Semiconductor device having doping multilayer structure

A bipolar transistor comprising:

• a semiconductor substrate;
• an emitter layer of a first conductivity type, a base layer of a second conductivity type, opposite to said first conductivity type, and a collector layer of said first conductivity type, formed on said substrate;
• an emitter electrode, a base electrode, and a collector electrode for electrically connecting to said emitter layer, said base layer and said collector layer, respectively;
• a doping multilayer structure placed in contact with said base layer and with said base electrode for electrically interconnecting said base electrode and said base layer;
• wherein said doping multilayer structure has an opening through which said emitter layer is connected with said base layer, and wherein said doping multilayer structure comprises first semiconductor layers doped heavily with impurity of said second conductivity type, and second semiconductor layers formed of a same semiconductor material as that of said first semiconductor layers and doped lightly with impurity of said first conductivity type, said first and second semiconductor layers being alternately stacked, the ratio of impurity concentration in said first semiconductor layers to that in said second semiconductor layers being at least ten, and each of said first and second semiconductor layers having a thickness in a range from 10 .ANG. to 1000 .ANG..