in one disclosed embodiment , a collector is deposited and a base is grown on the collector , for example , by epitaxially depositing either silicon or silicon - germanium . an emitter is fabricated on the base followed by implant doping an extrinsic base region . for example , the extrinsic base region can be implant doped using boron . the extrinsic base region doping diffuses out during subsequent thermal processing steps in chip fabrication , creating an out diffusion region in the device , which can adversely affect various operating characteristics , such as parasitic capacitance and linearity . the out diffusion is controlled by counter doping the out diffusion region . for example , the counter doped region can be implant doped using arsenic or phosphorous . also , for example , the counter doped region can be formed using tilt implanting or , alternatively , by implant doping the counter doped region and forming a spacer on the base prior to implanting the extrinsic base region .