A thermodynamic scheme for drawing energy band diagrams of ferroelectric heterostructures is presented. It incorporates band bending of the semiconductor and the ferroelectric due to spontaneous polarization into the procedure used for the semiconductor heterojunction. Using the diagram, the thermodynamic properties of the ferroelectric/insulator/semiconductor are investigated. It successfully explains the stability and the value of the semiconductor charge space. For thick insulator layers, the space charge is almost independent of conventional ferroelectric parameters and is mainly determined by the properties of the ferroelectric as a semiconductor. Additionally, the results explain the bistability of the ferroelectric on oxides related to high-T-c superconductors.