Heterojunction bipolar transistor

Abstract

The bipolar transistor of the present invention includes a Si collector buried layer, a first base region made of a SiGeC layer having a high C content, a second base region made of a SiGeC layer having a low C content or a SiGe layer, and a Si cap layer 14 including an emitter region. The C content is less than 0.8% in at least the emitter-side boundary portion of the second base region. This suppresses formation of recombination centers due to a high C content in a depletion layer at the emitter-base junction, and improves electric characteristics such as the gain thanks to reduction in recombination current, while low-voltage driving is maintained.

What is claimed is:  
     
         1 . A heterojunction bipolar transistor comprising: 
 a first-conductivity type collector region made of a semiconductor material including Si formed on a substrate;    a second-conductivity type base region made of a Si 1−x−y Ge x C y  layer (0<x<1, 0<y<1) having a nonuniform C content formed on the collector region; and    a first-conductivity type emitter region made of a semiconductor material including Si formed on the base region, the emitter region forming heterojunction with the base region,    wherein in the base region, a portion having the largest C content is separated from a portion adjacent to the emitter region.    
     
     
         2 . The heterojunction bipolar transistor of  claim 1 , wherein the C content of the portion of the base region adjacent to the emitter region is less than 0.8%.  
     
     
         3 . The heterojunction bipolar transistor of  claim 1 , wherein the C content of the portion of the base region adjacent to the emitter region is 0.01% or more.  
     
     
         4 . The heterojunction bipolar transistor of  claim 1 , wherein the heterojunction bipolar transistor is constructed so that a depletion layer formed at an emitter-base junction is within a region in contact with the emitter region in the base region.  
     
     
         5 . The heterojunction bipolar transistor of  claim 1 , wherein the Ge content of a region in contact with the emitter region in the base region constant.  
     
     
         6 . The heterojunction bipolar transistor of  claim 1 , wherein at least the center of a portion of the base region other than the portion adjacent to the emitter region has a uniform Ge content.  
     
     
         7 . The heterojunction bipolar transistor of  claim 1 , wherein the thickness of the portion of the base layer adjacent to the emitter region is 5 nm or more.  
     
     
         8 . The heterojunction bipolar transistor of  claim 7 , wherein the thickness of the portion of the base layer adjacent to the emitter region is 10 nm or more.  
     
     
         9 . The heterojunction bipolar transistor of  claim 1 , wherein a portion of the base region other than the portion adjacent to the emitter region is constructed so that the band gap decreases as the position in the base region is farther from the emitter region and closer to the collector region.  
     
     
         10 . The heterojunction bipolar transistor of  claim 1 , wherein a portion of the base region other than the portion adjacent to the emitter region has a composition of which the C content increases as the position in the base region is farther from the emitter region and closer to the collector region.  
     
     
         11 . The heterojunction bipolar transistor of  claim 1 , wherein the base region is divided into a first base region including the portion adjacent to the collector region and a second base region including the portion adjacent to the emitter region, and the band gap of at least the boundary portion of the first base region on the side of the second base region is equal to or smaller than the band gap of the second base region.  
     
     
         12 . The heterojunction bipolar transistor of  claim 11 , wherein a relationship 
 Δx≧4.288Δy    is established where Δx denotes the difference in Ge content between at least a boundary portion of the first base region on the side of the second base region and the second base region, and Δy denotes the difference in C content between at least the boundary portion of the first base region on the side of the second base region and the second base region.    
     
     
         13 . The heterojunction bipolar transistor of  claim 11 , wherein a portion of the first base region other than the boundary portion on the side of the second base region is constructed so that the band gap decreases as the position in the first base region is farther from the second base region and closer to the collector region.  
     
     
         14 . The heterojunction bipolar transistor of  claim 13 , wherein a relationship 
 Δx≧4.288Δy    is established where Δx denotes the difference in Ge content between at least a boundary portion of the first base region on the side of the second base region and the second base region, and Δy denotes the difference in C content between at least the boundary portion of the first base region on the side of the second base region and the second base region.