Abstract: A semiconductor device structure comprises an active layer and a buffer layer. The active layer is a quantum well structure. There is a lattice mismatch between the buffer layer and the active layer which places the active layer under biaxial compressive strain. Uniaxial tensile strain is applied to the active layer to reduce compressive strain on the active layer in a second direction but not in a first direction. This favours hole and electron mobility in the first direction, rendering the semiconductor device structure suitable for the formation of both p-channel and n-channel devices.