Bipolar junction transistor with improved extrinsic base region and method of fabrication

Abstract

A bipolar transistor and its fabrication are described. The extrinsic base region is formed by growing a second, more heavily doped, epitaxial layer over a first epitaxial layer. The second layer extends under, and is insulated from, an overlying polysilicon emitter pedestal.

1 - 16 . (canceled)  
   
   
       17 . A bipolar transistor comprising: 
 a monocrystalline extrinsic base region having relatively vertical sidewalls adjacent a monocrystalline link base region, the extrinsic base region being more heavily doped than the adjacent link base region.    
   
   
       18 . The transistor defined by  claim 17 , wherein the extrinsic base region is doped to a level 10 times or greater than the doping level in the adjacent link base region.  
   
   
       19 . The transistor of  claim 17 , including an emitter pedestal structure having, an emitter pedestal, sidewall spacers on the sides of the pedestal, and oxide regions under the pedestal having an opening communicating with an emitter region of the transistor, wherein the extrinsic base region extends under the spacers.  
   
   
       20 . The transistor defined by  claim 17 , wherein the monocrystalline extrinsic base region is disposed on an underlying monocrystalline layer having the link base region and an intrinsic base region for the transistor.  
   
   
       21 . The transistor defined by  claim 20 , wherein the monocrystalline extrinsic base region is doped to a level 10 times or greater than the doping level in the adjacent link base region.  
   
   
       22 . The transistor defined by  claim 21 , wherein the extrinsic base region and underlying monocrystalline layer are silicon-germanium.