology is being actively conducted. [22][23][24][25][26][27][28] The OIST uses transition metal oxide (TMO) as a channel, which can change its electrical conductivity depending on the extraction/injection of oxygen ions. In a recently published

Ion-Driven Electrochemical Random-Access Memory-Based Synaptic Devices for Neuromorphic Computing Systems: A Mini-Review

“…Channel quality was investigated in addition to the effect of the electrolyte [ 28 ]. With the given YSZ electrolyte acting as a good ion conductor, two binary oxides, TiO x and WO x , were compared.…”

Strategies to Improve the Synaptic Characteristics of Oxygen-Based Electrochemical Random-Access Memory Based on Material Parameters Optimization

“…Therefore, it is very difficult to uniformly control atomic-level changes in local filaments by applying identical voltage pulses. To solve this problem, three-terminal (3-T)-based electrochemical RAM (ECRAM) devices have been developed in which conductance is linearly changed by the bulk channel region. − In early ECRAM studies, channel conductivity was changed by using protons and lithium ions. − Excellent synaptic properties were achieved through facile ion migration based on a small ion mass. However, the main disadvantage is that conventional complementary metal–oxide–semiconductor (CMOS) processes cannot be used because a polymer- or lithium-ion-battery-based material is used as both the channel and electrolyte materials. , Moreover, because a battery stack is used as a memory device, lithium-ion-based ECRAM devices require a selector device because of open-circuit potentials …”

“…To overcome these limitations, oxygen-based ECRAM (O-ECRAM) devices are being studied as next-generation synaptic devices. − O-ECRAM devices have advantages such as CMOS compatibility and improved retention. Because RRAM-based materials are used as both electrolytes and channels in O-ECRAM devices, conventional CMOS processes can be utilized .…”

On-Chip Integrated Atomically Thin 2D Material Heater as a Training Accelerator for an Electrochemical Random-Access Memory Synapse for Neuromorphic Computing Application

“…ECRAM devices in which the dopant ion is O 2– (O-ECRAM) are compatible with complementary metal-oxide-semiconductor devices (CMOS), so processes can be integrated with silicon-based circuits in certain instances. − However, at room temperature, O-ECRAM suffers from poor training and inference due to nonlinear switching, limited number of analog states, low switching speed, and poor retention. External heating of the O-ECRAM device lowers the activation energy barrier for the transport of O 2– , which controls the channel conductance in an analog manner. , However, an external hot plate as a temperature source is a challenge to integrate into devices.…”