# State-of-the-Art Techniques in RF Energy Harvesting Circuits
## 3 Results
### Design Specification
#### 3.2.4 Rectifier
For instance, in [29] we can find a self-startup RF power scavenging system with storage capability. The authors in [75] designed a reconfigurable 12-stage rectifier with MPPT (Maximum power point tracking) for input power from \(-22\) to 4 dBm at 915 MHz. A reconfigurable multistage RF rectifier circuit at 900 MHz can be found in [76] with an efficiency of 45% for input power from \(-15\) to 20 dBm. In [77] the authors designed a single-stage receiver in which they used a 3-mode reconfigurable rectifier. The circuit was fabricated in a standard CMOS 0.35 um. This circuit works at 6.78 MHz and achieves 92.2% power efficiency. Designers constantly try to take advantage of lower input voltages looking for new ways of designing. In a standard CMOS process, a diode is usually replaced more easily with a transistor MOS connected as a diode. The conventional way to implement the MOS-connected (BTMOS) diode with parasitic diodes is shown in Figure 10(a), in which the gate is connected to the drain and the bulk to the source. This common connection displays a disadvantage during OFF mode, allowing high reverse current. To overcome the limitation, one solution is to use transistors with a dynamically adjustable threshold voltage, \(V_{\text{TH}}\). In the DTMOS (Dynamic Threshold MOSFET) architecture shown in Figure 10(b) the substrate (bulk) and the gate are connected to dynamically change the \(V_{\text{TH}}\) of the transistor, using the following equation. In which \(V_{\text{TH}}\) is the threshold voltage, \(V_{\text{T0}}\) is the zero bias  
Figure 10: CMOS rectifier topologies: (**a**) Dickson Charge Pump (**b**) Differential Voltage Multiplier. threshold voltage with \(V_{\text{SB}}\) = 0, \(\gamma\) is called body-effect coefficient or body factor and \(\varphi_{\text{F}}\) is the inversion layer voltage. \[V_{\text{TH}}=V_{\text{T0}}+\gamma\sqrt{|2\Phi_{\text{F}}|+V_{\text{SB}}}-\sqrt{ |2\Phi_{\text{F}}|} \tag{2}\]  
This not only improves power flow but also minimizes reverse leakage current. Figure 12 shows how the threshold voltage changes to BTMOS and DTMOS for various \(V_{\text{SB}}\) voltage values [78, 79]. The authors in [80] designed a N(1,2,3)-stage rectifier used the DTMOS technique. They achieved for 1-stage at 900 MHz efficiency 79.4% and sensitivity \(-32\) dBm. Figure 11: Diode connected transistors: (**a**) BTMOS (**b**) DTMOS. Figure 12: \(V_{\text{T}}\)-\(V_{\text{SB}}\) (The red color refers to DTMOS and the dark blue to BTMOS). All rectifiers of this section are listed in Table 5. Table 6 consists of the same rectifiers separated by topology. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Rectifier’s Topology** & **Diodes** & \begin{tabular}{c} **Efficiency** \\ **(Max)** \\ \end{tabular}  & **Input Power** & **Frequency** \\ \hline
[29] & CMOS Reconfigurable & - & 25\% & \(-14.8\) dBm & 915 MHz \\
[35] & \begin{tabular}{c} Dual Band Voltage \\ Dual Band Voltage \\ Dual Band Voltage \\ Doubler \\ \end{tabular}  & SMS-7630 & 30.40\% & \(-20\) dBm & 925 MHz \\
[43] & \begin{tabular}{c} Dual Band Voltage \\ Doubler \\ \end{tabular}  & SMS-7630 & 57.10\% & \(-10\) dBm to \(-30\) dBm & 2.45 GHz, 5.8 GHz \\
[49] & \begin{tabular}{c} Single Diode Rectifier \\ Voltage Multiplier \\ Greinacher Rectifier \\ with Rat-Race coupler \\ Greinacher Rectifier \\ with Rat-Race coupler \\ \end{tabular}  & \begin{tabular}{c} HSMS-2860 \\ 81.65\% \\ \end{tabular}  & \ \begin{tabular}{c} 1.3\% \\ 0 dBm \\ 0 dBm \\ \end{tabular}  & \begin{tabular}{c} -20 dBm \\ 2.45 GHz \\ 868 MHz \\ \end{tabular}  \\
[51] & \begin{tabular}{c} GF11 \\ \end{tabular}  & SMS-7630 & 5\% & \(-20\) dBm to \(-10\) dBm & 2.45 GHz \\
[52] & \begin{tabular}{c} GF11 \\ \end{tabular}  & SMS-7630 & 5\% & \(-20\) dBm to \(-10\) dBm & 2.45 GHz \\
[54] & \begin{tabular}{c} Double Diode Rectifier \\ Voltage Quadrupler \\ \end{tabular}  & SMS7630-040 LF & 21\% & \(-15\) dBm & 97.5 MHz \\
[55] & \begin{tabular}{c} GF11 \\ \end{tabular}  & SMS-2862 & 75.108\% & 20 dBm & 2.4 GHz, 5.8 GHz \\
[56] &