Abstract: A bipolar transistor and method of making the same is disclosed. The transistor has an emitter region which is diffused from polysilicon into the intrinsic base region, where the polysilicon is doped with two dopant species of different diffusivity. The impurity concentration of the higher diffusivity species, for example phosphorous, can be selected to define the emitter junction depth, which is preferably shallow, while the impurity concentration of the lower diffusivity species, for example arsenic, can be selected to provide a high conductivity emitter electrode, as well as reduce the sensitivity of the emitter electrode to counterdoping from the implantation of the extrinsic base region. The structure is compatible with BiCMOS processing, as the same anneal can be used to diffuse the emitter and the source/drains of the MOS transistors, with the emitter junction depth optimized via the implant conditions of the higher diffusivity species.

Abstract: A metal-oxide-semiconductor transistor gas sensor including a gate oxide film of a dielectric material having a permittivity of more than about ten between a substrate and a gate is provided. The existence of a particular gas is detected by the change in threshold voltage of the transistor.