# 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)  
Although there have been continuous improvements, the current state indicates that there is no extensive and detailed literature on this type of diode. Only a few authors have provided a significant improvement over the state of the art. This paragraph presents a review of the most recent progress and the latest results and issues for this asymmetric structure. In [83], the authors present an infrared optical response of geometric diode rectenna solar cells. The geometric diode is an inverse arrowhead-shaped graphene device  
Figure 18: A schematic of the graphene geometric diode (GD). coupled to bowtie antenna (5.1 \(\upmu\)m \(\times\) 2.3 \(\upmu\)m). GD was fabricated with the e-beam lithography (EBL) technique. Cr/Au metal contact pads formed a four-point probe configuration. Design parameters indicate a neck width of 70 nm, a shoulder width of 400 nm, and MFPL estimated around \(\sim\)50 nm. With a capacitance of the order of 1 aF and an RC time constant of \(\sim\)1.6 fs, this device has achieved an operating frequency of 28.3 THz. A current \(I_{DS}\) (\(\upmu\)A) was measured as a function of drain-source bias \(V_{DS}\) (V). For a gate voltage \(V_{G}\) = 20 V and a \(V_{DS}\) = 1.5 V, 150 \(\upmu\)A is obtained. Zero bias resistance measured as a function of drain-source bias \(V_{DS}\) (V) indicates 13 K\(\Omega\) for \(V_{DS}\) = 0 V. Finally, responsivity measured as a function of the input AC power indicates 0.18 V\({}^{-1}\) for \(V_{DS}\) = 0 V and 0.2 V\({}^{-1}\) for \(V_{DS}\) = 1.5 V. In [84], the authors present an optical rectenna solar cell using graphene geometric diodes. The authors fabricated this device using graphene obtained by exfoliation that adheres to a 90 nm thick oxidized silicon wafer. The only design parameter is a \(d_{neck}\) = 150 nm. This diode has the advantage of having very low capacitance of the order of 0.06 fF due to its planar structure with a low RC time constant of \(\sim\)7 fs, allowing an operating frequency in the optical range. The results indicate for \(I_{DS}\) (\(\upmu\)A), measured as a function of \(V_{DS}\) (V), a peak around 0.01 \(\upmu\)A for \(V_{DS}\) = 0 V. For zero-bias resistance measured as a function of \(V_{DS}\) (V), there is a peak around 19 K\(\Omega\) for \(V_{DS}\) = 0 V. The responsivity \(S\) measured as a function of \(V_{DS}\) (V) indicates a peak around 0.3 V\({}^{-1}\) for \(V_{DS}\) = 0 V and a peak around 0.8 V\({}^{-1}\) for \(V_{DS}\) = 0 V. By keeping all other parameters fixed, \(I_{DS}\) (mA) and asymmetry are obtained as a function of \(d_{neck}\) (50, 200, and 600 nm). The best values are obtained for \(d_{neck}\) = 600 nm and \(d_{neck}\) = 50 nm. In fact, \(I_{DS}\) (mA) of 1.5 mA is obtained in correspondence of \(V_{D}\) = 0.1 V and \(d_{neck}\) = 600 nm, whereas an asymmetry of 3.2 is obtained in correspondence of \(V_{D}\) = 0.05 V and \(d_{neck}\) = 50 nm. In [85], the authors present graphene geometric diodes for terahertz rectennas. This device, although designed to resonate at 28 THz, provides low resistance and a low RC time constant of \(10^{-15}\)s, required for operation at optical frequencies. For \(V_{DS}\) = 0.1 V and \(d_{neck}\) = 600 nm, \(I_{DS}\) (mA) assumes the maximum value of 1.5 mA. For \(d_{neck}\) = 75 nm, \(d_{shoulder}\) = 400 nm, and an MFPL = 45 nm, the diode obtains a responsivity \(S\) of 0.12 V\({}^{-1}\) at zero-bias voltage, and a responsivity over 0.2 V\({}^{-1}\) around \(V_{DS}\) (V) =1.4 V. \(I_{DS}\) (\(\upmu\)A) of 0.01 at zero-bias voltage, and an \(I_{DS}\) (\(\upmu\)A) over 150 at \(V_{DS}\) (V) = 1.4 V are obtained. Other results indicate a rectifying antenna coupling efficiency of 12% with diode resistance of 3 K\(\Omega\), an antenna impedance of 100 \(\Omega\), and an antenna radiation efficiency of 37%. In addition, quantum efficiency of 0.01% and zero-bias responsivity of 0.0285 V\({}^{-1}\) are obtained.