SHAPED TERMINALS FOR A BIPOLAR JUNCTION TRANSISTOR
Device structure and fabrication methods for a bipolar junction transistor. An emitter layer is formed on a base layer and etched to form an emitter of the device structure. The emitter layer has a concentration of an element that varies as a function of the thickness of the emitter layer. The etch rate of the emitter layer varies as a function of the concentration of the element such that the emitter has a variable width over the thickness of the emitter layer.
1 . A method of fabricating a device structure, the method comprising:
forming a base layer;
forming an emitter layer with a thickness on the base layer; and
etching the emitter layer to form an emitter of the device structure,
wherein the emitter layer has a concentration of an element that varies as a function of the thickness of the emitter layer, and an etch rate of the emitter layer varies as a function of the concentration of the element such that the emitter has a variable width over the thickness of the emitter layer.
2 . The method of claim 1 wherein the concentration of the element increases with increasing distance from an interface between the emitter layer and the base layer.
3 . The method of claim 2 wherein the concentration of the element increases linearly with the increasing distance.
4 . The method of claim 1 wherein the concentration of the element decreases with increasing distance from an interface between the emitter layer and the base layer.
5 . The method of claim 4 wherein the concentration of the element decreases linearly with the increasing distance.
6 . The method of claim 1 wherein the element is germanium, and the emitter layer is comprised of silicon-germanium.
7 . The method of claim 1 comprising:
etching the base layer to form an intrinsic base of the device structure.
8 . The method of claim 7 wherein the base layer is formed on a substrate, and further comprising:
etching the substrate to form a plurality of trenches extending through the base layer and bounding a collector of the device structure.
9 . The method of claim 8 wherein the base layer and the emitter layer are laterally etched to respectively form the intrinsic base and the emitter layer.
10 . The method of claim 9 wherein the emitter layer has a first peak concentration of the element that is less than a second peak concentration of the element in the base layer such that a first lateral etch rate of the emitter layer is greater than a second lateral etch rate of the base layer.
11 . A device structure for a bipolar junction transistor, the device structure comprising:
a base layer; and
an emitter on the base layer,
wherein the emitter has a height and a concentration of an element that varies as a function of the height, and the emitter has a width that varies as a function of the height in relation to the concentration of the element.
12 . The device structure of claim 11 wherein the concentration of the element increases with increasing distance from an interface between the emitter and the base layer.
13 . The device structure of claim 12 wherein the concentration of the element increases linearly with the increasing distance.
14 . The device structure of claim 11 wherein the concentration of the element decreases with increasing distance from an interface between the emitter and the base layer.
15 . The device structure of claim 14 wherein the concentration of the element decreases linearly with the increasing distance.
16 . The device structure of claim 11 wherein the element is germanium, and the emitter is comprised of silicon-germanium.
17 . The device structure of claim 11 wherein the base layer includes an intrinsic base of the device structure.
18 . The device structure of claim 17 wherein the base layer is formed on a substrate, and further comprising:
a plurality of trenches in the substrate; and
a collector bounded by the trenches.
19 . The device structure of claim 11 wherein the emitter has a first peak concentration of the element that is less than a second peak concentration of the element in the base layer.
20 . The device structure of claim 19 wherein the element is germanium, the emitter is comprised of silicon-germanium, and the base layer is comprised of silicon-germanium.