# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: State of the Art of Metal Insulator Metal (MIM) Diode
With a contact area of 3.1 \(\times\) 10\({}^{-4}\) um\({}^{2}\) and an insulator thickness (NiO) of 6 nm, the results obtained indicate a current of 150 uA around 1.0 V, \(Asym\) of 5 around 1.0 V, \(NL\) of 3.0 around 0.5 V, zero-bias responsivity \(S\) (V\({}^{-1}\)) of 5.8, and maximum responsivity \(S\) (V\({}^{-1}\)) of 8.5 around 0.1 V. These results indicate great potential for high-frequency applications. In [49], the authors present metal-insulator-metal diodes based on alkyltrichlorosilane self-assembled monolayers (SAMs) with different alkyl chain lengths. The insulator SAM is in between two metal contacts, Pt (5.65 eV) and Ti (4.33 eV). The electronic properties of the MIM diodes can be tuned by controlling the alkyl chain length of the SAMs. Alkyltrichlorosilane (SiCI\({}_{3}\)-(CH\({}_{2}\))\({}_{n-1}\)-CH\({}_{3}\)) SAMs are ultrathin high-quality dielectric films with a well-controlled structure. The diodes were fabricated on 2-inch Si wafers with a 300 nm thermally grown SiO\({}_{2}\) layer. Standard photolithography was used to pattern the metal contacts. The alkyltrichlorosilane SAM is sandwiched between two metal contacts, Pt and Ti. The overlapping region between the Ti and Pt contacts determines the effective area of the diode, which is 100 um\({}^{2}\). The zero-bias responsivity \(S\) for the diode based on 1.20 nm, 1.34 nm, 1.57 nm, and 2.23 nm films was found to be 4.1, 4.6, 4.7, and 8.0 V\({}^{-1}\), respectively. In addition, the zero-bias dynamic resistance \(R_{d}\) for the diodes based on 1.20 nm, 1.34 nm, 1.57 nm, and 2.23 nm films was found to be 32 k\(\Omega\), 71 k\(\Omega\), 464 k\(\Omega\), and 5 G\(\Omega\), respectively, as the thickness increases. In addition, parameters such as asymmetry, nonlinearity, and responsivity indicate that the diode based on 2.23 nm has the highest \(Asym\), strongest \(NL\), and highest \(S\), the values of which are 117.8, 6.8, and 20.8 V\({}^{-1}\), respectively. The only negative aspect for this asymmetric diode is a high \(R_{d}\) value that involves a potential mismatch between radiative element and diode rectifier. In [29], the authors present metal-insulator-metal diodes. The best results were obtained for two diodes, Nb/Nb\({}_{2}\)O\({}_{5}\)/Pt and Nb/TiO\({}_{2}\)/Pt, with 13 nm of insulator layer. These two types of diodes show a low TOV of 100 mV and an operating frequency up to 30 THz. At a voltage bias of 0.5 V, the Nb/Nb\({}_{2}\)O\({}_{5}\)/Pt MIM structure exhibits \(NL\) and \(Asym\) values of \(\sim\)3.8 and \(\sim\)130, respectively, while the Nb/TiO\({}_{2}\) /Pt MIM structure exhibits \(NL\) and \(Asym\) values of \(\sim\)3.5 and \(\sim\)80, respectively. In [27], the authors fabricated several metal-insulator-metal diodes based on the Nb/Nb\({}_{2}\)O\({}_{5}\)/X material system. The symbol X represents the second electrode (M2) realized by several materials combinations: Nb, Ag, Cu, Ni, Au, and Pt. The thickness of the Nb\({}_{2}\)O\({}_{5}\) was typically 15 nm. The top contact metals were deposited and patterned using a lift-off procedure on the Si/Nb/Nb\({}_{2}\)O\({}_{5}\) samples to create devices with a 6400 um\({}^{2}\) active area. Six different top contact metals were combined: Ag, Cu, Ni, Au, and Pt to Nb/Nb\({}_{2}\)O\({}_{5}\); the highest FOMs at 0.5 V are obtained for Nb/Nb\({}_{2}\)O\({}_{5}\) (15 nm)/Ni (300 K) and Nb/Nb\({}_{2}\)O\({}_{5}\) (15 nm)/Au (300 K). _Asym, NL, and S_ are 396.5, 7.1, 8.5 V\({}^{-1}\) for Nb/Nb\({}_{2}\)O\({}_{5}\)(15 nm)/Ni (300 K), and 1430.8, 8.0, 7.0 V\({}^{-1}\), respectively, for Nb/Nb\({}_{2}\)O\({}_{5}\) (15 nm)/Au (300 K). In [50], the authors present a new metal-insulator-metal capacitor based on SrTiO\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\)/SrTiO\({}_{3}\) for RF applications. The MIM capacitors were fabricated on a 5 \(\upmu\)m SiO\({}_{2}\) deposited on silicon wafers. The thinner Al\({}_{2}\)O\({}_{3}\) induces a higher capacitance density at the price of degrading leakage current while the thicker Al\({}_{2}\)O\({}_{3}\) would cause the opposite effects.