bipolardietransistors
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:
Pretty close.
You should give the PNP a lot more base current to make sure it saturates. Use a forced beta of 10-20, which means that the resistor R2 should be more like 470R.
The value of R3 is not critical, 10K would be fine, so would 20K.
You don't need a Darlington to drive the PNP transistor, a regular NPN will be fine. Use a base resistor of more like 10K. The base resistor limits the current- it is determined by the desired base current, not by the gain of the transistor- you do always need it.
Keep in mind these Darlington things won't switch instantly and if you don't allow some dead time in the drive firmware you'll get 'ghosting'.
This doesn't satisfy everything that Dwayne speaks of, but for a simple tester it should serve your purposes.
simulate this circuit – Schematic created using CircuitLab
Q1 can be either an NPN or a PNP. This is really just testing the polarity of the base terminal by assuming that current will flow out of the base in a pnp and into the base for an npn.
This will dimly light the LEDs depending on the base current the transistor can handle. If you need more brightness, I would go ahead and use an op-amp or a comparator to detect current flow between the voltage divider and Q1. That might look like this:
simulate this circuit
Note that this circuit doesn't detect if there's a short in your transistor or not so it's still a rudimentary transistor checker.
Best Answer
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.