# THz Rectennas and Their Design Rules
## 2 Historical Notes
### Targeting Higher Frequencies
Hence, it reflects the importance of matching between the antenna and the successive parts the diode in this case. Finally, \(\eta_{j}\) is the diode efficiency measured in A W\({}^{-1}\). Thus, it sets the unit of the final efficiency \(\eta\) to A W\({}^{-1}\) because all other efficiencies are dimensionless. This framework helps with viewing the recent work in this field such that developing the antenna or nantenna part is related to optimizing the values of \(\eta_{a}\) and \(\eta_{s}\). On the other hand, increasing the values of \(\eta_{c}\) and \(\eta_{j}\) happens only by using well-matched and efficient diodes; thus suggesting that the increased efficiency is the result of both actions. In order to bring more balance to the terms of (1), it might be favorable to increase the values of \(\eta_{c}\) and \(\eta_{j}\) to get a real improvement as compared to earlier findings. Indeed, the diode's poor efficiency has been addressed as the bottleneck towards reaching an efficient rectenna design in the THz region [29]. In fact, improving the diode efficiency and matching is related to many other factors, which will be discussed in more detail later on. In order to avoid confusion, these factors are categorized as shown in Figure 3. From the authors' point of view, the following reasons have a major impact on the rectifier diode efficiency:  
* Frequency represents the frequency of the impinging electromagnetic wave on the antenna element. Recent trials in the THz range extend over a very wide frequency range for both modeled and realized systems with frequencies starting from \(1\,\mathrm{THz}\)[30], reaching the ultra violet range and exceeding \(800\,\mathrm{THz}\)[31]. Strictly speaking, over this very wide range, the proposed mechanisms and expected efficiencies for implementation vary largely as will be explained later. * The theory of operation is a controversial issue for rectennas since there are many theories of operation and so many diode types that can be structurally different based on their different theoretical framework. The difference is not only between different diode types, but also extends to having more than one theory to explain the same diode behavior. One good example is the tunneling diode as mentioned earlier in this section. A review of the widespread theoretical frameworks will be provided in Section 4. * Power level refers to the expected power level of the received signal, which can vary largely for irradiation intensities ranging from \(60\,\mathrm{W}\,\mathrm{m}^{-2}\) for ground radiation [32] up to more than \(1000\,\mathrm{W}\,\mathrm{m}^{-2}\) for visible light during daylight [33]. This factor can have a significant impact both on the design of the antennas and on the correct choice of the rectifying diode. Furthermore, it does not refer only to the signal input amplitude, but also largely depends on whether the signal is narrow or wide band, the coherence between different frequency components and even the distance between the source and the rectenna and how this can affect the coherence of the received wideband signal. [PERSON] and [PERSON] [34] provide a detailed account of this issue as will be discussed later. * Thermal effect: Although it might be argued that this factor is better considered with the theory of operation, the thermal effect is present in almost every diode application and can even take more than one form in the same application as will be seen later; the most common of which is the thermionic effect [35]. * Fabrication aspects: After a theoretical model is adopted, it would be realized using the available methods for fabricating the rectenna system at the nano-scale. This can be a challenging task, particularly, with small components whose dimensions are in the nm range. Although there are some promising prototypes efficiency-wise [36], which involves a vacuum clearance at the  
Figure 3: Factors affecting the rectifier diode efficiency. nanometer scale between the diode electrodes, realizing them in practice is a real challenge.