  We present a physically consistent interpretation of the dc electrical
properties of niobiumnitride (NbN)-based superconducting hot-electron bolometer
(HEB-) mixers, using concepts of nonequilibrium superconductivity. Through this
we clarify what physical information can be extracted from the resistive
transition and the dc current-voltage characteristics, measured at suitably
chosen temperatures, and relevant for device characterization and optimization.
We point out that the intrinsic spatial variation of the electronic properties
of disordered superconductors, such as NbN, leads to a variation from device to
device.
