A number of important commercial applications would benefit from the introduction of easily manufactured devices that exhibit current-controlled, or “S-type,” negative differential resistance (NDR). A leading example is emerging non-volatile memory based on crossbar array architectures. Due to the inherently linear current vs. voltage characteristics of candidate non-volatile memristor memory elements, individual memory cells in these crossbar arrays can be addressed only if a highly non-linear circuit element, termed a “selector,” is incorporated in the cell. Selectors based on a layer of niobium oxide sandwiched between two electrodes have been investigated by a number of groups because the NDR they exhibit provides a promisingly large non-linearity. We have developed a highly accurate compact dynamical model for their electrical conduction that shows that the NDR in these devices results from a thermal feedback mechanism. A series of electrothermal measurements and numerical simulations corroborate this model. These results reveal that the leakage currents can be minimized by thermally isolating the selector or by incorporating materials with larger activation energies for electron motion.
Use of aluminum alloyed zinc oxide (AZO) as a transparent contact to p-GaN has received significant attention for GaN/InGaN light emitting diodes applications. Reports show that AZO as deposited on p-GaN forms a Schottky contact given the large work-function difference between AZO and p-GaN [1]. However, utilization of a thin nickel layer inserted between the AZO and p-GaN can result in an ohmic contact [2], given that Ni forms an ohmic contact to p-GaN upon annealing. Here, we undertake simulation studies of this AZO/Ni interface as a function of Nickel layer. Simulation studies of the transport mechanism in this contact indicate the likelihood of a tunneling junction at the AZO/Ni interface, thus the entire AZO/Ni forming an ohmic contact to p-GaN. Potential to better control injected current density uniformity utilizing such a contact structure is also discussed.
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