Patent ID: 8293608

Claim:
Intermediate product for the manufacture of a multiple-channel FET device, said intermediate product comprising a stack of semiconductor layers, said stack including: (a) a first epitaxial silicon layer; (b) a first epitaxial silicon-germanium layer which has a composition of formula Si x Ge 1−x , said first silicon-germanium layer overlying said first silicon layer; (c 1 ) a second epitaxial silicon layer overlying said first silicon-germanium layer; (d 1 ) a second epitaxial silicon-germanium layer overlying said second silicon layer, wherein said layer has a composition of formula Si x−y 1 Ge 1−x+y i ; wherein layers (c 1 ) and (d 1 ) are followed in ascending order by one or more pairs of layers (c n ) and (d n ) consisting, respectively, of an epitaxial silicon layer and an epitaxial silicon-germanium layer of formula Si x−y n Ge 1−x+y n , where n is a positive integer of at least 2; wherein, in each SiGe layer, the Ge concentration is substantially constant throughout the layer; wherein the values y n for all values of n are chosen such that 0<y 1 <y n <y n+1 and the difference in the percentage of Ge atoms between successive SiGe layers lies in the range of 2% to 10%, such that (y n+1 −y n ) is comprised in the range 0.02 to 0.10.