US 6,982,442 B2
Structure and method for making heterojunction bipolar transistor having self-aligned silicon-germanium raised extrinsic base
Kevin K. Chan, Staten Island, N.Y. (US); Marwan H. Khater, Poughkeepsie, N.Y. (US); Kathryn T. Schonenberg, Wappingers Falls, N.Y. (US); and Panda Siddhartha, Beacon, N.Y. (US)
Assigned to International Business Machines Corporation, Armonk, N.Y. (US)
Filed on Jan. 06, 2004, as Appl. No. 10/707,712.
Prior Publication US 2005/0151165 A1, Jul. 14, 2005
Int. Cl. H01L 29/737 (2006.01)
U.S. Cl. 257—198 9 Claims
OG exemplary drawing
 
1. A heterojunction bipolar transistor (HBT), comprising:
a collector;
an intrinsic base overlying said collector, said intrinsic base including a layer of a single-crystal semiconductor alloy;
a raised extrinsic base including a first semiconductive layer overlying said intrinsic base and a second semiconductive layer formed on said first semiconductive layer, said first semiconductive layer being etch distinguishable from said second semiconductive layer; and
an emitter overlying said intrinsic base, said emitter disposed in an opening of said first and second semiconductive layers, such that said raised extrinsic base is self-aligned to said emitter,
wherein said first semiconductive layer has a first composition Six1Gey1, where x1 and y1 represent precentages of silicon and germanium of said first composition, respectively, and said second semiconductive layer has a second composition Six2Gey2, where x2 and y2 represent precentages of silicon and germanuim of said second composition, respectively, said precentages y1 and y2 of germanium being sufficiently different to make said semiconductive layer etch distinguishable from said second semiconductive layer and at least one of said precentages y1 and y2 of germanium varies as a function of vertical position over a thickness of said first semiconductive layer or said second semiconductive layer, respectively.