Generally, and in one form of the invention a method is disclosed for forming a subcollector for bipolar transistors comprising the steps of epitaxially depositing a subcollector layer 22 on a substrate 20, the subcollector containing a co-deposited dopant; etching the subcollector layer to define an active device region; depositing a collector layer 24 above the subcollector layer; depositing a base layer 25 above the collector layer 24; and depositing an emitter layer 27 above the base layer 25, whereby the subcollector layer does not extend beyond the active device region and is of low resistance.

A scalable and relatively easily manufacturable heterojunction bipolar transistor (HBT) comprises a thin (exemplarily 5-25 nm) emitter layer that serves as an etch stop layer and that furthermore passivates the extrinsic base region. The portion of the emitter layer that overlies the extrinsic base region is essentially fully depleted at all bias voltages in the normal operating range of the transistor. Base contact is established through the emitter layer, exemplarily by means of a metallized region on the emitter layer. A novel technique for carbon doping is also disclosed. Use of the novel technique makes possible a further embodiment of the inventive HBT, wherein base contact is made by means of Be implantation into the emitter layer.

This is a method of fabricating a heterojunction bipolar transistor on a wafer. The method can comprise: forming a doped subcollector layer 31 on a semi-conducting substrate 30; forming a doped collector layer 32 on top of the collector layer, the collector layer doped same conductivity type as the subcollector layer; forming a doped base epilayer 34 on top of the collector layer, the base epilayer doped conductivity type opposite of the collector layer; forming a doped emitter epilayer 36, the emitter epilayer doped conductivity type opposite of the base layer to form the bipolar transistor; forming a doped emitter cap layer 37 on top of the emitter epilayer, the emitter cap layer doped same conductivity as the emitter epilayer; forming an emitter contact 38 on top of the emitter cap layer; forming a base contact on top of the base layer; forming a collector contact on top of the collector layer; and selective etching the collector layer to produce an undercut 45 beneath the base layer.

In a bipolar transistor including a semi-insulating substrate, a collector layer formed on the semi-insulating substrate and a base layer formed on the collector layer, a base contact layer is in contact; with a part of a lower surface of the base layer.

A semiconductor device includes a collector layer comprising a first conductivity type semiconductor layer, a base layer comprising a second conductivity type semiconductor layer produced on the collector layer, an emitter layer comprising a first conductivity type semiconductor layer produced on the base layer, a contact layer comprising an undoped semiconductor layer produced on the emitter layer, second conductivity type first implantation regions produced at regions each consisting of the contact layer, the emitter layer, and the base layer, so as to leave a central region therebetween, base electrodes produced on the first implantation regions, a first conductivity type second implantation region produced by implanting impurities from the surface of the contact layer extending into the emitter layer, in a region between the first implantation regions, and an emitter electrode produced on the second implantation region. Or, a semiconductor device includes an emitter layer comprising undoped semiconductor layer and a first conductivity type second implantation region produced by implanting impurities from the surface of the undoped semiconductor layer extending into the base layer, at a region between the first implantation regions.