**4. Tunnel barrier driven resistive switching**

The tunnel ReRAM based on oxides is peculiarly known as the nonvolatile memory that demonstrate area dependent resistive switching properties. Unity Semiconductor Company first introduced the tunnel ReRAM and then Rambus Labs followed the work. ReRAM employs thin insulator type oxide layer to control the current through quantum-mechanical tunneling as magnetic RAM. In contrast to other oxide based RRAM system showing movement of oxygen ions, the quantum-mechanical tunneling effect within the insulator or barrier provides a more regulated current values in different processes such as SET, RESET and

Read-out which facilitate improved device performance. The mobile oxygen ion traveling across the insulator or barrier tunnel shows considerable impact onto the tunnel properties. Also, an appropriate change in thickness of tunnel barrier or oxide effectively helps into regulate the device current levels. Moreover, such a control over *I-V* characteristics for SET and RESET states offers transistor or diode-like selector less fabrication of passive cross-point configuration (1R) based working memory devices. This type of operation memory devices exhibit self-select feature. A precise and optimized tunnel device geometry with regulated current conduction serves high device yeild due to small variability in cell-to-cell and wafer-to-wafer operation. Further, the multibit storage can also be realized even in a single memory cell owing to an analog transition of the SET and RESET processes. The area dependent switching properties also enables smaller technology nodes in near future. Since tunnel ReRAM is still in development stage, it suffers from a few critical issues like ease of processing, integration and optimum retention time [34].

The tunnel ReRAM is also a stratified structure containing a very thin tunnel oxide layer (in the range of 2–3 nm only) placed between conducting oxide and metal electrode. The conducting meal oxide is usually a metallic type perovskite oxides including (Pr, Ca)MnO3 or highly conducting La or Nb doped SrTiO3. The stable electrical insulators possessing high-*k* oxides such as ZrO2 or HfO2 are employed generally as the tunnel oxide barrier layer [35]. The noble metals like Pt are utilized as electrodes for electrical biasing. The use of noble electrode metal is of utmost since they do not

