  This thesis describes the development of low-energy gap superconducting
tunnel junctions (STJs) for use as photon detectors, with as a main goal the
improvement of the energy resolution in both the optical and the x-ray energy
domain. A new model for the photon detection process with STJs is presented,
which includes the full energy dependence of all the quasiparticle processes
occurring in the junctions. This model allows for the calculation of the time-
and energy-dependent quasiparticle distribution from the moment of generation
of the quasiparticles by the photon absorption process until the end of the
current pulse, when all the quasiparticles have disappeared.
