Patent ID: 7135721

Claim:
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 plural Si 1-x-y Ge x C y layers (0<x<1, 0≦y<1) having a different C content from a Ge 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 plural Si 1-x-y Ge x C y layers constructing the base region, the C content of a Si 1-x-y Ge x C y layer adjacent to the collector region is larger than that of a Si 1-x-y Ge x C y layer adjacent to the emitter region, and the Ge content of the Si 1-x-y Ge x C y layer adjacent to the collector region is larger than that of the Si 1-x-y Ge x C y layer adjacent to the emitter region, the Ge content of the Si 1-x-y Ge x C y layer adjacent to the collector region increases from the emitter region toward the collector region and the C content of the Si 1-x-y Ge x C y layer adjacent to the collector region increases from the emitter region toward the collector region, and a rate of increase of the Ge content of the Si 1-x-y Ge x C y layer adjacent to the collector region, from the emitter region toward the collector region, is larger than a rate of increase of the C content of the Si 1-x-y Ge x C y layer adjacent to the collector region, from the emitter region toward the collector region.