A novel artificial synapse with dual modes using bilayer graphene as the bottom electrode

Resistive Random Access Memory (RRAM) shows great potential to be used as an artificial synapse for neuromorphic applications. The resistance can be gradually reduced during reset, which can enable enough states to mimic the forgetting process. However, the abrupt set (Mode I) cannot generate enough states to mimic the learning process, which results in depression-only behavior. In this work, we introduce another mode (Mode II) in an Al/AlOx/graphene 'RRAM' stack by using oxygen vacancies as trapping centers and bottom electrode bilayer graphene as the channel material. In this way, since the pulse can gradually create the oxygen vacancies, post-synaptic current (PSC) can be gradually potentiated or depressed. By introducing Mode II, we can realize 166 potentiation states, which is higher than the previously reported conventional RRAM with insufficient potentiation states due to the abrupt set. Moreover, Mode II can help realize an inhibitory synapse. By combining modes I and II, we can realize both excitatory and inhibitory synapses in a single device. This work shows great potential to enable neuromorphic computations with greater learning and reconfigurability.