A self-aligned heterojunction bipolar transistor is disclosed which includes a semiconductor substrate having the (100) plane as a main surface, and at least a collector region, a base region, and an emitter region having a bandgap greater than the base region. The emitter region has an under-cut mesa structure and its crystal orientation is defined in a direction other than that parallel to the [011] direction. In neither the [001] direction nor the [011] direction has the transistor any outwardly slanted structure that could cause leakage current between the emitter and base and, hence, the transistor has improved electric isolation between the emitter and base although it is self-aligned.
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