Patent ID: 6717188
Filing Date: 2004-04-06
Classification: H01L

Abstract:
A semiconductor device, comprising:a substrate having a first semiconductor layer; a second semiconductor layer provided on the first semiconductor layer, wherein the second semiconductor layer has a smaller band gap than the first semiconductor layer and is made of a mixed crystal semiconductor; and a third semiconductor layer, which is provided on the second semiconductor layer and has a larger band gap than the second semiconductor layer; wherein the semiconductor device functions as a heterojunction bipolar transistor in which at least a portion of the first semiconductor layer is a collector region including first conductive-type impurities; at least a portion of the second semiconductor layer is a base region including second conductive-type impurities; and at least a portion of the third semiconductor layer is an emitter region including the first conductive-type impurities; wherein the second semiconductor layer comprises a graded composition layer having a composition in which the band gap becomes larger in a direction from the collector region toward the emitter region, and a upper layer having a composition in which the band gap change ratio is smaller than the band gap change ratio of the graded composition layer; a band gap in the upper layer of the second semiconductor layer decreases gradually in a direction from the emitter region toward the collector region, and the impurity concentration in the upper layer is substantially constant, and an emitter-base junction is formed in the upper layer of the second semiconductor layer.