We report the transformation of an ordinary hydrogenated amorphous silicon nitride based heterojunction pin diode into a strong visible light emitting electroluminescent device. This forming process (FP) comprises Joule heating induced crystallization during the application of sufficiently high forward bias to the fresh diode. FP starts at an arbitrary point and continues until the accompanying visible light is uniformly emitted from the whole diode. This remarkable phenomenon is presented by real-time photography of the lateral propagation of the formed region. Finally, both the role of window electrode for a successful FP and the luminescence mechanism after FP are briefly discussed. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3592232] All rights reserved.