All semiconductors are light sensitive, some more than others. Even LEDs can be used as photodiodes.
Back in the Paleozoic, when I was fresh out of school on my first job, one of the QA guys called me over to where he was testing radio receivers in the parking lot. The receiver enclosures were open so he could measure voltages. Bob said "look at this, why do you think it's doing this?" He'd stand in one place and the radio was working fine. Then he'd move a foot to his left and the radio would stop working. I was at a loss. No clue why it did that. Then he said "now look at this" and he held one finger so that it cast a shadow on a 1N4148. The radio started working again. Bob was an old Ham radio operator. He told me "silicon diodes are photosensitive. When sunlight hits 'em, the go into full conduction." At the same place we had some Motorola microcontrollers. They contained UV eraseable PROM and had a clear window on top for erasing them. The PROM and microcontroller were on the same chip. We had to cover the window when they were powered, otherwise normal room light was enough to make the microcontroller malfunction.
