**3. Conclusion**

We overviewed some essential techniques to fabricate analog memristive devices. Based on the techniques that we covered here, we noticed that most of the analog memristors tend to have or work at a lower current or conduction than that of digital memristors; the resistance of the On and the Off states is higher prior to the digital-to-analog transformation. Henceforth, we can assume that the employment of low voltage or current favors the exhibition of analog behavior, and thus, the synaptic characteristics can be observed easier. We hypothesized that some other way that might work to induce digital-to-analog transformation in the memristors is to engineer the partial formation of the conductive filament; this can be done by intentionally decrease the compliance current and the voltage operation. **Table 1** summarizes the switching characteristics and synaptic behavior of several devices in reported studies. It is clear that the devices that endure digital-to-analog switching transformation often suffer from low memory window and dynamic


#### **Table 1.**

*Several important examples of digital-to-analog switching transformation in published literature.*

*Practical Approach to Induce Analog Switching Behavior in Memristive Devices: Digital… DOI: http://dx.doi.org/10.5772/intechopen.98607*

range. We assume that analog switching in memristor device can only work when the switching region in the cell is small or no major structural changes happen due to the repeated switching cycles or pulses.

We also addressed several challenges in the analog memristor devices that could hinder the commercialization of these devices. First, the non-linear switching modes, such as complementary and diode-like modes, have unique advantages in integrating the memristors in the array configurations; however, the synaptic reliability of the devices having these modes is still less understood, and extensive investigation is needed to study the efficacy of these modes on the synaptic plasticity. Secondly, analog memristors often suffer from short retention [14], which may not be suitable for data storage applications; however, this property opens another opportunity to modulate the long- and short-term memory plasticity of the devices that can be useful for neuromorphic computing applications. Hence, the trade-off between short retention and synaptic plasticity should be carefully managed to fit with the desired application. Third, the cycle-to-cycle operation can promote internal cell resistance variability that degenerates the analog behavior and could lead to the occurrence of digital switching [32]. Analog devices made with interfacial layer techniques may suffer from this problem due to the repeated redox reactions at the interfacial layer. Therefore, we hypothesized that a careful choice of the insertion layer plays a crucial role in achieving long cycle endurance and, thus, may prolong the exhibition of analog behavior.
