# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: State of the Art on Metal Multi-Insulator Metal (MI\({}^{n}\)M) Diode
By optimizing the film thickness ratio, it is possible to achieve current density \(J_{\text{ON}}\) of approximately 10\({}^{8}\) A/m\({}^{2}\) with maximum _Asym_ of 9. In [66], the authors realized and characterized an asymmetric diode MI\({}^{2}\)M capable of rectifying up to 30 THz. The investigated diode is Cr (100 nm)/Cr\({}_{2}\)O\({}_{3}\) (3 nm)/Al\({}_{2}\)O\({}_{3}\) (3 nm)/Ag (100 nm), printed on 500 nm of SiO\({}_{2}\) substrate deposited on 500 \(\upmu\)m Si. The fabrication process for this diode is reported in [66]. The main results obtained in terms of FOMs indicate a density current (J-V) response \(J_{\text{ON}}\) of 3 mA/cm\({}^{2}\) at 0.5 V, and _Asym_ > 280 at 0.4-0.5 V. In [67], the authors present MI\({}^{2}\)M quantum electronic tunneling devices suitable for high-speed rectifiers. In this context, parameters such as nonlinearity and asymmetry are obtained through cascaded potential barrier architecture and similar metallic electrodes. Considering this, the authors present an operational cascaded potential barrier Cr/Al\({}_{2}\)O\({}_{3}\)-HfO\({}_{2}\)/Cr diode in Figure 14a. Although metal electrodes are of the same material, the cascaded potential barrier profile is asymmetric at zero-bias voltage. In forward bias condition, the electrons pass through a single potential barrier with a high probability of tunneling. In fact, the electrons have to cross the thickness of only one oxide layer and not two oxide layers as in the case of a reverse bias condition. This alteration of tunneling mechanism in forward and reverse bias voltages is attributed to the enhanced nonlinearity peculiar to multi-insulators diode. This diode is constituted by 2 nm Al\({}_{2}\)O\({}_{3}\) layer and 2 nm HfO\({}_{2}\) layer, which provides an overall thickness of 4 nm. The potential barrier in Figure 14b is \(\varphi_{1}=3.05\) eV corresponding to Al\({}_{2}\)O\({}_{3}\) and \(\varphi_{2}=1.75\) eV corresponding to HfO\({}_{2}\). In detail, both electrodes are selected to be of the same metal M1 = M2 = Cr with 100 nm thick Cr layer deposited on a SiO\({}_{2}\) substrate. The results indicate in the forward direction (\(V>0\)) a current of 64 uA at \(V=3\)V and in the reverse direction at \(V=-3\) V only 7 uA. Although the diode is made of the same metals, the diode shows high asymmetry as the oxide layer is modified to form a cascaded potential barrier layer; therefore, the barrier height symmetry is broken and asymmetric I-V behavior is observed. The results indicate an _Asym_ of 1.5 around zero-bias voltage and an _Asym_ of 10 around 3 V. The _NL_ is of 4 around 1 V and of 10 around 2.3 V. The addition of another insulator layer in the MIM configuration provides an increase in asymmetry and nonlinearity. In forward bias condition, resonant tunneling leads to great conductivity while maintaining nonlinearity. In reverse bias condition, step tunneling results in high asymmetry with a sharp turn-on. In [68], the authors realized and characterized an asymmetric diode MI\({}^{2}\)M capable of rectifying up to 30 THz. The diode is ZCAN (ZrCuAlNi 150 nm)/HfO\({}_{2}\) (5 nm)/Al\({}_{2}\)O\({}_{3}\) (3 nm)/Al (150 nm), printed on 100 nm of SiO\({}_{2}\) substrate deposited on Si substrate. The fabrication process for this diode is reported in [68]. The main results obtained in term of FOMs indicate an _Asym_ > 10 at 0.8 V, and a _NL_ > 5 at 0.8 V. In [69; 70], the authors realized and characterized one asymmetric diode MI\({}^{2}\)M capable of rectifying up to 30 THz. The diode is Pt (150 nm)/HfO\({}_{2}\) (1.5 nm)/TiO\({}_{2}\) (1.5 nm)/Ti (150 nm), with 20 \(\upmu\)m \(\times\) 20 \(\upmu\)m junction area, printed on 500 nm of SiO\({}_{2}\)/Si substrate. The fabrication process for this diode is reported in [69; 70]. The main results obtained in term of FOMs indicate an _Asym_ of 10 at 0.8 V, _NL_ > 5.5 at 0.8 V, peak responsivity \(S\) (V\({}^{-1}\)) of 2 \(\times\) 10\({}^{4}\), and zero-bias differential resistance of 0.1 M\(\Omega\).