). However due to a lack of asymmetry, the second Ni electrode must be replaced with a metal of different work function in order to achieve rectification. This has been investigated by Hoofring et al, Krishnan et al and Esfandiari et al with Au, Cr and Pt contacts respectively, all displaying good asymmetrical I-V response. Other structures of dissimilar materials which have shown promising results are Al/Al2O3/Ni, Nb/Nb2O5/Pt, ZrCuAlNi/Al2O3/Al [60] and also Nb/Nb2O5/Ag [61]. More tested metal insulator combinations are listed in [57]. The highest FOMs were observed for Nb/Nb2O5/Pt, which displayed 1500 asymmetry, 4 nonlinearity and 20 (V−1) responsivity meeting the criteria outlined in Equations 31, 32 and 33. The results above show that using different metals with great work function difference between them asymmetric I-V can be improved. High barrier asymmetric (work function difference between metals) diodes provide a large responsivity. On the contrary, the MIM requires a low resistance, which is possible only with low barrier heights on both sides, that limits the asymmetry. A possible solution between asymmetry and diode junction resistance seem be Metal Insulator Insulator Metal. Finally, with Nonlinearity seem there is not correlation with Δ Ф.
4.7.2 Metal–insulator–insulator–metal diodes
Different Metal on the two sides with great work function difference between them (different barrier heights) can be used to improve requirements as Asymmetric I–V and responsivity. Another requirement, low resistance mandatory for matching antenna-diode requires low barrier heights on both sides (which limits the asymmetry). A solution introduced to respect the requirements above is Metal-Insulator-Insulator Metal (MIIM) itself a quantum tunneling device, not based on semiconductors. Adding another insulating layer between equal metals rectification properties can be improved moreover different electron affinity of insulators can improve asymmetry. The figure 14 shows a band structure of Metal-Insulator-Insulator-Metal junction with