Double-barrier Nb/Al2O3/Al/Al2O3/Nb tunnel junctions are used as mixin
g elements in a 345 GHz waveguide mixer. Noise temperatures (double si
de band) down to 720 K at 3.0 K are obtained without the need to apply
a magnetic field to suppress the Josephson current. It is shown that
the composite barrier acts as a single barrier for photon-assisted tun
neling. Surprisingly, at these frequencies the capacitance of two stac
ked tunnel barriers is only determined by one barrier.