  We formulate a theoretical framework to describe multiparticle current
transport in planar superconducting tunnel junctions with diffusive electrodes.
The approach is based on direct solving of quasiclassical Keldysh-Green
function equations for nonequilibrium superconductors, and consists of a
combination of a circuit theory analysis and improved perturbation expansion.
The theory predicts much greater scaling parameter for the subharmonic gap
structure of the tunnel current in diffusive junctions compared to the one in
ballistic junctions and mesoscopic constrictions with the same barrier
transparency.
