A unique set of GaP semiconductor samples studied for over 50 years has exhibited significant improvement in their properties through the formation of the perfect host crystal lattice and the N‐impurity crystal superlattice. This article reviews this evolution of properties and discusses their novel utility in advanced optoelectronic devices. More specifically, nitrogen‐doped gallium phosphide (GaP:N) crystals that were originally prepared in the 1960s were theorized to form an excitonic crystal (1970s), and the best methods of their bulk, film, and nanoparticle crystal growth have subsequently been developed. The excitonic crystals yield novel and useful properties including enhanced stimulated emission and very bright and broadband luminescence at room temperature, which have been observed. These results provide a new approach to the selection and preparation of “perfect” materials for optoelectronics and offer a unique opportunity to realize a new form of solid‐state host – the excitonic crystal – as a high‐intensity light source with low thresholds for nonlinear optical effects.