# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: State of the Art of Metal Insulator Metal (MIM) Diode
The fabrication of the nanocretena is shown in Figure 6. The diode is composed of two different metals, gold and copper, with a copper oxide in between as an insulator. Gold (the lower antenna arm) and copper (the upper antenna arm) have a work function of 5.1 eV and 4.7 eV, respectively. The simulation results are carried out by considering a normally incident plane wave (z-axis) with an electric field intensity of 1 V/m and linear polarization parallel to the antenna axis (\(x\)-axis). Bias pads (20 \(\upmu\)m \(\times\) 32 \(\upmu\)m) are connected to the antenna. The results indicate maximum responsivity \(S\) (V\({}^{-1}\)) equal to 6, zero-bias responsivity equal to 4, and zero-bias resistance equal to 505 \(\Omega\). \begin{table}
\begin{tabular}{c c c c} \hline
**Material** & \(J_{ON}\) & \(Asym\) & NL & \(\mathbf{S}\) (V\({}^{-1}\)) \\ \hline Al/AlO\({}_{x}\)/Gr [23] & 1.0 A/cm\({}^{2}\) @ \(\pm\) 1 V & 2500 @ \(\pm\) 1 V & 3.8 & - \\ Gr/TiO\({}_{2}\)/Ti [42] & 7.5 A/cm\({}^{2}\) @ \(\pm\) 1 V & 520 & 15 & 26 \\ Ti/TiO\({}_{2}\)/Bilayer Graphene [43] & 0.1 A/cm\({}^{2}\) @ \(\pm\) 1 V & 9000 & 8 & 10 \\ Gr-h-BN-Gr [44] & 0.02 A/cm\({}^{2}\) @ \(\pm\) 1 V & 1000 & 40 & 12 \\ \hline \end{tabular}
\end{table} 
Table 2: MIG configurations and their contributions. Figure 5: THz antenna integrated with a Cu (100 nm)-CuO-Au (100 nm) (0.0045 \(\upmu\)m\({}^{2}\)) rectifier (rectenna) for harvesting infrared energy. Reprinted with permission from ref. [8]. Copyright 2014 Scientific Reports. The fabrication and characterization of Ti-TiO\({}_{2}\)-Al and Ti-TiO\({}_{2}\)-Pt tunnel diodes are given in [45]. In both cases, the thickness of the oxide is approximately 9 nm. The dioxide is formed by means of two different techniques, native oxide growth and plasma oxidation. In the first case, a silicon substrate is used for the MIM. The thickness of the titanium lower metal layer is of 70 nm. The thickness of the aluminum upper metal is of 150 nm. In the second case, the thickness of the titanium is of 80 nm. Measurements indicate for Ti-TiO\({}_{2}\)-Al (21,287 um\({}^{2}\) area) an \(S\) of 18 V\({}^{-1}\), at DC voltage of 0.09 V, an \(NL\) of 6.5 at 0.33 V, and a current density \(J_{\rm ON}\) of 10\({}^{-1}\) A/cm\({}^{2}\) at 1 V. The results for Ti-TiO\({}_{2}\)-Pt indicate an \(S\) of 15 V\({}^{-1}\) at DC voltage of 0.495 V, an \(NL\) of 15 at 0.5 V, and a current density \(J_{\rm ON}\) of 10\({}^{0}\) A/cm\({}^{2}\) at 1 V. In [46], the authors present fabrication and characterization of eight Nb/Nb\({}_{2}\)O\({}_{5}\)-based MIM diodes. The thickness of the Nb\({}_{2}\)O\({}_{5}\) layer is 15 nm, whereas the thickness of the Nb layer is around 100 nm. For all eight MIM combination diodes, the highest reported rectification speed (\(\sim\)150 THz) has been obtained. Samples were fabricated on p-type Si wafers. Measurement indicates promising results for Nb/Nb\({}_{2}\)O\({}_{5}\)/Cu, Nb/Nb\({}_{2}\)O\({}_{5}\)/Ag, Nb/Nb\({}_{2}\)O\({}_{5}\)/Au, and Nb/Nb\({}_{2}\)O\({}_{5}\)/Pt. In particular, Nb/Nb\({}_{2}\)O\({}_{5}\)/Pt exhibits an \(Asym\), an \(NL\), and an \(S\) of 1500, 4, and 20 V\({}^{-1}\) at 0.5 V, respectively, whereas Nb/Nb\({}_{2}\)O\({}_{5}\)/Cu, Nb/Nb\({}_{2}\)O\({}_{5}\)/Ag, and Nb/Nb\({}_{2}\)O\({}_{5}\)/Au exhibit an \(Asym\), an \(NL\), and an \(S\) of 1500, 8, and 20 V\({}^{-1}\) at 150 mV, respectively. The design, fabrication, and characterization of an asymmetric diode Au/Al\({}_{2}\)O\({}_{5}\)/Pt operating up to 28.3 THz printed on 1.5 um of Si substrate are given in [47]. The Au/Al\({}_{2}\)O\({}_{3}\)/Pt diode exhibits a highly nonlinear I-V characteristic due to which high zero-bias responsivity of 10 V\({}^{-1}\) is achieved with an on current of 6 \(\times\) 10\({}^{-7}\) A around 1 V. In [48], the authors present high-frequency Ni-NiO-Ag metal-insulator-metal tunnel diodes fabricated via anodic aluminum oxide (AAO) templates.