Heterojunction bipolar transistor with crystal orientation

DETAILED DESCRIPTION An object of the present invention is to obviate the above-described defects of the prior art and provide a method for fabricating HBTs which ensures electric isolation between the emitter and the base even when selective etching of the emitter layer with a solution is carried out for a short time, and an HBT which can be fabricated with ease, shows low leakage current between the emitter and the base, and is excellent in its high frequency characteristics.
Another object of the present invention is to provide a method for fabricating an HBT which can make an under-cut utilizing a mask on the emitter cap.
Still another object of the present invention is to provide a self-aligned HBT in which the base layer is doped with a dopant having a small diffusion constant, which has different conductivity types between the emitter and base electrodes, and which ensures sufficient electrical isolation between the emitter and the base.
Therefore, according to a first aspect of the present invention, there is provided a method for fabricating a heterojunction bipolar transistor, comprising the steps of: (a) forming a first semiconductor layer serving as a base layer on a collector layer formed on a compound semiconductor substrate; (b) forming on the first semiconductor layer a second semiconductor layer serving as an emitter layer; (c) forming on the second semiconductor layer a first conductive material layer which constitutes an emitter electrode, all edges of the emitter electrode being oriented in a direction other than the ?011! direction; (d) etching the second semiconductor layer by an etching process in which the etching rate differs depending on an orientation of a crystal due to anisotropy of the crystal, utilizing the first conductive material layer as a mask, to expose the first semiconductor layer; and (e) depositing on the substrate a second conductive material to form a base electrode.
Here, the first conductive material constituting the emitter electrode may differ from the second conductive material constituting the base electrode