Isolated nmos transistor fabricated in a digital bicmos process

Abstract

A method for making an isolated NMOS transistor ( ) in a BiCMOS process includes forming an N− conductivity type DUF layer ( ) in a P conductivity type semiconductor substrate ( ), followed by forming alternate contiguous N+ and P conductivity type buried regions ( ) in the substrate ( ). A layer of substantially intrinsic semiconductor material ( ) is then formed on the substrate ( ) in which alternate and contiguous N and P conductivity type wells ( ) are formed, respectively above and extending to the N+ and P conductivity type buried regions ( ). Finally, NMOS source and drain regions ( ) are formed in at least one of the P conductivity type wells ( ). The method is preferably performed concurrently with the construction of a bipolar transistor structure ( ) elsewhere on the substrate ( ). More particularly, the steps of forming the P conductivity type buried layer ( ) may be performed a part of a simultaneous formation of a collector element of the PNP transistor ( ) elsewhere on the substrate ( ).