Brain-inspired neuromorphic computing
emulates the biological functions
of the human brain to achieve highly intensive data processing with
low power consumption. In particular, spiking neural networks (SNNs)
that consist of artificial synapses can process spatiotemporal information
while enabling energy-efficient neuromorphic computations. Artificial
synapses are a key element of sophisticated neuromorphic hardware,
so a significant amount of research has been conducted to develop
various materials and device structures. Of these, we assess amorphous
InGaZnO (IGZO)-based synaptic transistors that have exhibited properties
suitable for emerging hybrid optoelectronic neuromorphic systems.
Here, we describe the fundamental principles of neuromorphic computations,
neuron circuits, and synaptic devices according to recent studies.
IGZO-based transistors are discussed, from their material properties
to various device physics for electronic- and/or photonic-neuromorphic
systems with extraordinary biological emulations.
We investigated the hysteresis and off-current (I off ) of amorphous In-Ga-Zn oxide thin-film transistors illuminated by 400 nm light at various intensities. Both hysteresis and I off are induced by the ionized oxygen vacancy (Vo 2+ ) that forms at the interface between the gate insulator and active layer. In our measurements, I off was much less than the estimated photocurrent. I off showed a rapid nonlinear increase with light intensity, while the photocurrent of a conventional crystalline semiconductor is expected to show a linear relationship. Furthermore, a numerical analysis suggested that the response time of Vo 2+ should be considered when analyzing the hysteresis of these devices.
This study investigated the effects of fluorine (F) diffusion from a CYTOP passivation layer into amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). The F contained in the CYTOP passivation layer was diffused into a-IGZO through 350 °C annealing. The similar ionic radii of F and oxygen (O) allowed the passivation of oxygen vacancy (Vo) and weakly bonded oxygen by F. As a result, the a-IGZO TFTs with CYTOP passivation were highly stable under various stresses. The threshold voltage (Vth) shifts of a-IGZO TFTs without CYTOP passivation and with CYTOP passivation under a negative bias stress test for 10 000 s were −6.7 V and −2.5 V, respectively. In addition, the Vth shifts of each device under a negative bias illumination stress test for 4000 s were −10.9 V and −5.3 V, respectively. This improvement was caused by a reduction of Vo and a widened band gap of a-IGZO through the F diffusion effect. In addition, the CYTOP passivation layer maintained excellent properties as a barrier against moisture after 350 °C annealing.
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