PURPOSE: To minimize recombination leak current generated at the isolation boundary caused by ion plantation in a p-n junction in order to prevent the deterioration of characteristics such as amplification factor at the time of manufacturing a heterojunction BT element and permit high-resistance and stable deep isolation.

CONSTITUTION: In an area which is not covered by resist on a multilayer semiconductor epitaxial film which has the p-n junction of a p+-AlGaAs layer 2 and an n-AlGaAs layer 3 on a GaAs substrate 1, an Ar ion which is heavier than oxygen is implanted by the energy which permits the p-n junction to be at a depth between an ion projection range Rp and Rp+δRp and as a result of forming the Ar ion implanting part 6, a p-n junction active area 7 is separated. After annealing is performed in a annealing furnace 8 under the prescribed conditions, an O ion is implanted in the multilayer semiconductor film using a resist pattern as a mask, an O ion implanting part which reaches the substrate 1 is formed and an element area is separated. Annealing is performed again in the furnace 8, a p+ layer is exposed by mesa-etching, (n) and (p) electrodes are formed and an element is annealed to be completed.