Heterojunction bipolar transistors with reduced parasitic capacitance

1. A method of forming a device structure for a heterojunction bipolar transistor, the method comprising:

forming a first semiconductor layer including a first section disposed on a collector and on a trench isolation region coextensive with the collector;

forming a second semiconductor layer including a first section disposed on the first section of the first semiconductor layer and having an outer edge that overlies the trench isolation region; and

removing a second section of the first semiconductor layer from between the first section of the second semiconductor layer and the trench isolation region to define a cavity extending laterally from the outer edge of the first section of the second semiconductor layer to the first section of the first semiconductor layer,

wherein the first section of the first semiconductor layer is comprised of a first single crystal semiconductor material, the first section of the second semiconductor layer is comprised of a second single crystal semiconductor material, the collector has a first width, the first section of the first semiconductor layer has a second width greater than the first width, and the first section of the second semiconductor layer has a third width greater than the second width.

2. The method of claim 1 comprising:

before the second section of the first semiconductor layer is removed, removing a second section of the second semiconductor layer peripheral to the first section of the second semiconductor layer to define the outer edge of the first section of the second semiconductor layer.

3. The method of claim 1 wherein the second section of the first semiconductor layer is removed by etching, and comprising:

before the second semiconductor layer is formed, masking the first section of the first semiconductor layer; and

implanting the second section of the first semiconductor layer to introduce an element at a concentration effective to provide a first etch rate for the second section of the first semiconductor layer that is greater than a second etch rate of the first section of the first semiconductor layer or greater than a third etch rate of the first section of the second semiconductor layer.

4. The method of claim 1 wherein the second section of the first semiconductor layer is removed by etching, and comprising:

before the second semiconductor layer is formed, implanting the first semiconductor layer to introduce an element at a concentration effective to provide a first etch rate for the first semiconductor layer that is greater than a second etch rate of the first section of the second semiconductor layer.

5. The method of claim 1 wherein the second section of the first semiconductor layer is removed by etching, and forming the second semiconductor layer comprises:

before the second semiconductor layer is formed, depositing the first semiconductor layer with a first composition different from a second composition of the second semiconductor layer so as to include an element at a concentration effective to provide a first etch rate for the first semiconductor layer that is greater than a second etch rate of the first section of the second semiconductor layer.

6. The method of claim 1 comprising:

applying a mask to the second semiconductor layer that covers the first section of the second semiconductor layer and that exposes a second section of the second semiconductor layer overlying the trench isolation region; and

after the mask is applied, removing the second semiconductor layer to at least partially expose the second section of the first semiconductor layer.

7. The method of claim 6 wherein etching the second section of the first semiconductor layer from between the first section of the second semiconductor layer and the trench isolation region define the cavity comprises:

after the second section of the second semiconductor layer is removed, removing the mask from the second semiconductor layer; and

after the mask is removed, etching the second section of the first semiconductor layer to remove the second section of the first semiconductor layer selective to the first section of the first semiconductor layer and selective to the first section of the second semiconductor layer.

8. The method of claim 1 further comprising:

after the second semiconductor layer is formed, realigning the first section of the first semiconductor layer and the first section of the second semiconductor layer to form respective single crystal materials.

9. The method of claim 1 comprising:

forming at least one spacer peripheral to the first section of the second semiconductor layer and positioned to close an entrance to the cavity.

10. The method of claim 9 comprising: forming an intrinsic base layer with a single crystal first section and a non-single crystal second section, the first section of the intrinsic base layer conterminous with the outer edge of the first section of the second semiconductor layer and coupled by the first section of the first semiconductor layer and the first section of the second semiconductor layer with the collector, and the second section of the intrinsic base layer located on the trench isolation region adjacent to the at least one spacer and cooperating with the at least one spacer to close the cavity.