## 4 Conclusions  
This study proposed an auxiliary power integrated chip (IC) to supply power to the WSN with a Power cast transmitter of ISM 915 MHz. The RF-energy-harvesting IC was designed and fabricated using the standard TSMC 0.18 \(\upmu\)m 1P6M CMOS process. The externally matched capacitors and inductors were manufactured in the matching network by MuRata Company. On integrating the externally matched components with the designed RF-energy-harvesting IC, the simulated results showed that the maximum _PCE_s of the rectifier and harvesting IC were 43.6% and 29.873%, respectively, at an ISM band of 915 MHz, an input power of \(-7\) dBm, and a load of 1 M\(\Omega\). The output voltage of the RF-DC rectifier with the over-voltage protection circuit was limited from 1.773 V to 1.809 V with the varying input power from \(-10\) dBm to \(+20\) dBm. A stable voltage of 1.5 V was supplied to the charge control circuit passing through the LDO circuit. Measurements validated that the designed RF-energy-harvesting IC works successfully. The output voltage V\({}_{\rm dc}\) varied from 0.291 V to 1.725 V with respect to the distance from 5.45 m to 0.55 m. The large load resistor reduced load current; however, it sharply increased the charging current to charge the battery abruptly. This phenomenon provided the maximum system _PCE_ at low input power. A trade-off was observed between the load resistor and the maximum system _PCE_. Furthermore, the maximum rectifier _PCE_ of this study was superior to that in [30], and the maximum _PCE_ of the RF-energy-harvesting IC was superior to that in [31]. Measurements indicate that the IC used in this study exhibited low power consumption and a small chip area. The proposed energy harvesting IC can be used in both ambient source and dedicated \begin{table}
\begin{tabular}{c c} \hline
**Parameters (unit)** & **Values** \\ \hline RF frequency (MHz) & 915 \\ Input impedance (\(\Omega\)) & \(51.464-\mathrm{j}\times 1.3071\) \\ Input power (dBm) & 0 \\ Distance between Powercastand PCB (m) & 2.45 \\ Matching components & L = 7.5 nH and C = 22 pF \\ Load resistor (k\(\Omega\)) & 30 \\ Measured PCB (cm\({}^{2}\)) & 15.57\(\times\) 7.906 \\ Maximum _PCE_ & 40.566\% @\(P_{in}=-6\) dBm \\ Output voltage (V) & 1.50 \\ Power consumption (\(\upmu\)W) & 42 \\ Chip area (mm\({}^{2}\)) & \(0.58\times 0.49\) \\ \hline \end{tabular}
\end{table} 
Table 4: Measured electrical characterizations of the proposed RF-energy-harvesting IC. \begin{table}
\begin{tabular}{c c c c c} \hline
**Reference** & [30] & [31] & [32] & [33] & **This** \\
**(year)** & **(2018)** & **(2018)** & **(2016)** & **(2017)** & **Work** \\ \hline Process (\(\upmu\)m) & 0.13 & 0.18 & 0.18 & 0.13 & 0.18 \\ RF frequency (MHz) & 915 & 900 & 900 & 200 & 915 \\ Matching circuits & Off-chip & Off-chip & Off-chip & Off-chip & Off-chip \\ Input power range & \(-35\sim-15\) & \(-30\sim+0\) & \(-26\sim-8\) & \(-35\sim+5\) & \(-20\sim+10\) \\ (dBm) & 10 & 5 & 2 & 3 & 6 \\ Stages (Rectifier) & 42.8\%@ \(P_{in}=\) & 78.2\%@ \(P_{in}=\) & 73.9\%@ \(P_{in}=\) & 43.6\%@ \(P_{in}=\) \\ (Rectifier) & \(-16\) dBm & — & \(-12\) dBm & 4.34 dBm & \(-7\) dBm \\ Output voltage (V) & 2.32 & — & 1.0 & 3.5 & 1.725 \\ (Rectifier) & & 32.8\%@ \(P_{in}=\) & — & — & 40.56\%@ \(P_{in}=\) \\ Maximum _PCE_ & — & \(-2\) dBm & — & — & \(-6\) dBm \\ (RF harvesting IC) & — & 1.77 & — & — & 1.50 \\ Output voltage (V)(RF & & & & 2 & 30 \\ harvesting IC) & — & 16.56 & — & 0.954 & 0.2842 \\ Load resistor (k\(\Omega\)) & — & — & — & — & 42 \\ Chip area (mm\({}^{2}\)) & 0.0296 & 16.56 & — & 0.954 & 0.2842 \\ Power (\(\upmu\)W) & — & — & — & — & 42 \\ \hline \end{tabular}
\end{table} 
Table 5: Summary of performance and its comparison with those of other RF-DC rectifiers and RF-energy-harvesting ICs. source [34]. Another possibility offered by the use of microelectronic substrates is on-chip photovoltaic generation with integrated photodiodes [35; 36]. This study was completed by four authors.