The light emission process from a p-n junction in the forward-bias region is slow to respond to modulation signals due to the indirect band structure of silicon. Experimental results for a reverse-bias region showing light modulation in the range of tens of gigahertz are observed for the first time. For such a light emitter, the limiting speed of light modulation is shown to be determined by the transit time of the minority carriers across the junction during the filament formation of breakdown currents, which has been demonstrated by simulation of the propagation of a shockwave-like pattern in the breakdown field. © 2004 IEEE.