# THz Rectennas and Their Design Rules
## 8 Conclusions  
In this paper, some recent and earlier work on rectennas and their applications has been reviewed. More focus has been given to the factors that contribute to the decision-making design process, giving particular attention to the frequency, theory of operation and specific types of diodes. While sunlight provides an extremely high radiation intensity, we think it is unlikely to obtain an efficient design for frequencies in the several hundred THz range with the available technology for today. Lower frequency resources referred to as waste heat sources that radiate in the 30 THz range offer good abundance and have more encouraging experimental results, although their power is limited. While many forms of THz rectennas having been suggested, designed and even fabricated, there is no common ground among researchers on the best design or even the theory of operation. As some researchers assume tunneling as the dominant mechanism for their operation, others rely on the ballistic effect to build their models. Be it this or that, there is also the thermal effect, which is always present in more than one form. In order to simplify the election process, relying on the figures of merit given by (21)-(23) would yield an easier and clearer decision-making process. Therefore, out of the many available designs, tunneling, geometric and hybrid diodes hold high potentiality for deployment for this frequency range in any future design because their figures of merit are close to those imposed by Equations (21)-(23), and reaching the required figures is a matter of optimizing the diode design. Having said that, the general trend in the recent research overlooks the importance of matching between the diode and the antenna and just focuses on the proof of concept for the physical phenomenon, be it tunneling or ballistic carrier transportation. On the other hand, these diodes' \(I(V)\) relationship is not only nonlinear, but it can also be nonlocal with respect to time, so imposing more importance on accounting for this behavior in any future simulation. In other words, to optimize the models to reach efficient designs, the future model must take into account the physical aspect from the perspective of charge transport and the electrical aspect from the perspective of dynamic signal models together to have a more accurate and realistic simulation. Doing so is not easy, however, because the carrier transport is often subject to quantum physics at high frequencies. Especially, as the wave (Schrodinger) equation is used to describe the carrier transport at the mesoscopic scale, it incorporates many parameters when solving a quantum problem. Therefore, the modeled systems are not only quantumly complicated, but there is also a subjectivity issue. In other words, each device would need a separate physical model to describe its operation. We support this view, for example, with the models of [PERSON] and [PERSON] et al., who tried to formulate one model linking physical and electrical parameters in a few equations. Further tests on the models of [PERSON] and [PERSON] et al. approved that they are neither expandable nor amendable to account for higher frequencies as they have structural limitations in their formalism. On the other hand, having a separate simulation for each build of a system would be both computer and resource intensive and can be an intimidating work to do for many researchers. Therefore, we think that for any future and efficient development of the rectenna device, it is of primary importance to reach an accuracy-cost-compromising model such that acceptably accurate results can be reached without imposing too complicated physical models. Another culprit that can also distort the experimental output of a rectenna system is the presence of thermal effects. While the thermionic effect is expected to be present in many diode designs, Seebeck and thermally-assisted tunneling effects can be present, as well.