Method of fabricating an ultra-thin active region for high speed semiconductor devices

A method of fabricating a semiconductor device to retard diffusion of a dopant within a center region between two regions comprising,

• forming an epitaxial alloy layer on a first silicon layer, said step of forming said epitaxial alloy layer including selectively adjusting epitaxial procedure variables during said forming such that said epitaxial alloy layer includes a concentration profile of germanium tailored to minimize diffusion, thereby providing an epitaxial alloy layer of Si.sub.1-x Ge.sub.x wherein x is selectively varied through said epitaxial alloy layer, said adjusting of said epitaxial procedure variables including introducing said germanium throughout said step of forming said epitaxial alloy layer so that x remains above zero, said step of forming an epitaxial alloy layer further comprising introducing a dopant, and
• forming a second silicon layer on said epitaxial alloy layer,
• thereby providing a structure in which the center epitaxial alloy layer has a lower diffusivity with respect to said dopant than either of said first and second silicon layers.

A method of fabricating a semiconductor device to retard diffusion of a dopant from a center region into adjacent regions comprising,

• forming a collector region having a first conductivity type,
• epitaxially forming a base region of an epitaxial alloy on said collector region, including selectively increasing and decreasing an introduction of a diffusion-suppressing material into a semiconductor material so that concentration of said diffusion-suppressing material is maximized at a first side contacting said collector region and at a second side opposite to said first side, said increasing and decreasing said introduction being carried out in a manner to provide a lesser concentration of said diffusion-suppressing material at areas between said first and second sides while maintaining said diffusion-suppressing material above zero percent by content, said base region having a second conductivity type, and
• epitaxially forming an emitter region of epitaxial material on said second side of said base region, said emitter region having said first conductivity type.