Improved Synaptic Behavior of CBRAM Using Internal Voltage Divider for Neuromorphic Systems

In this paper, we demonstrate the linear conductance-change characteristics of a conductive-bridging RAM (CBRAM) to be employed as an artificial synapse device in neuromorphic systems. The CBRAM with a bilayer electrolyte structure (Cu/Cu
_2-x
S/WO
_3-x
/W) exhibits analog switching behavior during the depression process due to the well-controlled dissolution of the conductive filament. To analyze the origin of this motion, we investigate the effective voltage applied to Cu
_2-x
S and WO
_3-x
. Our findings reveal that Cu
_2-x
S, acting as a voltage divider, helps in suppressing the large voltage drop in WO
_3-x
, where the formation/dissolution of filament occurs. Furthermore, due to the diode-like characteristics of Cu
_2-x
S and the division of voltage drop between WO
_3-x
and Cu
_2-x
S, an optimum programming energy is applied to WO
_3-x
during the depression process. This leads to linear conductance-change characteristics under identical pulses. However, abrupt conductance-change characteristics are observed during the potentiation process. Thus, we use only the device characteristics of the depression part for the neuromorphic system. An excellent classification accuracy is achieved due to the linear conductance-change characteristics and optimized pulse conditions.