  The tunneling spectrum of an electron and a hole in
metal-superconductor-metal junctions is computed using the
Blonder-Tinkham-Klapwijk theory. The incident and the outgoing currents finally
balance each other by an interface charge inside the superconductor and metal
junction. The present computation shows a more abundant structure compared to
that on a metal-superconductor junction, such as the resonance at bias voltages
above the energy gap of the superconductor. The density of the interface charge
shows a quantum-like oscillation.
