Transistors are manufactured by growing germanium source and drain regions, implanting dopant impurities into the germanium, and subsequently annealing the source and drain regions so that the dopant impurities diffuse through the germanium. The process is simpler than a process wherein germanium is insitu doped with p-type or n-type impurities. The dopant impurities diffuse easily through the germanium but not easily through underlying silicon, so that an interface between the germanium and silicon acts as a diffusion barrier and ensures positioning of the dopant atoms in the regions of the device where they improve transistor performance.