*3.1.1 Electrochemical mechanism (ECM)*

Here, an electrochemical redox reaction, at the active electrode under an external voltage applied, produces the RS characteristics. Initially, a positive voltage was applied to the active electrode (top electrode Ag), which caused the metal atoms (Ag) to oxidize and transform into the corresponding ions (Ag<sup>+</sup> ), which can subsequently diffuse through the active layer to the bottom inert electrode and reduced to Ag atoms at the interface. There are two resistance states in memristor: low resistance state (LRS) and high resistance state (HRS), which refers to the formation and rapture of the conduction filaments in the active layer between the top (Ag) and bottom electrode, respectively. This reaction represents a common electrochemical oxidation–reduction process. Some other materials used as active electrodes are Cu, Au, Pt, and Al [21, 22].

**Figure 2.** *Illustration of types of switching mechanisms in various memristors.*

#### *3.1.2 Valence charge mechanism (VCM)*

In terms of integration and scaling, filamentary VCMs are the most sophisticated mechanism. The two electrodes are shorted by the formation and bursting of conductive filaments (CF), which are caused by a concentrated localized area of defects. There may be two or more stable resistance states, depending on how the CF diameter and/or dissolution are modulated or controlled. The conductance of interfacial VCM devices is assumed to scale with the device junction area through a homogenous oxygen ion flow across the oxides, either at the electrode/oxide or oxide/oxide interface. Bilayer stacks of difficult oxides, such as TiO2/TaO2 [23] and a-Si/TiO2 or complex oxides as bismuth ferrite [24] and praseodymium calcium manganite, form the foundation of reference material systems [25].

#### **3.2 Ferroelectric**

Despite the fact that ferroelectric memory has been around for a while, it has not captured the attention due to various scale-up issues. In 2006, the first device as ferroelectric tunnel junctions (FTJs) [26] was realized and led as novel concept for data storage and neuromorphic computing [27, 28]. An FTJ comprises two metal electrodes separated by a thin ferroelectric insulator. Quantum electron tunneling, in which electrons pass through a potential barrier of the ultrathin insulator, is the dominating process in this ferroic nanostructure, which is made up of the ultrathin ferroelectric barrier. The alignment of ferroelectric polarization in the insulator can change the stream of electrons, producing the enormous tunnel electro-resistance (TER) effect. Depending on the polarity of the ferroelectric layer, the tunneling electrons are either attracted or repulsive. An energy band profile becomes asymmetric when two distinct metals are placed across the ultrathin ferroelectric layer. When voltages are applied across the device, the electric potential at the interface increases or decreases depending on the polarization direction. As a result, the modified energy potential control the electrons transport through these contacts [29]. This screening phenomena causes the FTJ to exhibit significant resistance changes, allowing for the storage and processing of data [30].

#### **3.3 Phase change**

A device can be referred to as a hysteretic memristor when the joule heating led the phase changes between two states: amorphous and crystalline. Among all new memristor technologies, the phase-change-based memristor is the most developed and commercialized in the storage class memory (SCM) sector. One could say that phase-change memory technology has made a significant contribution to the growth of new electronic technologies. The first time, phase-change technique originally described was nearly 50 years ago [31]. However, recently, the phase-change technology has only gained popularity as a result of research on chalcogenide materials such as Ge2Sb2Te5 [32] or Ag- and In-doped Sb2Te [33]. **Figure 2** illustrates a phase-change memory instance [34]. The narrow metal heater's current saturation promotes the joule heating process when an electrical field is applied between the top electrode and bottom metal heater. An internal temperature change gradually heats the phase-change material [35]. The resistance contrast results from the phasechange between an HRS that is amorphous and an LRS that is crystalline. The distribution of internal voids determines the variation in structural disorder between two states. The band structure is redesigned as a result of ordered vacancies, and the conductivity rises as a result of the localization of charge carriers in a crystalline state [36].
