Abstract: Prior fabricating the transistors, a phase of forming a deep insulative trench in the substrate is followed by a phase of forming a shallow insulative trench in the substrate and extending the deep trench. The phase of forming the deep trench includes coating the inside walls of the deep trench with an initial oxide layer and filling the deep trench with silicon inside an envelope formed from an insulative material. The phase of forming the shallow trench includes coating the inside walls of the shallow trench with an initial oxide layer and filling the shallow trench with an insulative material.