an algaas / gaas heterojunction bipolar transistor in which a base doping concentration is set to be as very high as 5 × 10 19 cm - 3 or more to cause a band gap narrowing effect and to reduce a band gap difference by an amount corresponding to the band gap narrowing effect , an al composition ratio , i . e ., x in an al x ga 1 - x as material of an emitter layer is set to be less than 0 . 25 , whereby an emitter resistance can be made small , an operating speed can be made fast and a current gain can be made high . preferably , the base doping concentration is set to be 1 . 2 × 10 20 cm - 3 or more so that the operation speed can be made remarkably fast .