Epi layer advantages. Doping level and conductivity type of epitaxial layer is controlled independently of the substrate. Resistivity and thickness can be controlled very accurately, and multi-layer structures are possible. Chemically purer than the substrate → e.g. sensitive detector solutions. No OISF which benefits CMOS and bipolar device technology. Very high dopant levels available, e.g. BGe epi at 0.0006 Ohm-cm. Constant doping profile across the wafer.