Abstract: This invention provides a method for fabricating a BiCMOS device, in which said device has a Si substrate of a first conductivity in which there is formed a first substrate region of a second conductivity for a bipolar transistor, a second substrate region of said second conductivity for a first MOSFET, having a source and drain of the first conductivity, and in which a part of said Si substrate is formed to provide a second MOSFET which has a source and drain of the second conductivity. A first nitride layer is used to prevent the substrate under a masking layer from oxidizing during the following oxidation processes, wherein the masking layer is composed of a oxide layer and the nitride layer. After some processes, the masking layer is removed. Implanting As impurities, a new oxide layer and a new nitride layer are deposited, wherein the role of the nitride layer is to protect a shallow emitter region.