Method of producing Si-Ge base heterojunction bipolar device

A method for making a bipolar transistor comprising the steps of:

• providing a semiconductor substrate having a surface;
• forming a device isolation layer on the surface for Isolating device regions;
• forming a polysilicon film on the surface and device isolation layer;
• forming at least one opening in the polysilicon film at a location corresponding to a said device isolation region;
• forming a base semiconductor film layer having a first conductivity type on the polysilicon film and surface;
• forming an emitter semiconductor film layer having a first concentration of impurity of a second conductivity type different from the first conductivity type on the base semiconductor film layer;
• forming a dielectric film on the emitter semiconductor film layer, said dielectric film having a higher concentration of impurity of the second conductivity type than is present in said emitter semiconductor film layer;
• performing solid-state diffusion so that impurities from the dielectric film diffuse into the emitter semiconductor film layer so that the concentration of impurities of the second conductivity type Is increased above the first concentration in a surface region of the emitter semiconductor film layer;
• patterning the dielectric film to provide an etching mask;
• and
• thereafter, patterning the surface region of the emitter semiconductor film layer by etching using the etching mask to form an emitter contact layer and patterning remaining portions of the emitter semiconductor film layer by etching using the etching mask to form an emitter.