  We consider ferroelectric phase transitions in both short-circuited and
biased ferroelectric-semiconductor films with a space (depletion) charge which
leads to some unusual behavior. It is shown that in the presence of the charge
the polarization separates into `switchable' and `non-switchable' parts. The
electric field, appearing due to the space charge, does not wash out the phase
transition, which remains second order but takes place at somewhat reduced
temperature. At the same time, it leads to a suppression of the
ferroelectricity in a near-electrode layer. This conclusion is valid for
materials with both second and first order phase transitions in pure bulk
samples. Influence of the depletion charge on thermodynamic coercive field
reduces mainly to the lowering of the phase transition temperature, and its
effect is negligible. The depletion charge can, however, facilitate an
appearance of the domain structure which would be detrimental for device
performance (fatigue). We discuss some issues of conceptual character, which
are generally known but were overlooked in previous works. The present results
have general implications for small systems with depletion charge.
