I've posted a similar question to the high energy forum. As I understand it, LEDs emit photons when electrons and holes have sufficient energy to cross a particular "well". I'm sure this explaination is lacking in many key ways. Why can't we "tune" the gap to create electron-positron pairs and shape the crystalline structure to draw electrons to one side and positrons to the other? IOW, can a PED (positron emitting diode) be physically possible?