# THz Rectennas and Their Design Rules
## 1 Introduction  
In recent years, there has been an increasing demand to reduce the pollution of the environment to meet the stringent measures posed by the governmental bodies throughout the world. In 2020, the cutting back of greenhouse gases should be 20% compared to 1990 levels [1]. In a related development, the British government's National Energy Efficiency Action Plan (NEEAP) set an even higher figure to meet for residential housing carbon emission, where a reduction by 31% compared to 1990 levels should be met [2]. Consequently, it becomes desperate to look for more efficient and, of course, less carbon-emissive energy sources that can stand up to these measures within this relatively limited time. It is, therefore, expected that the renewable energy sources such as wind and sun radiation can verify help with achieving this goal without producing carbon emissions. While there can be some good mechanical solutions to this issue, this article is concerned about the recent development in proposed electrical solutions to harvest energy in the THz region using antennas that receive AC energy and rectify the waveform using a nonlinear diode. This system is often referred to in the literature as rectenna. The idea of harvesting radiation directly and converting into an electrical current has been around for many decades starting from the photovoltaic effect earlier referred to as the Becquerel effect and moving forward to optimize the photovoltaic cells and look for other alternatives to achieve better performance in addition to minimizing their production costs. The motivation to achieve this goal is what fuels the demand for increasing the research on rectennas. [PERSON] et al. [3] theoretically argue for rectennas, working in the solar frequency range, for which power efficiency in excess of 80% can be achieved. In fact, efficiency figures in excess of 90% have been reported by [PERSON] [4] by using fast-switching diodes and optimizing the match between the antenna and the rectifying diode for his2.45 GHz rectenna system. However, as the operation frequency increases, many critical limitations come into play and limit the achievable efficiency to well below 1% [5]. The aim of this paper is to review the past and present trials on rectennas focusing on the important parameters and figures of merit for researchers to consider in their endeavors to reach more efficient rectenna systems. In Section 2, a historical background to the current state-of-the-art situation is given starting from the discovery and development of the photoelectric effect. The pronounced [PERSON] prototype of the rectenna helicopter [4] is, also, briefly discussed. Then, we move on to higher frequencies for energy harvesting from the naturally-available sources. Section 3 is dedicated to the relation between the operation frequency and the naturally-available sources' power levels. The correct choice for the theory of operation based on the frequency will be justified. Section 4 will elaborate on the different theories to explain and model the diodes. Their main features and drawbacks will be discussed with a follow up of the most recent amendments to them and to the available models for rectifier simulation. The models will be recognized based on the operation condition (i.e., with/without insulator illumination) and the suitable models for each diode design (i.e., tunneling and geometric diodes differ in their suitable models). Section 5 is dedicated to analyzing the impact of the incident signal power on the correct choice of the diodes. This is a very generic term and refers to the received signal amplitude, frequency bandwidth, the received signal polarization and the impact of these factors on the best modeling and diode structure. The thermal effect, which is always present at above zero temperatures is discussed in Section 6 with more focus on the available thermal effects such as the thermionic emission and Seebeck effect, which has a promising future for its use for rectenna detectors.