Show that substitution of an arsenic atom with 5 valence electron for a silicon atom with it's 4 valence electron in the donor layer gives a site with an excess of 1 electron, whereas substitution of a boron atom with only 3 electron for a silicon atom in the acceptor layer give a site "hole" that is deficient in 1 electron. The surface of a donor layer in contact with an acceptor layer contains electrons that are attracted to the acceptor layer.