A method for forming a silicon ingot includes the following steps: providing a silicon ingot of variable electrical resistivity and containing interstitial oxygen, determining the interstitial oxygen concentration in different areas of the silicon ingot, calculating the concentration of thermal donors to be created in the different areas to reach a target value of the electrical resistivity, and subjecting the different areas of the silicon ingot to annealing so as to form the thermal donors. The annealing temperature in each area is determined from the thermal donor and interstitial oxygen concentrations of the area and from a predefined annealing time.