  Configuration space heat-kernel methods are used to evaluate the determinant
and hence the effective action for an SU(2) doublet of fermions in interaction
with a {\it covariantly constant} SU(2) background field. Exact results are
exhibited which are applicable to {\it any} Abelian background on which the
only restriction is that $(B^{2}-E^{2})$ and $E\cdot B$ are constant. Such
fields include the uniform field and the plane wave field. The fermion
propagator is also given in terms of gauge covariant objects. An extension to
include finite temperature effects is given and the probability for creation of
fermions from the vacuum at finite temperature in the presence of an electric
field is discussed.
