Double-barrier Josephson junctions, with outer electrodes of superconductin
g Nb and a central thin layer of non-superconducting Al, are being develope
d as self-shunted junctions for high-frequency superconducting integrated c
ircuits. This paper presents a simplified phenomenological model of the beh
aviour of such junctions (often called SINIS, for superconductor-insulator-
normal-insulator-superconductor) based on the resistively shunted junction
model, using Josephson coupling between the layers and the time-dependent G
inzburg-Landau equation to describe the dynamics of the weak induced energy
gap in the Al layer. This analysis shows how non-hysteretic I-V characteri
stics can be obtained but suggests some limitations in their high-frequency
performance. Predictions for critical current generally agree with prior e
xperiments. The technological implications of this model are discussed.