  We investigate Schwinger pair production in spatially inhomogeneous electric
backgrounds. A critical point for the onset of pair production can be
approached by fields that marginally provide sufficient electrostatic energy
for an off-shell long-range electron-positron fluctuation to become a real
pair. Close to this critical point, we observe features of universality which
are analogous to continuous phase transitions in critical phenomena with the
pair-production rate serving as an order parameter: electric backgrounds can be
subdivided into universality classes and the onset of pair production exhibits
characteristic scaling laws. An appropriate design of the electric background
field can interpolate between power-law scaling, essential BKT-type scaling and
a power-law scaling with log corrections. The corresponding critical exponents
only depend on the large-scale features of the electric background, whereas the
microscopic details of the background play the role of irrelevant perturbations
not affecting criticality.
