A method to improve transistor performance uses a wafer (100) of single-crystalline semiconductor with a first zone (102) of field effect transistors (FETs) and circuitry at the wafer surface, and an infrared (IR) laser with a lens for focusing the IR light to a second depth (112) farther from the wafer surface than the first depth of the first zone. The focused laser beam is moved parallel to the surface across the wafer to cause local multi-photon absorption at the second depth for transforming the single-crystalline semiconductor into a second zone (111) of polycrystalline semiconductor with high density of dislocations. The second zone has a height and lateral extensions, and permanently stresses the single-crystalline bulk semiconductor; the stress increases the majority carrier mobility in the channel of the FETs, improving the transistor performance.