Abstract: A method of forming a well of one conductivity type in a silicon substrate having a first surface region thereof which is doped with a dopant of one conductivity type and a second surface region thereof which is doped with a dopant of opposite conductivity type. The first and second regions are covered by respective first and second portions of an oxide layer which has been grown on the silicon substrate, the first portion being thicker than the second portion. The substrate is oxidized thereby to increase the thickness of the oxide layer such that the difference in thickness between the first and second portions is reduced. The substrate is also heated to cause diffusion of the dopant of one conductivity type thereby to form a well and of the dopant of opposite conductivity type down into the substrate. The heating step is carried out before, during or after the oxidizing step.