| 摘要 |
A heterostructure bipolar transistor has an emitter layer, a base layer and a collector layer, the emitter layer being formed of a semiconductor material whose energy gap is wider than that of the base layer, so that a heterojunction is formed between the emitter layer and the base layer. One of the emitter layer and the base layer has first and second layers which are sequentially formed, and the first layer constituting the heterojunction has a lower impurity concentration than that of the second layer. When the impurity concentration and the thickness of the first layer are defined as N1 and W1, respectively, the following relation is satisfied: N1W12</=(2 epsilon s epsilon 0/q)Vbi where q: the absolute value of electron charge (=1.6x10-19 Coulombs), epsilon 0: the free space permittivity (=8.86x10-14 farads/cm), epsilon s1: the dielectric constant of the first layer, and Vbi: the built-in potential at the heterojunction.
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