Heterojunction bipolar transistor

Claim 2

A heterojunction bipolar transistor comprising: an emitter layer of a first conductivity type; a base layer of a second conductivity type adjacent to said emitter layer; a collector buffer layer of the first conductivity type; and a collector layer arranged between said collector buffer layer and said base layer, wherein said collector layer includes a first collector sub-layer formed at the side of said base layer and a second collector sub-layer arranged at the side of said collector buffer layer, said first collector sub-layer is a semiconductor layer having an impurity concentration lower than that of said base layer, and said second collector sub-layer is a semiconductor layer of the second conductivity type having an impurity concentration higher than that of said first collector sub-layer, wherein said first collector sub-layer and said second collector sub-layer are depleted in an operating state and form an electric field distribution determined by parameters of said base layer, said first collector sub-layer, said second collector sub-layer and said collector buffer layer.

Claim 3

A transistor according to claim 1, wherein said collector buffer layer is a collector electrode layer.

Claim 4

A transistor according to claim 1, wherein said first collector sub-layer is the first conductivity type and its impurity concentration is designed so that the impurity concentration compensates for a space charge produced by free carriers injected into the collector depletion layer upon transistor operation.

Claim 5

A transistor according to claim 1, further comprising a third collector sub-layer between said base layer and said first collector sub-layer, wherein said third collector sub-layer is the first conductivity type, and wherein said first, said second and said third collector sub-layers are depleted in an operating state and form an electric field distribution determined by parameters of said base layer, said first, said second, said third collector sub-layers and said collector buffer layer.

Claim 6

A heterojunction bipolar transistor comprising: an emitter layer of a first conductivity type; a base layer of a second conductivity type adjacent to said emitter layer; a collector buffer layer of the first conductivity type; and a collector layer arranged between said collector buffer layer and said base layer, wherein said collector layer includes a first collector sub-layer formed at the side of said base layer and a second collector sub-layer arranged at the side of said collector buffer layer, said first collector sub-layer is a semiconductor layer having an impurity concentration lower than that of said collector buffer layer, a band gap energy of which is formed in a predetermined distribution from said base layer to said second collector sub-layer, said second collector sub-layer is a semiconductor layer of the second conductivity type having an impurity concentration higher than that of said first collector sub-layer, a band gap energy which is formed in a predetermined distribution from said first collector sub-layer to said collector buffer layer, and said collector buffer layer has a band gap energy larger than that of said second collector sub-layer, wherein said first collector sub-layer and second collector sub-layer are depleted in an operating state and form an electric field distribution determined by parameters of said base layer, said first collector sub-layer, said second collector sub-layer and said collector buffer layer.

Claim 7

A heterojunction bipolar transistor comprising: an emitter layer of a first conductivity type; a base layer of a second conductivity type adjacent to said emitter layer; a collector buffer layer of the first conductivity type; and a collector layer arranged between said collector buffer layer and said base layer, wherein said collector layer includes a first collector sub-layer formed at the side of said base layer and a second collector sub-layer arranged at the side of said collector buffer layer, said first collector sub-layer is a semiconductor layer having an impurity concentration lower than that of said collector buffer layer, a band gap energy of which is formed in a predetermined distribution from said base layer to said second collector sub-layer, said second collector sub-layer is a semiconductor layer of the second conductivity type having an impurity concentration higher than that of said first collector sub-layer, a band gap energy which is formed in a predetermined distribution from said first collector sub-layer to said collector buffer layer, and said collector buffer layer has a band gap energy larger than that of said second collector sub-layer, wherein said first collector sub-layer and second collector sub-layer are depleted in an operating state and form an electric field distribution determined by parameters of said base layer, said first collector sub-layer, said second collector sub-layer and said collector buffer layer.

Claim 8

A transistor according to claim 5, wherein the band gap energy of said first collector sub-layer is increasing from said base layer to said collector buffer layer.

Claim 9

A transistor according to claim 5, wherein distribution of the band gap energy of said first collector sub-layer and distribution of said second collector sub-layer form a pattern which is gradually increased as a whole.