US Patent 7538387 Stack SiGe for short channel improvement

A semiconductor structure includes a first compound layer including an element, and a first impurity having a first impurity concentration; and a second compound layer including the element and a second impurity of a same conductivity type as the first impurity, wherein the second impurity has a second impurity concentration, and wherein the second compound layer is on the first compound layer. The semiconductor structure further includes a third compound layer including the element and a third impurity of a same conductivity type as the first impurity, wherein the third impurity has a third impurity concentration, and wherein the third compound layer is on the second compound layer, and wherein the second impurity concentration is substantially lower than the first and the third impurity concentrations.