  The semiclassical geometry of charged black holes is studied in the context
of a two-dimensional dilaton gravity model where effects due to pair-creation
of charged particles can be included in a systematic way. The classical
mass-inflation instability of the Cauchy horizon is amplified and we find that
gravitational collapse of charged matter results in a spacelike singularity
that precludes any extension of the spacetime geometry. At the classical level,
a static solution describing an eternal black hole has timelike singularities
and multiple asymptotic regions. The corresponding semiclassical solution, on
the other hand, has a spacelike singularity and a Penrose diagram like that of
an electrically neutral black hole. Extremal black holes are destabilized by
pair-creation of charged particles. There is a maximally charged solution for a
given black hole mass but the corresponding geometry is not extremal. Our
numerical data exhibits critical behavior at the threshold for black hole
formation.
