  We study electron-positron pair production by spatially inhomogeneous
electric fields. Depending on the localization of the field, a critical point
(critical surface) exists in the space of field configurations where the pair
production probability vanishes. Near criticality, pair production exhibits
universal properties similar to those of continuous phase transitions. We
extend results previously obtained in the semi-classical (weak-field) critical
regime to the deeply critical regime for arbitrary peak field strength. In this
regime, we find an enhanced universality, featuring a unique critical exponent
$\beta=3$ for all sufficiently localized fields. For a large class of field
profiles, we also compute the non-universal amplitudes.
