Back in the days, when phototransistors were hard to get (especially in some parts of the world like the communist eastern block), one could brew their own by just filing/sawing off the top of a TO 18 NPN or PNP to expose its junction. The the collector-emitter pair would serve as a photosensitive device.
Since I'm just an amateur, I'd like to know whether it's possible to obtain something out of an unjunction transistor via the same treatment (e.g. the universal 2n2646 is an ideal, cheap and common candidate). The goal would be to obtain a light controlled saw-tooth/relaxation oscillator (probably an on-off modulation might be the most intuitive behavior).
Later edit: I managed to build a simple audio saw-tooth oscillator with a home-brewed unijunction phototransistor. To test it, I used both visible range and IR light from a remote control, and, as in this question's hypothesis, the transistor seemed to shift its curve slightly, the only testimony being the modulation of the audio frequency by the strength of the light shone onto the exposed junction. Anybody interested can try it out using such a circuit.
UPDATE A theoretical back-up of the observations/hypothesis of this question is provided here.