# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: State of the Art on Metal Multi-Insulator Metal (MI\({}^{n}\)M) Diode  
Various MI\({}^{n}\)M diodes with different combinations of oxide layers (in particular \(n=2,3\), and 4) and electrodes have been reviewed. FOMs values are obtained, considering selection of materials, thickness of oxides, overlapping area, and fabrication techniques. By considering two or more insulator layers with different electron affinities, the asymmetry and nonlinearity values can influence the behavior of MI\({}^{n}\)M diodes. A study conducted by [59] shows two MI\({}^{2}\)M W/Nb\({}_{2}\)O\({}_{5}\) (3 nm)/Ta\({}_{2}\)O\({}_{5}\) (1 nm)/W and W/Nb\({}_{2}\)O\({}_{5}\) (1 nm)/Ta\({}_{2}\)O\({}_{5}\) (1 nm)/W able to rectify up to 150 THz. The most relevant results indicate an \(S\) of 11 V\({}^{-1}\) at 0.02 V. A study conducted by [60] aims to observe the impact of the number of insulators on performance of the diode. The authors fabricated and characterized a metal-insulator-insulator-metal (MI\({}^{2}\)M), and a metal-insulator-insulator-insulator-metal (MI\({}^{4}\)M). The selection of metals and insulators is conditioned by parameters such as work functions and difference in the electron affinities. For MI\({}^{2}\)M diode, chromium (Cr) constitutes the first electrode M1, and titanium (Ti) constitutes the second electrode M2. Insulator layers such as TiO\({}_{2}\) and Al\({}_{2}\)O\({}_{5}\) are chosen for their difference in the electron affinity. Cr (60 nm) and Ti (100 nm) are deposited on a SiO\({}_{2}\) substrate. For MI\({}^{4}\)M diode, the used metals are the same as MI\({}^{2}\)M. The total thickness of the insulator layers in the MI\({}^{4}\)M diode is 3 nm, with each single insulator layer thickness of 0.75 nm. Cr/TiO\({}_{2}\)/Al\({}_{2}\)O\({}_{5}\)/Ti for MI\({}^{2}\)M diode and Cr/TiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)/TiO\({}_{2}\)/Al\({}_{2}\)O\({}_{5}\)/Ti for MI\({}^{4}\)M diode are indicated in Figure 9.  
For MI\({}^{2}\)M diode, there are three different interfaces between two metals. Each interface has a different potential barrier value. Figure 9b shows MI\({}^{2}\)M without applying any voltage. The geometry of the energy band diagram is influenced by work functions and electron affinities. In Figure 9c, by applying a bias to one of the metals, the shape of the energy band diagram changes. When a negative bias is applied to the Ti, electron flow occurs from Ti to Cr and is prevented from the opposite side (from Cr to Ti). In Figure 9d, when a positive bias is applied to the Ti, electron flow occurs from Cr to Ti and is prevented from the opposite side (from Ti to Cr). For MI\({}^{4}\)M diode, there are five potential barrier interfaces between the Cr and Ti. Figure 9e shows a schematic diagram of the MI\({}^{4}\)M diode. Figure 9f shows MI\({}^{4}\)M without applying any voltage (zero-bias). Under the bias voltage of \(-\)1 V, Figure 9g shows a current higher in the MI\({}^{4}\)M diode than the MI\({}^{2}\)M diode, owing to the difference in the tunneling distances. The same compartment is valuated at +1 V in Figure 9h. MI\({}^{4}\)M diode shows a positive current density higher than the MI\({}^{2}\)M diode. The results observed in Figure 10 indicate, I-V curve, asymmetry, and nonlinearity of the Cr/TiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)/Ti and Cr/TiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)/TiO\({}_{2}\)/Al\({}_{2}\)O\({}_{5}\)/Ti. MI\({}^{4}\)M offers more current than MI\({}^{2}\)M above +1 V bias. At +1.8 V, the current is 0.17 \(\mu\)A for MI\({}^{2}\)M and 0.85 \(\mu\)A for MI\({}^{4}\)M. Figure 10a indicates that MI\({}^{4}\)M diode shows a higher turn-on voltage than that of MI\({}^{2}\)M. In Figure 10b, the _Asym_, the 3 for MI\({}^{2}\)M and 90 for MI\({}^{4}\)M diode at +1.6 V. In Figure 10c, the _NL_ for MI\({}^{4}\)M diode is 6 around  
Figure 8: Step tunneling: (**a**) forward bias, (**b**) reverse bias. 0.8 V whereas for MI\({}^{2}\)M, the \(N\)\(L\) is 3.8 at the same voltage.