An electronic or optoelectronic device fabricated from a crystalline material in which a parameter of a bandgap characteristic of said crystalline material has been modified locally by introducing distortions on an atomic scale in the lattice structure of said crystalline material and the electronic and/or optoelectronic parameters of said device are dependent on the modification of said bandgap is exemplified by a radiation emissive optoelectronic semiconductor device which comprises a junction (10) formed from a p-type layer (11) and an n-type layer (12), both formed from indirect bandgap semiconductor material. The p-type layer (11) contains an array of dislocation loops which create a strain field to confine spatially and promote radiative recombination of the charge carriers.