We demonstrate single- and double-gate synaptic operations of a thin-film transistor (TFT) with double-gate stack consisting of an Al-top-gate/SiOx/TaOx/n-IGZO on a SiO2/n+-Si-bottom-gate substrate. This synaptic TFT exhibits a tunable drain current, mimicking synaptic weight modulation in the biological synapse, upon repeatedly applying gate and drain voltages. The drain current modulation features are analog, voltage-polarity dependently reversible, and strong with a dynamic range of multiple orders of magnitude (∼104). These features occur as a consequence of the changes in mobility of the IGZO channel, gate insulator capacitance, and threshold voltage. The drain current modulation responsive to the timing of the voltage application emulates synaptic potentiation, depression, paired-pulse facilitation, and memory transition behaviors depending on the voltage pulse amplitude, width, repetition number, and interval between pulses. The synaptic motions can be realized also by a double-gate operation that separately tunes the channel conductance by top-gate biasing and senses it by bottom-gate biasing. It provides the modulated synaptic weight with a wide level of synaptic weight through the read condition using a bottom-gate stack without read-disturbance. These results verify the potential application of TaOx/IGZO TFT with single- and double-gate operations to artificial synaptic devices.
Analog synaptic weight modulation that is linear, symmetric, and exhibits long-term stability is demonstrated by the resistance changes in a Pt/indium-tin-oxide (ITO)/CeO2/Pt memristor. Distinct from a Pt/CeO2/Pt memristor without the ITO layer, which shows highly nonlinear and asymmetric resistance changes, the Pt/ITO/CeO2/Pt memristor exhibits linear and symmetric resistance changes in proportion to the number of voltage applications with opposite polarities for potentiation and depression behaviors. The Pt/CeO2/Pt memristor also displays high long-term stability of modulated synaptic weight over time, which originates from the ITO layer acting as a reservoir of oxygen ions drifted from the CeO2 layer to retain the resistance change. Comparison of the results for the Pt/CeO2/Pt and Pt/ITO/CeO2/Pt memristors confirms the role of ITO in the linearity, symmetry, and long-term stability of the resistance change in CeO2-based memristors for use as artificial synapses in neuromorphic systems.
We demonstrate strong, analog, reversible, and nonvolatile memcapacitance in a Si-based MOS (metal-oxide-semiconductor) memcapacitor with an ITO (In-Sn-O)/HfOx/Si structure. Both accumulation and depletion capacitances change sequentially and reversibly upon repeating voltage application with respect to voltage polarity. This memcapacitance is thought to be induced by oxygen ions' migration between ITO and HfOx layers, which changes the HfOx permittivity and the depletion states in Si and ITO. The Si-based memcapacitor has potential to be applied to the gate stack of the MOS field-effect-transistor for nonvolatile memory and synaptic transistors through modulating drain current determined by the capacitance change of the MOS gate stack.
Nonvolatile memory and synaptic characteristics in thin‐film transistors (TFTs) with HfOx gate insulator and ZnO channel are investigated for the application to nonvolatile memory and artificial synapse in neuromorphic systems. Nonvolatile change of drain current induced by modulated gate stack properties is demonstrated to be applicable to nonvolatile memory operation. It also emulates synaptic weight change for learning and memory functions in artificial synapses. The TFTs with HfOx and ZnO layers deposited by sputtering or atomic layer deposition (ALD) at low temperatures exhibit tunable drain current upon applying gate pulses, featuring analog, reversible, nonvolatile changes with respect to pulse amplitude, width, interval, and repetition number. However, the TFTs with HfOx and ZnO by ALD at high temperatures show negligible change. The structural and chemical analyses reveal similarities in defective nature of sputter‐deposited and low‐temperature ALD HfOx and ZnO layers, leading to analogous drain current modulation. Also, the results of temperature‐ and voltage polarity‐dependent drain current changes and capacitance changes verify that the drain current modulation is driven by oxygen ion migration associated with defective states of HfOx and ZnO layers. It demonstrates feasibility of application of ALD‐HfOx/ZnO TFTs to nonvolatile memory and artificial synapses using modulated gate stack properties.
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