RRAM is a type of device that exhibit resistive switching between a high-resistance state (HRS) and a low-resistance state (LRS). Its structure is simply an active material sandwiched between two metal electrodes, which is called metal-insulator-metal (MIM) structure. The advantages of RRAM are high read/write speed, low power consumption, high reliability, and scalability with simple structure. The expectation for RRAM is that: (1) it will be a new memory technology that can be integrated with conventional CMOS technology, and (2) it can be stacked in a 3D crosspoint architechture with a memory cell area of 4F2. Although there are numerous materials and approaches being reported, but property optimization based on deep understanding about resistive switching mechanism is required.

We studied the behavior of artificial synapses. By controlling the filament formation and rupture, neuromorphic properties such as analog memory switching, short-term plasticity (STP), long-term plasticity (LTP), spike-rate-dependent plasticity, and short-term to long-term transition were demonstrated. Also, we demonstrated flexible neuromorphic devices by utilizing biomaterials. Biomaterial-based flexible artificial synapse devices emulated STP, paired-pulse facilitation, and transition from STP to LTP of a biological synapse without noticeable degradation under mechanical bending test. The above-mentioned neuromorphic characteristics were realized by exploiting the similarities between the silver or oxygen ion dynamics in the active materials and the Ca2+ dynamics in a biological synapse. The development of electronic devices possessing the functionality of biological synapses is a crucial step toward mimicking the capabilities of the human brain.

Cross-point array is a good candidate for future high density memory device due to 3D configuration with small cell size of 4F2. However, the cross-point array has critical problems with interconnected series resistance and undesired current leakage path, which can lead to misreading of information stored in the selected cell. Selector device has been introduced to alleviate these problems.