  We have modified Landau-Khalatnikov approach and shown that both the polar
lattice and the screened charged defects determine the response of disordered
ferroelectric-semiconductors. This system exhibits the spatially inhomogeneous
switching under the external field while Landau-Khalatnikov model describes
homogeneous switching with the sharp pyroelectric coefficient peak near the
thermodynamic coercive field value. Our model gives more realistic pyroelectric
hysteresis loop shape without any peaks near the coercive field and describes
both qualitatively and quantitatively typical Pb(Zr,Ti)O_3 and (Sr,Ba)Nb_2O_6
films pyroelectric hysteresis loops.
