# Progress in THz Rectifier Technology: Research and Perspectives
## 4 Perspectives in THz Rectifier Technology for Energy Harvesting Applications  
The widespread use of rectennas in RF and microwave range has suggested the inspiration for extending the concept of rectenna to THz, from infrared (IR) to solar optical frequencies [4]. The rectenna tuned at IR frequency can also be employed at night and has no particular efficiency limits [88]. Efficiencies above 85% at microwave frequencies have been observed in the laboratory; however, as the frequency increases, the device cannot simply be scaled down, due to the significant changes in material properties; consequently, the efficiency will be very low [4]. Schottky diodes are currently the building block in THz  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Diode Configuration** & **Nanoantenna** & \begin{tabular}{c} **Operating** \\ **Frequency** \\ **(THz)** \\ \end{tabular} & \begin{tabular}{c} **Maximum** \\ **Respusivity** \\ **(V\({}^{-1}\))** \\ \end{tabular} & \begin{tabular}{c} **Zero-Bias** \\ **Respusivity** \\ **(V\({}^{-1}\))** \\ \end{tabular} & \begin{tabular}{c} **Zero-Bias** \\ **Resistance** \\ **(f\(\Omega\))** \\ \end{tabular} \\ \hline \begin{tabular}{c} Exfoliated monolayer graphene- based \\ arrowhead-shaped diode [83] \\ \end{tabular} & \begin{tabular}{c} metal bowtie 15 nm \\ Cr/40 nm Au \\ \end{tabular} & 28.3 & 0.2 for \(V_{DS}\) = 1.5 V & 0.18 for \(V_{DS}\) = 0 V & 13 K \\ \hline \begin{tabular}{c} Exfoliated monolayer graphene- based \\ arrowhead-shaped diode [84] \\ \end{tabular} & \begin{tabular}{c} metal bowtie 15 nm \\ Cr/40 nm Au \\ \end{tabular} & Up to 160 & 0.8 for \(V_{DS}\) = 0.4 V & 0.3 for \(V_{DS}\) = 0 V & 19 K \\ \hline \begin{tabular}{c} Exfoliated monolayer graphene- based \\ arrowhead-shaped diode [85] \\ \end{tabular} & \begin{tabular}{c} metal bowtie 15 nm \\ Cr/40 nm Au \\ \end{tabular} & 28.3 & 0.2 for \(V_{DS}\) (V) = 1.4 V & 0.12 for \(V_{DS}\) = 0 V & 3 K \\ \hline \begin{tabular}{c} (CVD) monolayer graphene- based \\ arrowhead-shaped diode [86] \\ \end{tabular} & \
\begin{tabular}{c} metal bowtie Ti \\ (10 nm)/Au \\ (40 nm) \\ \end{tabular} & 28.3 & 0.3 for \(V_{DS}\) (V) = 0.5 V & 0.1 for \(V_{DS}\) (V) = 0 V & 5 K \\ \hline
\begin{tabular}{c} Z-Shaped graphene geometric diodes [87] \\ \end{tabular} & - & 28.3 & 2.4 for \(V_{0}\) (V) = 0.5 V & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 7: Summary of recent achievements in the field of graphene-based geometric diode for THz rectennas.  
Figure 24: Asymmetry dependence on carrier mean-free-path length for a graphene \(Z\)-diode. Reprinted with permission from ref. [87]. Copyright 2021 Nanomaterials.  
technologies up to 5 THz [33; 81]; however, as the frequency increases, this diode becomes slow [89]. Technological maturity has already been reached for this type of diode, so now no major changes can be expected with the current technological process to reduce the problems of series resistance and parasitic capacitance [90; 91; 92; 93]. Therefore, if the current trend continues, the diodes examined in this review, based on the principles of quantum tunneling effect [4], ballistic and quasi-ballistic transport [87], will have the possibility of surpassing the Schottky diode. The future of these diodes will be determined by the ability to improve their efficiency. The future of THz rectifier technology for EH applications promises to be intriguing. The main impact is that it will become possible to harvest and convert low-level energy that cannot be harvestable today. The possible perspectives advanced by the authors are highlighted as follows.