# State-of-the-Art Techniques in RF Energy Harvesting Circuits
## 3 Results
### Design Specification
#### 3.2.3. Diodes  
In rectifier design, one important step is the selection of a diode. The characteristics of the diode determine the overall efficiency of the circuit as it can be the main source for losses because depending on their values, changes the voltage drop across the diode [36]. Conventionally, a Schottky diode was selected because of its low junction capacitance, low threshold voltage, low barrier (high saturation current), and high switching capacity [37]. Low threshold voltage allows more efficient operation at frequencies in which we operate. The junction capacitance will determine the maximum operating frequency at which the diode will operate. These characteristics (Cj is the junction capacitance, Rs is the series resistance, Rj is the junction resistance) are presented in Figure 7a. Figure 7b illustrates the I-V characteristic of a Schottky diode and Figure 7c depicts the symbol of Schottky diode [38]. Table 3 presents the Schottky diodes and their parameters, which are used in several papers in the literature. There are some issues that we need to consider for the proper diode selection. For example, the diode has a nonlinear behavior [39], hence the RF to DC conversion efficiency varies with different power levels of the input. The main goal of the designers is to maximize the harvested RF power; hence they have to minimize the forward voltage drop of the diode and maximize the input voltage level of the rectifier. If the input voltage level of the rectifier is not larger than the forward voltage drop of the diode, then the rectifier circuit doesn't work. In addition to that, the diode doesn't work if the reverse breakdown voltage is exceeded [40]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Matching-Network** & **Type of Elements** \\ \hline
[29] & L-Type & Lumped \\
[30] & T-type & Lumped \\
[31] & L-Type & Distributed \\
[32] & Multiband & Distributed \\
[33] & L + μstrip Line Network & Distributed \\
[34] & Combination of different types of Matching Networks & Distributed \\
[35] & 2 Reverse L & Distributed \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Different types of Impedance Matching Networks. Figure 7: The characteristics of a diode (**a**) electronic equivalent circuit (**b**) I-V characteristic of Schottky diode (**c**) symbol of Schottky diode. The most commonly used Schottky diodes are the HSMS-285X (n-type) and the HSMS-286X (p-type). The first one has a high fence and low values of R\({}_{\text{s}}\), whereas the second one has a low barrier and high values of R\({}_{\text{s}}\)[7]. In [41] a dual-stage voltage doubler was manufactured for operation at a frequency of 3.5 GHz using HSMS-2820 diodes. In [42] the authors designed three rectifiers and connected them in series. Each rectifier is a single series diode rectifier. For this design, they used an SMS7630-079 diode. A voltage doubler was manufactured in [43] with an efficiency of 50.7% at 2.45 GHz and 20.1% at 5.85 GHz. The authors used the SMS-7630 diode [44]. According to the above, the diode greatly affects the efficiency of the rectifier. The energy conversion efficiency increases when we have a decrease of turn-ON threshold voltage or decrease of junction tension Vj and decreases when we have an increase of resistance R\({}_{\text{s}}\) or decrease of junction capacitance Cj [7].The authors in [45] designed a dual-band (1800 MHz, 2.1 GHz) rectenna in which the rectification was achieved with a single diode rectifier (by the use of an SMS7630 diode). The efficiency of this work is 33% for \(-7\) dBm input power. The authors have used HSMS-285C diodes in [46] to manufacture and compare a 1-stage Dickson rectifier and a 3-stage Dickson rectifier. The circuits tune in 1 GHz and for \(-7\) dBm input power. The first circuit achieved 70.5% efficiency while the second circuit achieved 77% efficiency. The authors in [47] designed a rectenna in which HSMS-286C diodes were used. The circuit resonates at 868 MHz. The maximum efficiency is 44.5% at \(-10\) dBm.