Shaped terminals for a bipolar junction transistor

Abstract

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.