  In quantum field theory, particle creation occurs, in general, when an
intense external field, such as an electromagnetic field, breaks time
translational invariance. This leads to an ambiguity in the definition of the
vacuum state. In cosmological backgrounds this ambiguity has been reduced by
imposing that the quantization preserves the symmetries of the system and that
the dynamics is unitarily implemented. In this work, we apply these
requirements to the quantization of a massive charged fermionic field coupled
to a classical time-dependent homogeneous electric field, extending previous
studies done for a scalar field. We characterize the quantizations fulfilling
the criteria above and we show that they form a unique equivalence class of
unitarily related quantizations, which provide a well-defined number of created
particles at all finite times.
