The increase in semiconductor conductivity that occurs when a hard indenter is pressed into its surface has been recognized for years(1-5), and nanoindentation experiments have provided numerous insights into the mechanical properties of materials. In particular, such experiments have revealed so called pop-in events, where the indenter suddenly enters deeper into the material without any additional force being applied; these mark the onset of the elastic-plastic transition(6-11). Here, we report the observation of a current spike-a sharp increase in electrical current followed by immediate decay to zero at the end of the elastic deformation-during the nanoscale deformation of gallium arsenide. Such a spike has not been seen in previous nanoindentation experiments on semiconductors(1-5), and our results, supported by ob initio calculations, suggest a common origin for the electrical and mechanical responses of nanodeformed gallium arsenide. This leads us to the conclusion that a phase transition is the fundamental cause of nanoscale plasticity in gallium arsenide(12), and the discovery calls for a revision of the current dislocation-based understanding of nanoscale plasticity(6-11).