# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: State of the Art on Metal Multi-Insulator Metal (MI\({}^{n}\)M) Diode
With the same thickness (3 nm), the results indicate a major performance of MI\({}^{4}\)M rather than MI\({}^{2}\)M.  
Figure 10: (**a**) Current-Voltage (I-V) curve: (**b**) asymmetry; (**c**) nonlinearity of the MI\({}^{2}\)M and MI\({}^{4}\)M diodes. Reprinted with permission from ref. [60]. Copyright 2014 Austin Publishing Group. Figure 9: (**a**) Schematic diagrams of the cross-sectional view of the MI\({}^{2}\)M diode. Energy band diagram of the MI\({}^{2}\)M diode: (**b**) no bias applied; (**c**) negative; (**d**) positive bias applied to Ti. (**e**) Schematic diagrams of the cross-sectional view of the MI\({}^{4}\)M diode. Energy band diagrams of the MI\({}^{4}\)M diode; (**f**) no bias applied; (**g**) negative; (**h**) positive bias applied to Ti. Reprinted with permission from ref. [60]. Copyright 2014 Austin Publishing Group. A study conducted by [61] evaluates the performance of Al/Ta\({}_{2}\)O\({}_{5}\)/Al\({}_{2}\)O\({}_{3}\)/Al and Al/Nb\({}_{2}\)O\({}_{5}\)/Al\({}_{2}\)O\({}_{3}\)/Al fabricated on 4 \(\upmu\)m \(\times\) 4 \(\upmu\)m glass substrates by considering a thickness between 1 and 6 nm. The results indicate for Al (60 nm)/Nb\({}_{2}\)O\({}_{5}\) (3-6 nm)/Al\({}_{2}\)O\({}_{3}\) (1 nm)/Al, a higher current due to reduced band gap and barrier height than Al/Ta\({}_{2}\)O\({}_{5}\)(3-6 nm)/Al\({}_{2}\)O\({}_{3}\)/Al. Al/Ta\({}_{2}\)O\({}_{5}\)/Al\({}_{2}\)O\({}_{3}\)/Al, which shows _Asym_ of 18 at 0.3 V, _NL_ of 7.5 for a bias \(<\) 0.8 V, and \(S\) of 6 V\({}^{-1}\) at 0.2 V. Al/Nb\({}_{2}\)O\({}_{5}\)/Al\({}_{2}\)O\({}_{3}\)/Al shows _Asym_ of 7.6 for 0 V, _Asym_ of 6 at larger voltages, _NL_ of 6.8 at voltage \(<\) 0.8 V, and \(S\) of 9 V\({}^{-1}\) at 0.2 V. In [62], the authors present MI\({}^{2}\)M integrated into a micron-scale antenna for converting MID-IR radiation into electrical power. The diode Co/Co\({}_{3}\)O\({}_{4}\)/TiO\({}_{2}\)/Ti shows an area of 0.071 \(\upmu\)m\({}^{2}\), and it is able to obtain high responsivity at zero-bias voltage and low resistance near zero-bias. These properties make it suitable for EH applications. The diode is constituted by two different metal contacts, Co and Ti, with work functions 5.0 and 4.3 eV, whereas the relative dielectric constants \(\varepsilon_{r}\) of 13 and 15-110, respectively, for Co\({}_{3}\)O\({}_{4}\) and TiO\({}_{2}\) allow having low diode resistance. The results obtained for an area of 0.071 \(\upmu\)m\({}^{2}\) and from \(-\)0.3 to +0.3 V at 222 \({}^{\circ}\)C indicate responsivity at 1.2 V\({}^{-1}\) at zero-bias voltage with a resistance of 14 k\(\Omega\). In [63], the authors present MI\({}^{2}\)M with low zero-bias resistance, high responsivity, and insulator layers with a small difference in their electron affinities. The MI\({}^{2}\)M proposed is a Ti/TiO\({}_{2}\) (1 nm)/ZnO (0.5 nm)/Al, with electron affinities of 3.9 eV and 4.1 eV for TiO\({}_{2}\) and ZnO and work functions of 4.33 eV and 4.28 eV for the metals Ti and Al, respectively. These metals are excellent candidates for contact pads due to their low barrier heights. With thicknesses fixed at 1 nm for TiO\({}_{2}\) and 0.5 nm for ZnO and a contact area of 0.01 \(\upmu\)m\({}^{2}\), MI\({}^{2}\)M is integrated on an antenna whose resistance is of 55 \(\Omega\). The cutoff frequency obtained by considering a capacitance equal to 16.6 fF/\(\upmu\)m\({}^{2}\) and a time constant of approximately 9.1 fs is of 17.4 THz. The results obtained are zero-bias resistance of 312 \(\Omega\) with zero-bias responsivity of 1.6 V\({}^{-1}\) and maximum responsivity of 5.1 V\({}^{-1}\) at a bias of \(-\)200 mV. Diodes governed by electron resonance tunneling mechanism, via multiple quantum wells formation at the oxides interfaces, exhibit extreme asymmetry and high nonlinearity such as ultrahigh speed. In [64], the authors fabricated and characterized two MI\({}^{3}\)M devices consisting of Cr/Cr\({}_{2}\)O\({}_{3}\)/HfO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)/Cr and Cr/Cr\({}_{2}\)O\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\)/HfO\({}_{2}\)/Cr.