Wo Licht ist, ist auch Schatten! ("More light, more shadow!" or simpler: "Nothing is perfect") -Johann Wolfgang von Goethe, from Got ] z von Berlichingen, Act I There exist already about ten books (e. g. [1-8]) - not counting the many conf- enceproceedingsvolumes- onthe differentaspectsofSi-basedphotonicsincluding also the issue of silicon-basedlight emission. Why is now anotherone neededabout this subject, and even more, exclusively about a special type of light emitters? This book summarizes all aspects of the development of rare earth (RE) c- taining MOS devices fabricated by ion implantation as the key technology and critically re ects the related referencesthroughoutthe different chapters. This work was mainly done in the course of the last 10 years. Preliminary work for this goal, undertaken mostly in the nineties, was based on the introduction of group IV e- ments (Si, Ge, Sn) into the thermally grown silicon dioxide leading to the highest power ef ciency values in the blue-violet wavelength range. This success inspired us to use the REs as means of exploring other wavelength ranges with the same or even higher power ef ciencies. After an historical introduction of the REs and silicon-based light emission, Chap. 1 presents a review of electroluminescence from MOS-type light emitters, based on silicon and its technology. The achievement of an optimized material for electrically driven light emission, that is, ef cient emission with reasonable reliability, is only possible with a deep knowledge of the materials properties det- miningtheelectro-optical(orphotonic)properties(seeChap. 2).