Development of Oxide Synapse Devices

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It is well known that the oxygen vacancies are formed to compensate vallency in nickel doped barium strontium titanate ceramics. At first, such vacancy's behavior upon electric field application was made clear. The vacancy was found through thermally stimulated current to move by hopping conduction with activation energy about 0.9eV.As a result, there occurs conduction current increment with time under electric field, 0.6kV/mm above 120 degree C.Since the conduction change takes place continuously and remains even after the field is removed, it works as if an analogue memory This phenomenon can be understood by the change of conduction type ; initially, nickel ions and oxygen vacancies are balanced while upon application of electric field forces to move the vacancies toward cathode direction forming pn junction. It is exactly forwardly biased pn junction. The non-linear current-voltage characteristics confirms the pn junction establishment.At last, a principal confirmation that such memory characteristics work as synapse devices in an analogue neuro- computer was carried out. An operational amplifier with iron-doped rutile ceramics, as a synapse, showed switching performance after application of 12 impulses by learning effect. Further effort is still now required to form thin film device in addition to accelerate writing operation by heat radiation.