Aluminum diffuses readily into silicon. for transistors with shallow source-drain regions, annealing frequently causes aluminum to diffuse through the source-drain region and short to the substrate. this problem, called "junction spiking," can be combated through the use of a thin titanium-tungsten (tiw) barrier between the aluminum and silicon. how might such a barrier affect device performance? (hint: read the section on contact resistance in the device analysis lab.)