The insulating film became a probe
Giaever’s 1960 experiments used tunneling through a thin insulating layer to probe superconducting electrodes. A sufficiently transparent barrier permitted measurable current while retaining sensitivity to the states on either side.
Suppressed low-voltage current and its recovery at higher bias made the superconducting excitation gap electrically accessible. This was different from Esaki’s semiconductor occupation-overlap effect.
Voltage selects accessible final states
For a normal metal coupled to a superconductor at low temperature, the onset is associated with the superconducting gap. With two superconducting electrodes, their densities of states are convolved and the prominent threshold involves both gaps. Thermal occupation rounds either result.
Sources: [1]
Junction defects can imitate extra states
Pinholes provide ordinary conduction that bypasses the intended tunnel process. Heating and finite temperature also create subgap current. A quantitative density-of-states inference therefore needs junction checks and temperature dependence, not a threshold voltage alone.
Sources: [1]
The related barrier model supplies the tunneling step
Barrier transmission explains why the insulating film need not stop every electron. It omits electrode occupations and superconducting coherence factors. The historical gap evidence belongs to the complete electrical junction experiment, recognized with Esaki’s work in the experimental half of the 1973 prize.