  In this work, we review and expand recent theoretical proposals for the
realization of electronic thermal diodes based on tunnel-junctions of normal
metal and superconducting thin films. Starting from the basic rectifying
properties of a single hybrid tunnel junction, we will show how the
rectification efficiency can be largely increased by combining multiple
junctions in an asymmetric chain of tunnel-coupled islands. We propose three
different designs, analyzing their performance and their potential advantages.
Besides being relevant from a fundamental physics point of view, this kind of
devices might find important technological application as fundamental building
blocks in solid-state thermal nanocircuits and in general-purpose cryogenic
electronic applications requiring energy management.
