A pristine memory device with high initial resistance state (IRS) can be switched in to a low-resistance state (LRS) by applying a high voltage stress. This process is called the ‘electroforming process’ or simply ‘forming process’ and alters the resistance

of the pristine device irreversibly [15, 37]. Some RRAM devices do not need the forming process and are called forming-free devices. Forming-free devices are highly required for RRAM practical application and are reported infrequently [38–41]. After the forming process, the RRAM device can be switched to a high-resistance state (HRS), generally lower than that of the IRS by the application of a particular voltage called reset voltage. This process is called ‘RESET process.’ Switching from a HRS to a LRS called ‘SET.’ In the SET process, generally, the current is limited by the current compliance (CC) in order to avoid device damage. AZD5582 The resistive switching in unipolar mode has been observed in many highly insulating oxides, such as binary metal oxides [10]. The unipolar devices suffer from high non-uniformity and poor endurance. In bipolar

resistive switching mode, the SET and RESET occur in the opposite polarity, i.e., if memory device find more can be set by applying positive voltage on TE, then only negative voltage can reset the device (Figure 3b). So, this type of resistive switching is sensitive to the polarity

of the applied voltage. For bipolar switching to occur, the MIM stack should be asymmetric generally, such as different electrodes or a dedicated voltage polarity for the forming process. Many oxides show bipolar resistive switching and will be also discussed later. The devices in which unipolar and bipolar modes can be changed by changing the operation conditions are called ‘nonpolar’ devices [42], and the resistive switching mechanism is explained below. Figure 3 Switching mode of the RRAM devices. (a) I-V curves for unipolar (nonpolar) switching where the switching direction is independent on the polarity of the applied Thiamet G voltage and (b) bipolar switching. In bipolar switching, SET and RESET occur at opposite polarity bias. Resistive switching mechanism Generally, depending on the conduction path, the switching mechanism can be classified as (1) filamentary-type and (2) interface-type, as shown in Figure 4. In the filamentary model, the switching originates from the formation/rupture of conducting filament in the switching material by the application of suitable external bias shown in Figure 4a [15, 17]. The filamentary paths are formed under SET and ruptured under RESET. Electrochemical migration of oxygen ions and redox reaction near the metal/oxide interface is widely considered as the possible mechanism behind the formation and rupture of the filaments [43].