US 5,177,583 AGranted

Heterojunction bipolar transistor

AssigneeKabushiki Kaisha Toshiba

InventorsTakahiko Endo, Riichi Katoh

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Abstract

In a first heterojunction bipolar transistor (HBT) of the present invention, base layers and collector layers are respectively divided into a plurality of layers and one of the base layers provided closer to the collector layer reiogn is set lower in impurity concentration than the other thereof provided closer to an emitter layer, thus solving a problem that thermal histories during epitaxial growth or during processes cause a set impurity distribution to be destroyed due to diffusion and thus a heterojunction is shifted from a p-n junction. Since minority carriers in the base can smoothly flow toward the collector, there can be realized an excellent HBT having a very high current gain and a very high cut-off frequency. In a second HBT of the invention, a base region comprises a first base layer of a low concentration having the same energy band gap as an emitter region and to be changed to a complete depletion layer in a thermally balanced state and a graded second base layer of a high concentration, and the first and second base layers form a heterojunction, thereby realizing an excellent HBT having a high speed performance which can exhibit a sufficient grading effect while preventing deterioration of an emitter-base voltage withstanding characteristic.

Claims17

Dependent claims

A heterojunction bipolar transistor of a double heterojunction structure comprising at least 5 layers including of a first semiconductor layer of a first electric conductivity type, a second semiconductor layer of a second electric conductivity type, a third semiconductor layer of the second conductivity type, a fourth semiconductor layer of the first conductivity type and fifth semiconductor layer of the first conductivity type, said first to fifth layers being sequentially formed as laminated, wherein forbidden band widths of said first, fourth and fifth semiconductor layers are set larger than forbidden band widths of said second and third semiconductor layers, a heterojunction between the third and fourth semiconductor layers is formed so that the forbidden band widths abruptly vary within a range of several atomic layers, the first semiconductor layer functions as an emitter, the second and third semiconductor layers function as a base, and the fourth and fifth semiconductor layers function as a collector,

said transistor satisfying relationships which follow:
W.sub.2 .gtoreq.W.sub.3 and N.sub.2 .gtoreq.N.sub.3 and N.sub.4 >N.sub.5
or
W.sub.2 .gtoreq.W.sub.3 and N.sub.2 >N.sub.3 and N.sub.4 .gtoreq.N.sub.5
where, said second semiconductor layer has an impurity doping concentration N.sub.2 and a thickness W.sub.2, said third semiconductor layer has an impurity doping concentration N.sub.3 and a thickness W.sub.3, said fourth semiconductor layer has an impurity doping concentration N.sub.4 and a thickness W.sub.4, and said fifth semiconductor layer has an impurity doping concentration N.sub.5.

2 more elements

Dependencies are inferred from the claim text.