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is it possible to tell from this photograph whether the transistors are npn or pnp? And if so, what is the line of reasoning? How to sort that out?
This photo I took is from a Sharp GP1FAV51RK0F Toslink reveiver and shows two bipolar transistors (at least I guess so).
Edit:
Some more photos of other transistors on that die:
These devices look like vertical bipolar transistors (as opposed to lateral) that are different sizes or multiple emitter/collector. They are probably all the same type of transistor. If I had to guess, the purple area is the emitter, the yellow area is the base, and the blurry light blue rectangle on the bottom is the collector. Devices with multiple purple regions have multiple emitters (probably going to different nets).
If you can't find a different transistor structure, it's possible that there are only NPN (or PNP) devices.
The best bet for figuring out which terminals are emitters or collectors is to look at the VCC and GND pads of the chip and see where the metal wires for VCC and GND go. If the devices are NPN, then there will likely be transistors with their emitters connected directly to GND (or through a resistor). Likewise, if there are any direct connections to VCC, then those are probably PNP emitters. It helps if you can try and find some easily identified topology, like a differential pair.
I'll bet that there are a few differential pairs in that part, see if you can identify them. That will probably help the most for figuring out how things are connected.
