  Electrical power generation from a moderate temperature thermal source by
means of direct conversion of infrared radiation is important and highly
desirable for energy harvesting from waste heat and micropower applications.
Here, we demonstrate direct rectified power generation from an unbiased
large-area nanoantenna-coupled tunnel diode rectifier, called a rectenna. Using
a vacuum radiometric measurement technique with irradiation from a
temperature-stabilized thermal source, a generated power density of 8 nW/cm$^2$
is observed at a source temperature of 450C for the unbiased rectenna across an
optimized load resistance. The optimized load resistance for the peak power
generation for each temperature coincides with the tunnel diode resistance at
zero bias and corresponds to the impedance matching condition for a rectifying
antenna. Current voltage measurements of a thermally illuminated large-area
rectenna show current zero crossing shifts into the second quadrant indicating
rectification. Photon-assisted tunneling in the unbiased rectenna is modeled as
the mechanism for the large short-circuit photocurrents observed where the
photon energy serves as an effective bias across the tunnel junction. The
measured current and voltage across the load resistor as a function of the
thermal source temperature represents direct current electrical power
generation.
