  Energy harvesting is essential for the internet-of-things networks where a
tremendous number of sensors require power. Thermoelectric generators (TEGs),
especially those based on silicon (Si), are a promising source of clean and
sustainable energy for these sensors. However, the reported performance of
planar-type Si TEGs never exceeded power factors of 0.1 ${\mu} Wcm^{-2} K^{-2}$
due to the poor thermoelectric performance of Si and the suboptimal design of
the devices. Here, we report a planar-type Si TEG with a power factor of 1.3
${\mu} Wcm^{-2} K^{-2}$ around room temperature. The increase in thermoelectric
performance of Si by nanostructuring based on the phonon-glass electron-crystal
concept and optimized three-dimensional heat-guiding structures resulted in a
significant power factor. In-field testing demonstrated that our Si TEG
functions as a 100-${\mu}W$-class harvester. This result is an essential step
toward energy harvesting with a low-environmental load and cost-effective
material with high throughput, a necessary condition for energy-autonomous
sensor nodes for the trillion sensors universe.
