## 2 Memristor: Models, Material, and Structure Design
### Memristor Material and Structure Design  
Resistive switching (RS) materials are the most important part of memristors, and different RS materials have different resistive characteristics. It is crucial to understand the properties and behaviors of different RS materials in order to design and fabricate memristors with the desired performance. By studying the resistive characteristics of various materials, researchers can tailor memristors to specific applications and optimize their functionality. Recently, researchers have introduced new resistive switching (RS) materials for memristors. These include common electrodes and specific electrode materials, as well as organics, two-dimensional (2D) materials, three-dimensional (3D) integration, resistive layers, binary oxides, and perovskites. These new materials can be generally classified as inorganic and organic materials and offer improved performance in terms of speed, energy efficiency, and reliability, making them promising candidates for future electronic applications. Memristor electrodes are materials used to carry electric current and participate in resistive reactions. They are made from various materials, including conventional metals, noble metals, alloys, carbon-based materials like graphene and carbon nanotubes, nitrides like TiN and TaN, and transparent conductive flexible oxides like indium tin oxide (ITO), doped ITO, and F-doped tin oxide (FTO) [23]. These electrodes can be classified into three types based on their role in resistive response (RS) behavior. First, electrodes conduct current without affecting RS behavior, using inert metals like Au and Pt. Second, electrodes create conductive filaments (CFs) through the chemical dissolution and deposition of active metal electrodes like Cu and Ag. Finally, certain new electrodes like ITO, FTO, graphene, TiN, and alloy electrodes aim to stabilize the RS behavior. Binary oxides, such as TiO\({}_{\mathrm{x}}\), SiO\({}_{\mathrm{x}}\), AlO\({}_{\mathrm{x}}\), NiO\({}_{\mathrm{x}}\), CuO\({}_{\mathrm{x}}\), ZnO\({}_{\mathrm{x}}\), HfO\({}_{\mathrm{x}}\), TaO\({}_{\mathrm{x}}\), WO\({}_{\mathrm{x}}\), ZrO\({}_{\mathrm{x}}\), and SnO\({}_{\mathrm{x}}\), are simple, highly stable, low in cost, and compatible with traditional CMOS processes [23]. HfO\({}_{\mathrm{x}}\) and TaO\({}_{\mathrm{x}}\) are promising due to their high on/off ratio, sub-ns operation speed, and endurance over \(10^{10}\) cycles. However, conventional binary oxide memories often have high power consumption or low uniformity. In [24], a bismuth-doped tin oxide (Bi: SnO\({}_{2}\)) memristor is prepared using magnetron sputtering, resulting in smaller self-compliance currents, switching voltages, and operating currents compared to the ITO/SnO\({}_{2}\)/TiN device. However, conventional binary oxide memories often have high power consumption or low uniformity. The high-resistance state and low-resistance state have higher resistance values, and the operating current is reduced by over an order of magnitude. The authors of [25] created a three-layer memristor made of HfO\({}_{2}\), BiFeO\({}_{3}\)(BFO), and HfO\({}_{2}\) to improve memory, mimic how synapses work, and perform neuromorphic calculations. The memristor improved the storage window and multi-level storage capability, and the recognition simulation showed 91.2% accuracy. The BFO inserting layer facilitated the formation of hexagonal lattice structures of hafnium crystals. Research on binary oxides has been conducted by academics and industry, with various prototypes and applications being considered. Examples include Unity's 64 Mb test chip, Panasonic's MN101 L microcontroller (Osaka, Japan), Sony's 16 Gb test chip (Tokyo, Japan), and TSMC's embedded oxide memristor modules (Hsinchu, Taiwan) [23]. Perovskites are materials with an ABX\({}_{3}\) structure, with a wide variety of physical and chemical properties achieved by partially doping the A and B sites with different valence or radius cations.