Why do some power transistors emit light?
It's probably a hot carrier avalanche condition. Normally a Si junction would not emit visible wavelengths of light (Si band gap is 1.1eV). It is of no help to you, but I vaguely recall seeing that photo years ago- a dim red light, if I recall correctly. It's complicated by the opacity of Si to visible light, so the junction must have been exposed. The phenomenon can be (indirectly) observed in small signal Si transistors. 

Interestingly, CMOS transistors emit a faint light which can be used as a kind of ideal (in the sense it does not affect the circuit) current probe to directly measure timing.