**3. Resistive switching in memory devices**

#### **3.1 Bipolar and unipolar switching**

The rapid switching speed, lower power consumption, and excellent scalability, ReRAM has emerged as the most encouraging choice for the future use of non-volatile devices [1, 2]. A dual terminal ReRAM device has an insulating layer wedged between two conducting electrodes. An external electric field can cause the memory cell to flip between two resistance states, known as the low resistance state (LRS, ON state) and high resistance (HRS, OFF state) state [16]. ReRAM is divided into two switching modes: unipolar and bipolar. The polarity of the switching voltage is inconsequential in unipolar switching, whereas in bipolar switching, the electrical polarity required to change from an HRS to an LRS is the inverse of that required to switch from an LRS to an HRS. Switching materials such as organics, binary oxides, and perovskite oxides, have been studied to achieve ReRAM with reliable switching, high ON/OFF current ratio, and long retention period [17–19].

Non-volatile memory (NVM) is a classification of computer memory that cannot be deleted or removed even when the power is turned off. In today's technology, NVMs are typically utilized for long-term data storage or secondary storage. Flash drives, magnetic storage devices, ferroelectric RAM, magnetoresistive RAM, ReRAM, and optical discs are examples of NVM devices. In today's storage technology, NAND (Boolean operator and logic gate) flash memory are widely used in most goods. However, the demand for faster writing speeds, higher density, and lower cost drives new research into developing technologies such as ReRAM. In the late 1990s and early 2000s, researchers investigated ReRAM technology, which allows RS between two resistance states to be exchanged via a thin film layer [20]. It works on the principle of applying a high voltage across a dielectric to transition from insulating to conductive qualities via a conduction filament pathway. Classification of ReRAM mechanism as memristors is controversial [21]. Theoretically, their RS curves differ from each other. **Figure 2** shows a typical current-voltage (*I-V*) characteristic obtained for a ReRAM device [2]. Such classifications of ReRAM and memristors are still a source of contention today.

The memristor is the fourth fundamental circuit element identified, after resistors, capacitors, and inductors. Chua et al. suggested the notion of memristor in 1971 [22], and it was validated in 2008. There are several materials having memristive characteristics which have been discovered since 1962. Memristors, like resistance in Ohms, are defined as:

$$\mathbf{M} = \frac{\mathbf{dq}}{\mathbf{dq}}\tag{1}$$

where φ is magnetic flux and *q* is electric charge. As observed in its theoretical memristor curves, memristor curves are typically non-linear devices. Real-world memristors, on the other hand, have a comparable *I-V* characteristic to ReRAM. Some researchers hypothesized that such behavior was caused by conducting filament [23], active memristor [24], and non-zero crossing [25]. ReRAM curves, on the other hand, can be classified as bipolar or unipolar switching. To generate the RS curve, unipolar switching uses the same polarity of the swept bias with variable magnitudes, but bipolar switching requires various polarities [26]. ReRAM devices, in general, consist of an insulator layer sandwiched between two electrodes. Conduction pathways were formed by the flow of charge carriers alternating between the cathode and anode,

**Figure 2.**

*(a) The predicted and typical types of (a) unipolar switching and (b) bipolar switching ReRAM curves. Adapted from Figure 4 [2].*

which was induced by several physical mechanisms that are still commonly utilized today. Furthermore, the conduction methods differ depending on the materials utilized and the device's overall architecture. Szot et al. were the first to detect conductive filament using an electron microscope, which they attributed to filament build-up and rupture [27].

### **3.2 Write-once-read-many (WORM)**

WORM devices, in general, are devices in which data that has been written cannot be manipulated or removed. WORM memory is used in fields such as healthcare, security, taxation, and accounting where the data cannot be tampered with or updated to secure information. The most prevalent WORM devices in use today are the Compact Disc Recordable (CD-R) and Digital Versatile Disc Recordable (DVD-R). Furthermore, the "read-many" element implies that the device's data can be read an infinite number of times, with the only limitation being the device lifetime. The WORM memory pixels are read according to the rows, with an unwritten pixel labeled as logical "0" and a written pixel labeled as logical "1" [3].

Today's WORM memory technology is based on the electrically or laser programmed fuse WORM type. However, in current WORM research, the emphasis was shifted to organic materials or solution process techniques in order to achieve rapid switching rate, lower power consumption,, large storage density, simplicity, and being cost-effective, which has been dubbed WORM RS [28, 29]. Unlike ReRAM, the intrinsic features of WORM RS were sufficient to oppose the applied electric field, resulting in an irreversible shift. A typical WORM RS *I-V* curve obtained when a voltage is supplied is shown in **Figure 3** [30]. The change between resistance states (OFF to ON) is an irreversible process, indicating WORM features.
