1. an integrated circuit structure comprising: a substrate having at least two types of crystalline orientations; first-type transistors formed on first portions of said substrate having a first type of crystalline orientation; second-type transistors formed on second portions of said substrate having a second type of crystalline orientation; and a straining layer above said first-type transistors and said second-type transistors, wherein said first portions of said substrate comprise a layer having said first type of crystalline orientation and a layer having said second type of crystalline orientation, and wherein said second portions of said substrate comprise said layer having said second type of crystalline orientation and are devoid of said layer having said first type of crystalline orientation.