MOBILITY ENHANCEMENT IN SiGe HETEROJUNCTION BIPOLAR TRANSISTORS

Abstract

The present invention relates to a high performance heterojunction bipolar transistor (HBT) having a base region with a SiGe-containing layer therein. The SiGe-containing layer is not more than about 100 nm thick and has a predetermined critical germanium content. The SiGe-containing layer further has an average germanium content of not less than about 80% of the predetermined critical germanium content. The present invention also relates to a method for enhancing carrier mobility in a HBT having a SiGe-containing base layer, by uniformly increasing germanium content in the base layer so that the average germanium content therein is not less than 80% of a critical germanium content, which is calculated based on the thickness of the base layer, provided that the base layer is not more than 100 nm thick.

1 . A heterojunction bipolar transistor comprising a collector region, a base region, an extrinsic base region, and an emitter region, wherein the base region comprises a SiGe-containing layer, wherein the SiGe-containing layer has a thickness of not more than about 100 nm and a predetermined critical germanium content associated with said thickness, and wherein the SiGe-containing layer has a germanium content profile with an average germanium content of not less than about 80% of the predetermined critical germanium content. 
   
   
       2 . The heterojunction bipolar transistor of  claim 1 , wherein the germanium content profile of the SiGe-containing layer is stepped or graded, and wherein the average germanium content in the SiGe-containing layer is determined by integrating germanium content over the entire SiGe-containing layer, so as to determine an integrated germanium content in the layer, and dividing the integrated germanium content over the thickness of said layer. 
   
   
       3 . The heterojunction bipolar transistor of  claim 1 , wherein the average germanium content in the SiGe-containing layer is not less than about 90% of the predetermined critical germanium content. 
   
   
       4 . The heterojunction bipolar transistor of  claim 1 , wherein the average germanium content in the SiGe-containing layer is not less than about 95% of the predetermined critical germanium content. 
   
   
       5 . The heterojunction bipolar transistor of  claim 1 , wherein the average germanium content in the SiGe-containing layer is not less than about 99% of the predetermined critical germanium content. 
   
   
       6 . The heterojunction bipolar transistor of  claim 1 , wherein the average germanium content in the SiGe-containing layer is substantially equal to the predetermined critical germanium content. 
   
   
       7 . The heterojunction bipolar transistor of  claim 1 , wherein the predetermined critical germanium content of the SiGe-containing layer is not less than about 10 atomic %. 
   
   
       8 . The heterojunction bipolar transistor of  claim 1 , wherein the SiGe-containing layer having a thickness of not more than about 50 nm, and wherein the predetermined critical germanium content of the SiGe-containing layer is not less than about 17 atomic %. 
   
   
       9 . The heterojunction bipolar transistor of  claim 1 , wherein the base region comprises two epitaxial semiconductor layers, and wherein the SiGe-containing layer is sandwiched between said two epitaxial semiconductor layers. 
   
   
       10 . The heterojunction bipolar transistor of  claim 9 , wherein the two epitaxial semiconductor layers both consists essentially of silicon. 
   
   
       11 . A heterojunction bipolar transistor comprising a SiGe-containing base layer having a thickness of not more than about 50 nm and a germanium content profile with an average germanium content ranging from about 16.5 atomic % to about 17.5 atomic %.