# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: Open Questions
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Description** & **Benefits** & **Common Issues** \\ \hline \multirow{3}{*}{_f_} & \multirow{3}{*}{Overall efficiency} & Up to 100\% under monochromatic illumination; Up to 44\% under broadband solar illumination & The real total efficiency is around 0.001\% due choice of materials and antenna design; antenna/diode mismatch; very low responsivity \\  & & & \\ \hline \multirow{3}{*}{_J_} & \multirow{3}{*}{Tunneling-current density} & \multirow{3}{*}{Able to convert AC to DC} & An insulator layer with a thickness less than 3 nm remains challenging \\  & & & \\ \hline \multirow{3}{*}{_fc_} & \multirow{3}{*}{Cut-off frequency} & At present the maximum cut-off frequency is 343 THz & Very low \(\tau_{D}\) involves thickness of the insulator layer beyond 5 nm which reduces the tunneling current and increases the diode resistance \\ \hline \multirow{3}{*}{_Asym_} & \multirow{3}{*}{Asymmetry} & \(Asym>1\) indicates rectification in the diode & An insulator layer with a thickness less than 3 nm remains challenging \\  & & & 3 nm remains challenging \\ \hline \multirow{3}{*}{_NL_} & \multirow{3}{*}{Nonlinearity} & \(NL>3\) indicates the measure of the deviation from a linear resistor & As the insulator layer increases, the value of the tunneling current decreases and diode resistance increases \\ \hline \multirow{3}{*}{_S_} & \multirow{3}{*}{Responsivity} & If \(S>3\) the diode rectifites the signal efficiently & An increase in high barrier involves large resistance \\  & & & \\ \hline \multirow{3}{*}{\(R_{0}\)} & \multirow{3}{*}{Diode resistance} & If \(R_{4}=R_{0}\) it involves: maximum transfer of the energy captured by the antenna toward the load; high tunneling current density & If the resistance value is very low, the diode shows very low values for responsivity and non-linearity \\ \cline{1-1} \cline{2-4}  & & & \\ \cline{1-1} \cline{2-4}  & & & \\ \cline{1-1} \cline{2-4}  & & & \\ \cline{1-1} \cline{2-4}  & & & \\ \hline \multirow{3}{*}{TOV} & It represents the positive voltage in order to “turn on” the diode & TOV has a low value for thin insulator layer & Very low nonlinearity \\ \cline{1-1} \cline{2-4}  & \multirow{3}{*}{Ultrathin insulator layer in between two dissimilar metal contacts} & The current state of the art indicates rectification signals up to 343 THz & Higher FOMs and low resistance cannot be obtained with a single insulator layer \\ \hline \multirow{3}{*}{\(\mathrm{M}^{2}\mathrm{M}\) or \(\mathrm{M}^{2}\mathrm{M}\) diodes} & Multiple insulator layers sandwiched in between two similar or dissimilar metal contacts & Higher efficiency compared to MIM; higher FOMs and low resistance compared to MIM; similar metals can also be used as contacts & Higher bias to move the electrons out of the quantum well (QW): by inserting two or more insulator layers, the tunneling resistance becomes high, by reducing the current. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of benefits and common issues for planar diodes at THz frequencies. moreover, these two parameters increase linearly by increasing the layer thickness. The thin dielectric layer and the low barrier height allow high transmission probability which increases exponentially with the decrease of the insulator thickness. To minimize TOV and maximize asymmetry and nonlinearity, the electron affinity of the insulator should be close to one of the metal work function values to produce a low barrier height. The cut-off frequency of the diode is governed by the RC (resistance-capacitance) time constant. The capacitance has to be kept as small as possible to increase the operating frequency of the MIM diode. The contact area and the thickness of the insulator layer mainly affect the capacitance. By increasing the thickness \(d\) of the insulator layer or reducing the contact area, it is possible to minimize the capacitance. However, as the thickness of the dielectric layer increases, the nonlinearity of I-V characteristics increases and the probability of tunneling decreases.