# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: State of the Art of Metal Insulator Metal (MIM) Diode  
The introduction of graphene as one of the possible electrodes is justified by virtues such as the band gap tunability, the very high electrical conductivity, and the thin-film fabrication process on a rigid or flexible substrate. In recent years, diodes based on graphene referred to as metal-insulator-graphene (MIG) have been realized. With the addition of boron nitride insulator (H-BN), it is possible to reduce the resistance of MIG. The work function of graphene is adjustable through the application of an electrical field, chemical doping, metal deposition, or plasma treatment. For EH application, the work functions of graphene must be adjusted using the chemical doping method [14; 40; 41]. The state of the art and the latest results for MIG and MIM are presented as follows. A study conducted by [23] shows an Al/AlO\({}_{\text{X}}\)/Gr printed on a Si/SiO\({}_{2}\) substrate able to rectify up to 30 THz. The results obtained in terms of FOMs indicate _Asym_ of 2500 at bias voltage of 1 V, an _NL_ of 3.8, and zero-bias resistance (ZBR) of 600 \(\Omega\) with a high current density \(J_{ON}\) up to 1 A/cm\({}^{2}\) for a 1 V bias. A Gr/TiO\({}_{2}\)/Ti printed on a Si/SiO\({}_{2}\) substrate with 1 \(\upmu\)m thermal SiO\({}_{2}\) is shown in [42]. The results obtained in terms of FOMs indicate an _Asym_ up to 520, a maximum _NL_ up to 15, a maximum \(S\) up to 26 V\({}^{-1}\), and a current density \(J_{ON}\) of 7.5 A/cm\({}^{2}\) for 1 V bias. For RF power detection, \(S\) is 2.8 V\({}^{-1}\) at 2.4 GHz and 1.1 V\({}^{-1}\) at 49.4 GHz. A Ti/TiO\({}_{2}\)/Bilayer Gr printed on a Si/SiO\({}_{2}\) substrate with 300 nm thermal SiO\({}_{2}\) is presented in [43]. The results obtained in terms of FOMs indicate a high _Asym_ up to 9000, a maximum _NL_ up to 8, a maximum \(S\) up to 10 V\({}^{-1}\), and a current density \(J_{ON}\) of 0.1 A/cm\({}^{2}\) for 1 V bias. The best result was obtained in [44], by considering a graphene/hexagonal boron nitride (h-BN)/graphene heterostructure (Gr-h-BN-Gr). This structure, printed on a Si/SiO\({}_{2}\) substrate with 90 nm thermal SiO\({}_{2}\), consists of graphene as contacts and hexagonal boron nitride (h-BN) as insulator with a thickness less than 6 nm. The results obtained in terms of FOMs indicate an _Asym_ of 1000, an _NL_ of 40, a zero-bias \(S\) of 2.75 V\({}^{-1}\), a peak \(S\) of 12 V\({}^{-1}\), and a current density \(J_{ON}\) of 0.02 A/cm\({}^{2}\) for a bias of 1 V. The latest studies on MIG configurations and related FOMs discussed in this subsection are summarized in Table 2. In [8], the authors present an antenna integrated with a rectifier for harvesting infrared energy. The antenna consists of a resonant bowtie antenna that has been optimized to resonate at operating frequency of 28.3 THz and to produce highly enhanced localized fields inside the gap. The authors fabricated a Cu (100 nm)-CuO-Au (100 nm) (0.0045 \(\upmu\)m\({}^{2}\)) MIM diode with a very small contact area (67 nm \(\times\) 67 nm) and a very small oxide thickness (0.7 nm) to decrease the values of its capacitance and resistance (a relatively low zero-bias resistance of 500 \(\Omega\)), respectively. The MIM diode, developed using electron beam lithography (EBL), guarantees the low resistance requirement and the THz signal rectification with no external electrical source. The structure of the THz antenna integrated to the diode is shown in Figure 5. It is composed of a four-layer stack-up. The first is a chromium thin-film layer (3 nm) for adhesion between the gold nanoantenna and the substrate. The second is a silicon dioxide layer (1.5 \(\upmu\)m), deposited by plasma-enhanced chemical vapor deposition (PECVD), to increase the THz radiation transmission into the silicon substrate. The third is a thick silicon layer (375 \(\upmu\)m) with high resistivity (2 K\(\Omega\)-cm) to reduce substrate losses. The fourth is a thin gold back reflector layer (200 nm) to enhance coupling to the antenna and the substrate.