Hetero-junction bipolar transistor

1 - 3 . (canceled)

4 . A hetero-junction bipolar transistor comprising:

a sub-collector layer formed on a substrate and having conductivity;

a first collector layer formed on the sub-collector layer;

a second collector layer formed on the first collector layer and having the same conductive type as a conductive type of the sub-collector layer; and

a semiconductor layer provided between the first collector layer and the second collector layer so as to have a composition ratio varying in the direction from part of the semiconductor layer located closer to the first collector layer to part of the semiconductor layer located closer to the second collector layer.

5 . The hetero-junction bipolar transistor of claim 4 , wherein the first collector layer contains InGaP,

wherein the second collector layer contains GaAs,

wherein the semiconductor layer contains a compound expressed by a general formula of Al x Ga |1-x| As where 0≦x≦1, and

wherein an x value in the general formula is reduced in the direction from an interface of the semiconductor layer with the first collector layer to an interface of the semiconductor layer with the second collector layer.

6 . The hetero-junction bipolar transistor of claim 5 , wherein the x value is 0.25 at the interface of the semiconductor layer with the first collector layer and the x value is 0 at the interface of the semiconductor layer with the second collector layer.

7 - 10 . (canceled)