# Progress in THz Rectifier Technology: Research and Perspectives
## 3 THz Diode Technology: General Information on Geometric Diode (GD)  
In light of the above discussion, a planar structure designed to operate at the THz frequency must meet two requirements, an extremely low RC time constant and a high coupling efficiency (the real part of the impedance of the diode has to be relatively close to that of the nanoantenna). These requirements cannot be obtained simultaneously; in fact, the reduction of the overlapping area of diode involves a low \(C\) whereas R increases proportionately.  
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Material**}} & \multicolumn{1}{c}{**Cut-Off**} & \multirow{2}{*}{_Jov_} & \multirow{2}{*}{_Asym_} & \multirow{2}{*}{_NL_} & \multicolumn{1}{c}{\(\mathbf{S}\)} & \multicolumn{1}{c}{**Zero-Bias**} & \multirow{2}{*}{**Resistance**} \\  & & & & & & & & \\ \cline{5-8}  
A completely different approach is a device referred to as a geometric diode. It shows rectification properties thanks to its asymmetric structure that induces different current levels for forward and reverse motion [73]. The asymmetric structure observable in Figure 18 forces one part of the charge carriers to flow in one direction only from left to right through \(d_{neck}\), whereas a second part of the charge carriers will be blocked and reflected away from \(d_{neck}\) constriction [7; 19; 20; 23; 24].  
During the design phase, the diode must meet some requirements. Long mean-free path length (MFPL) is the average distance that an electron travels before substantially changing its direction due to one or more successive collisions with other electrons, atoms, or defects. If MFPL is short, a faster response time is verified; however, as MFPL decreases, the collision time increases. When the condition \(d_{neck}<\) MFPL is verified, the electrons can move ballistically and the probability that the electrons pass in reverse direction will decrease significantly, obtaining a high rectification performance [74]. As \(d_{neck}\) increases, electrons can move more easily through the constriction with an increase in the magnitude of the current. Another design parameter is \(d_{shoulder}\). If it increases, an increase in current through \(d_{neck}\) and an increase in the asymmetry are observed simultaneously. However, above a certain shoulder width, both the current and the asymmetry change. Therefore, a larger \(d_{shoulder}\) will increase the forward current and a smaller \(d_{neck}\) will reduce the reverse current [14; 55; 56]. Due to asymmetry in the structure, no external voltage bias is applied (zero-bias turn-on voltage), making this device adequate for EH applications. This planar structure presents both low resistance and ultra-low capacitance, overcoming the capacitance/resistance tradeoff. The metals possess a high current density \(J_{ON}\) up to \(10^{7}\) A/cm\({}^{2}\); however, the MFPL is only a few nanometers, approximately from 10 to 60 nm at room temperature. A good choice to fabricate this diode type is represented by graphene, which, when placed on a silicon oxide substrate SiO\({}_{2}\) above a substrate Si layer, shows current density \(J_{ON}\) up to \(10^{8}\) A/cm\({}^{2}\) and MFPL up to 1 \(\upmu\)m. In addition, the graphene allows the electrons to move freely in the plane without encountering scattering points on the contrary in metals, where electrons move through a 3D plane encountering scattering points, impurities, and thermal vibrations. However, 2D material requires ultrafine lithography to form the device and this makes the manufacturing process challenging [75; 76; 77; 78; 79; 80; 81; 82].