# Progress in THz Rectifier Technology: Research and Perspectives
## 3 THz Diode Technology: General Information on Geometric Diode (GD)
#### THz Didee Technology: State of the Art on Geometric Didee (GD)
All diodes discussed above are made by considering a monolayer graphene obtained by the exfoliated graphene process. This technique, however, presents two inconveniences. First, graphene can be realized in a very small area through the exfoliation process; this makes it not suitable for mass production. Second, the exfoliated graphene is not uniform; therefore, some areas of its surface, may be multi-layer rather than monolayer. In [86], the authors present a CVD-grown monolayer graphene-based geometric diode for THz rectennas. This process allows realizing rectennas at a mass scale. The fabrication process of the graphene starts in the cleanroom through high-resolution EBL processes, as shown in Figure 19. A 300 nm layer of SiO\({}_{2}\) is deposited on a highly doped Si wafer through a plasma-enhanced (PE)--chemical vapor deposition (CVD) (Figure 19a). On SiO\({}_{2}\), a layer of e-beam resist ZEP520A is deposited. ZED-N50 was used to form two square-shaped trenches (Figure 19b). A Ti (10 nm)/Au (40 nm) layer was deposited on the sample through sputtering. The two contact pads were realized through the lift-off process (Figure 19c). The monolayer graphene was transferred on the sample with Ti/Au electrodes (Figure 19d). In the gap between the metal pads (Figure 19e), an e-beam resist PMMA 950 K A4 was spin-coated and patterned by EBL. The last step indicates an oxygen plasma etching utilized to realize the graphene geometric diode (Figure 19f). To verify the fabrication process of graphene GD, atomic force microscopy (AFM) was used with different \(d_{neck}\) as shown in Figure 20. The increase of \(d_{neck}\) is observed in the AFM images in Figure 20a-e. Figure 20f shows a graphene GD with \(d_{neck}\) = 50 nm. \(I_{DS}\) (mA) is measured as a function of \(V_{DS}\) (V) by varying \(d_{neck}\) from 50 nm to 250 nm and \(V_{DS}\) from \(-\)2 V to 2 V. For \(d_{neck}\) = 50 nm and \(V_{DS}\) (V) = 2, \(I_{DS}\) (mA) reaches a maximum of 0.7. Figure 19: The fabrication process of the graphene geometric diode. Reprinted with permission from ref. [86]. Copyright 2021 Nanomaterials. Figure 20: AFM images of the graphene geometric diode (the black area is graphene) with different neck widths: (**a**) 50 nm neck width, (**b**) 100 nm neck width, (**c**) 150 nm neck width, (**d**) 200 nm neck width, (**e**) 250 nm neck width, (**f**) SEM image of the 50 nm neck width geometric diode. Reprinted with permission from ref. [86]. Copyright 2021 Nanomaterials. For \(d_{neck}\) = 100 nm and \(V_{DS}\) (V) = 2, \(I_{DS}\) (mA) reaches a maximum of 1.3. For \(d_{neck}\) = 150 nm and \(V_{DS}\) (V) = 2, \(I_{DS}\) (mA) reaches a maximum of 1.4. For \(d_{neck}\) = 200 nm and \(V_{DS}\) (V) = 2, \(I_{DS}\) (mA) reaches a maximum of 1.5 and finally, for \(d_{neck}\) = 250 nm and \(V_{DS}\) (V) = 2, \(I_{DS}\) (mA) reaches a maximum of 1.8. For \(V_{DS}\) fixed, \(I_{DS}\) (mA) increases as \(d_{neck}\) increases. This is due to the greater flow of charges that carriers can pass through \(d_{neck}\) per second. It is observed that parameters such as nonlinearity and asymmetry that identify the performance of the diode begin to decrease as \(d_{neck}\) increases. For \(V_{DS}\) = 2 V, \(d_{neck}\) = 50 nm, and \(d_{neck}\) = 100 nm, the _Asym_ obtained is 1.40 and 1.25, respectively. For the other \(d_{neck}\) values, the _Asym_ is equal to 1; therefore, no rectification is observed. Zero-bias resistance and zero-bias responsivity tend to decrease as \(d_{neck}\) increases. The results indicate that zero-bias resistance (\(\Omega\)) reaches a maximum of \(5\times 10^{3}\) or \(d_{neck}\) = 50 nm, and a minimum of \(2\times 10^{3}\) for \(d_{neck}\) = 250 nm. Zero-bias responsivity (V\({}^{-1}\)) reaches a maximum of 0.06 for \(d_{neck}\) = 50 nm, and a minimum of 0.02 for \(d_{neck}\) = 250 nm. Finally, when a small \(V_{DS}\) is applied to GD, the resistance is reduced, and the responsivity is increased.