# Progress in THz Rectifier Technology: Research and Perspectives
###### Abstract  
Schottky diode (SD) has seen great improvements in the past few decades and, for many THz applications, it is the most useful device. However, the use and recycling of forms of energy such as solar energy and the infrared thermal radiation that the Earth continuously emits represent one of the most relevant and critical issues for this diode, which is unable to rectify signals above 5 THz. The goal is to develop highly efficient diodes capable of converting radiation from IR spectra to visible ones in direct current (DC). A set of performance criteria is investigated to select some of the most prominent materials required for developing innovative types of electrodes, but also a wide variety of insulator layers is required for the rectification process, which can affect the performance of the device. The current rectifying devices are here reviewed according to the defined performance criteria. The main aim of this review is to provide a wide overview of recent research progress, specific issues, performance, and future directions in THz rectifier technology based on quantum mechanical tunneling and asymmetric structure. energy harvesting; rectifying antenna; quantum tunneling; ballistic transport; quabatec  
rectenna, contraction of RECTifying antENNA [34]. This device is very attractive at MID-IR and visible frequencies and can have a significant impact in many areas. For terrestrial applications, solar radiation is null at night or in the presence of fog and is greatly reduced in humid areas. For the space applications, the success of any interplanetary exploration passes through correct generation of electricity in situ obtained by solar panels. However, they are not capable of producing electricity during night hours and dust storms. The IR radiation emitted by the Earth persists night and day; therefore, rectennas tuned in MID-IR could be implemented with rectennas tuned in the visible range to power small devices night and day. Rectennas at THz frequencies are considered potentially more efficient than solar cells that exceed the S&Q limit. Currently, the use of an antenna integrated with a rectifying diode has only been successfully demonstrated for microwave and radio frequency energy harvesting [4], because as the operation frequency increases (at infrared and visible frequencies), the conversion efficiency decreases. Cutting-edge efficiency of MID-IR rectennas even using 2D materials indicates values well below 1% [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. In the visible range with the use of carbon nanotubes (CNTs) it was possible to obtain conversion efficiency of up to \(\approx\)10\({}^{-6}\)%. This low efficiency value is due to several impediments such as fabrication challenges but also impedance mismatching between antenna (hundreds of ohms) and diode (\(\Omega\Omega\)) [16; 17; 18]. The architecture of the whole system with the related details is shown in Figure 1. It consists of three blocks: an antenna at the nanoscale, a diode optimized to rectify the high frequency signal, and a load. These components are commonly associated with other components not visible in Figure 1 that will be discussed in the text. In the first block, when the size of the antenna coincides with the wavelength of the incident EM wave, an energy transfer between them occurs. In this condition, called resonance condition, the collective oscillations of conduction electrons propagate at the metal-dielectric interface. This wave is referred to as surface plasmon polaritons (SPPs) and is localized at the interface between dielectric and metal with opposite sign permittivity. SPPs have the characteristic of decaying very rapidly as the distance from the metal surface increases. The movement of the SPPs induced by the intensity of the electric field (\(E\)) of the radiation of the incident EM wave generates an alternating current (AC) on the surface of the antenna.