# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: State of the Art on Metal Multi-Insulator Metal (MI\({}^{n}\)M) Diode
This diode is particularly suitable for EH applications due to a very small \(V_{bias}\). In [69], authors realized and characterized one asymmetric diode MI\({}^{2}\)M capable of rectifying up to 30 THz. The diode is Pt (150 nm)/Al\({}_{2}\)O\({}_{3}\) (1.5 nm)/TiO\({}_{2}\) (1.5 nm)/Ti (150 nm), with 20 \(\upmu\)m \(\times\) 20 \(\upmu\)m junction area, printed on 500 nm of SiO\({}_{2}\) substrate deposited on Si substrate. The main results obtained in term of FOMs indicate an _Asym_ of 17 at 5 V, _NL_ > 5.5 at 5 V, peak responsivity \(S\) (V\({}^{-1}\)) of 2 \(\times\) 10\({}^{4}\), and zero-bias differential resistance of 0.1 M\(\Omega\). Due to a very high \(V_{bias}\), this diode is not suitable for EH applications. In [71], the authors present a double insulator Ni-NiO\({}_{x}\)-ZnO-Cr tunnel diode. The surface roughness of the complete NiOx and ZnO stack is 3.2 nm over an area of 20 \(\upmu\)m \(\times\) 20 \(\upmu\)m. The turn-on voltage for this diode is nearly 250 mV. The band gap value of the NiO\({}_{x}\) is found to be 3.04 eV,while the band gap of ZnO at room temperature is 3.37 eV. The performance of two thin-film insulators is influenced by barrier heights, bias polarity, surface roughness of metal electrodes, uniformity of the dielectric, and junction area. Tunneling probability of the electrons increases with the decreasing insulator thickness but at the cost of asymmetry and nonlinearity. The work functions of Ni and Cr are 5.15 eV and 4.5 eV, respectively, whereas the electron affinity of NiO\({}_{\mathrm{x}}\) is of 0.6 eV. The barrier height difference is of 0.65 eV. The effective barrier thickness of 3 nm across the 20 \(\upmu\)m \(\times\) 20 \(\upmu\)m contact area is lower than the targeted thickness of 7 nm. The results obtained are an _Asym_ of 16 at 0.5 V and a current of 1.8 \(\times\) 10\({}^{-7}\) A at 0.5 V. Finally, in [72], the authors carried out a simulation study of the multi-junction insulator tunnel diode for solar energy harvesting applications. Specifically, in this article, the idea advanced by the authors [72] is to study various possible combinations of oxide materials and optimized thickness with fixed electrodes to obtain a high operating cutoff frequency (28.3 THz or 10.6 \(\upmu\)m). The authors' efforts are concentrated on considering only two insulators. Platinum (Pt; work function = 5.65 eV) as bottom metal and titanium (Ti; work function = 4.33 eV) as top metal for M11I2M2 diode were selected to obtain a high work function difference. In Figure 15a, the schematic of M11I2M2 is shown, where M1 and M2 represent the metals at the bottom and top and I\({}_{1}\) and I\({}_{2}\) represent the different oxide layers of the diode. Figure 15b represents the energy band diagram of an M11I2M2 diode with different barrier heights. These two insulators in Figure 15b have different electron affinities, which results in a discontinuity at their interfaces. Table 4 shows different combinations of oxide layers and optimized thickness. Figure 14: Schematic of (**a**) the layout of the MIIM device and (**b**) the energy band diagram. Figure 8:0 (**a**) Schematic of an M11I2M2 diode; (**b**) energy band diagram of M11I2M2 diode with different barrier heights. Reprinted with permission from ref. [72]. Copyright 2021 IOP Publishing. In the simulation model, the authors consider an overlap area of 100 nm\({}^{2}\), a standard antenna impedance of 50 \(\Omega\), and a voltage sweep range set from \(-1\) to 1 V. Figure 16 shows I-V response of diode for different oxide combinations. The zero-bias resistance and zero-bias responsivity are indicated in Figure 17a,b. In Table 5, the results for the oxide combination are reported. The difference in the electron affinities between the two insulator layers leads to an extra barrier at the interfaces of the oxide, which increases the asymmetry and the nonlinearity.