a semiconductor substrate may be formed by providing an providing a semiconductor - on - insulator substrate including a base semiconductor layer , a buried insulator layer above the base semiconductor layer , and a soi layer comprising a first semiconductor material above the buried insulator layer ; forming an isolation region in the soi layer isolating a first portion of the soi layer from a second portion of the soi layer ; removing the second portion of the soi layer to expose a portion of the buried insulator layer ; forming a hole in the exposed portion of the buried insulator layer to expose a portion of the base semiconductor layer ; and forming a semiconductor layer made of a second semiconductor material on the exposed portion of the base semiconductor layer , so that the replacement semiconductor layer covers the exposed region of the buried insulator layer .