Method for making buried isolation

A method for reducing punchthrough comprising:

• providing a p-type semiconductor substrate;
• providing a first buried collector region with a first junction and a second buried collector with a second junction, wherein the first and second junctions in the semiconductor substrate are positioned with a small region of the p-type semiconductor substrate between the first and second buried collectors;
• depositing a heavily doped layer of silicon on the semiconductor substrate, covering the region and the first and second buried collectors;
• depositing a lightly doped layer of epitaxial silicon over the heavily doped layer of epitaxial silicon;
• and
• completing subsequent thermal fabrication sequences for making at least a bipolar transistor which diffuses at least a portion of the dopant of the heavily doped layer into the region between the first and second buried collectors and into the first and second buried collector during subsequent thermal processing making a doped isolation barrier between the first buried collector and the second buried collector, thereby positioning a more heavily doped dopant barrier between the first and second buried collector thus reducing punchthrough.