  Control of heat flux at small length scales is crucial for numerous
solid-state devices and systems. In addition to the thermal management of
information and communication devices the mastering of heat transfer channels
down to the nanoscale also enable, e.g., new memory concepts, high sensitivity
detectors and sensors, energy harvesters and compact solid-state refrigerators.
Electronic coolers and thermal detectors for electromagnetic radiation,
especially, rely on the maximization of electro-thermal response and blockade
of phonon transport. In this work, we propose and demonstrate that efficient
electro-thermal operation and phonon transfer blocking can be achieved in a
single solid-state thermionic junction. Our experimental demonstration relies
on suspended semiconductor-superconductor junctions where the electro-thermal
response arises from the superconducting energy gap, and the phonon blocking
naturally results from the transmission bottleneck at the junction. We suspend
different size degenerately doped silicon chips (up to macroscopic scale)
directly from the junctions and cool these by biasing the junctions. The
electronic cooling operation characteristics are accompanied by measurement and
analysis of the thermal resistance components in the structures indicating the
operation principle of phonon blocking in the junctions.
