Abstract: A method for fabrication of a monolithically integrated SOI substrate capacitor has the steps of: forming an insulating trench (14), which reaches down to the insulator (11) and surrounds a region (13?) of the monocrystalline silicon (13) of a SOI structure, doping the monocrystalline silicon region, forming an insulating, which can be nitride, layer region (17?) on a portion of the monocrystalline silicon region, forming a doped silicon layer region (18) on the insulating layer region (17?), and forming an insulating outside sidewall spacer (61) on the monocrystalline silicon region, where the outside sidewall spacer surrounds the doped silicon layer region to provide an isolation between the doped silicon layer region and exposed portions of the monocrystalline silicon region. The monocrystalline silicon region (13?), the insulating layer region (17?), and the doped silicon layer region (18) constitute a lower electrode, a dielectric, and an upper electrode of the capacitor.