A HBT on a GaAs substrate is presented, wherein its base comprises a first base layer comprising IniGa1-iAs with an Indium content i with a slope s1 and a second base layer on the emitter side comprising InjGa1-jAs with an Indium content j with a slope s2, and an average of s1 is half of the average of s2 or smaller; or the base comprises a first base layer comprising InmGa1-mAs with an Indium content m and a second base layer on the emitter side comprising InnGa1-nAs with an Indium content n, and an average of n is larger than the m at a second base layer side; or the base comprises a first base layer pseudomorphic to GaAs with a bulk lattice constant larger than GaAs, and the emitter comprises a first emitter layer pseudomorphic to GaAs with a bulk lattice constant smaller than GaAs.