How do I identify transistors?

I'm a beginner hobbyist, I'm trying to get to the point where I can look at a basic circuit (physical, not a diagram) and speculate on what it does. Transistors are confusing, because there's so many types and some are visually similar. Obviously I could google whatever I find printed on the side of them, identify the manufacturer, etc. Assuming I don't want to bust out my laptop every time I see a transistor, what I can I determine?

What I'm asking is what I can know by:

• Looking. It has 3 pins on one side, it's D shaped, rectangular with through hole, hat shaped, round, Size, notches on the side, Any rules like "If it's D shaped, it's always a BJT" (is that true?)

• Testing. Things I can easily determine by poking it with a basic multimeter. Rules like "if there's high/infinite resistance between pins 1 and 2...". Whether or not such tests will work while it's in the circuit or if it needs to be removed to know for sure.

• Id numbers. Are there any naming schemes that are in wide enough usage to bother with?

I'm putting together some notes I can keep in the garage with things like resistor color codes, the various things each component is called, their diagram symbol, what each pin does, etc. I hope this isn't too open ended, if I knew more I'd ask something more specific. I've found a lot of resources that explain the types, what they do, and their symbols, which is great for design, but I'm having a rough time finding resources that explain how to identify the type of transistor by examining the actual thing.

• MOSFETs come in more varieties. Just not often. There are enhancement mode and depletion mode, for example.
– jonk
Commented Apr 7, 2021 at 18:47
• D-share absolutely does not guarantee that its BJT. If you don't have a number, you're shooting in the dark. If you have a transistor tester, you might get lucky (assuming what you're testing is a transistor and not a regulator or SCR or some such.) Commented Apr 7, 2021 at 19:01
• Ye olde linear regulators LM317 LM7805 etc use exactly the same TO 220 package. But it's usually quite easy to tell if something is a regulator or transistor by watching the surrounding PCB. TO 92 is almost certainly a transistor or MOSFET though. Commented Apr 8, 2021 at 10:33

Looking at the transistor reveals nothing. They all come in all shapes and have three pins.

Testing is possible, assuming the transistor is not broken, but testing does not work while the transistor is in circuit.

ID numbers or part numbers on the transistor tells what specific part it is, so looking it up reveals what it is and you can read the datasheet about important parameters.

The circuit around the transistor and how it is used may reveal clues about the type what it could be, or at least what it can't be.

Extremely coarse determination is possible by measuring the circuit or just figuring out what the transistor does in a circuit. Unless the circuit is somewhat specialized, in general the transistors are BJTs or MOS FETs. If it drives a load, it can be determined if it is a N-FET or NPN if the transistor switches load ground or P-FET or PNP if it switches load supply. If the transistor is controlled with current, then it's a BJT, if with voltage, it's a FET. JFETs are used to do very specialized things in analog circuits. But like I said, this is awfully general, and there may be many exceptions.

So all in all, it is simply easiest to read what is printed on the transistor and fetch the datasheet to know what you are working with and which pinout it uses.

• What if it's out of circuit, is there anything I could determine with a multimeter, or is a transistor tester required? Commented Apr 7, 2021 at 19:16
• nevermind, I found some good sources for that. Commented Apr 7, 2021 at 19:30
• Many multimeters have built-in transistor test socket for testing BJTs. Even if there isn't, you can use the multimeter diode measurement mode to determine a PNP or NPN BJT. Commented Apr 7, 2021 at 19:30

General answer to your question is: you don't (as alluded to in comments by Duston). Unfortunately you really can't definitively identify anything without looking up a part number and getting a datasheet.

Even among the same type of transistor in identical packages (say, a BJT in a TO-92 package), the order of the pins can be different. I just did a quick search on "TO-92 BJT" and picked the first two BJTs from Mouser, which demonstrate exactly my point:

Images captured from the datasheets of the respective parts (linked above).

Unless you know the details of the specific manufacturer and part number off the top of your head, there's no definitive way to know the specifics about a particular transistor package just by visual inspection.

No, you can't tell by the shape. And many ICs have packages that are the same.

Certain transistor types are more common, so you can use an ohmmeter to make an attempt at identification.

Most common are NPN and N-Channel MOSFET. From an ohmmeter's perspective, they look like the symbols on the right. For an NPN, you won't be able to tell which is the collector or emitter, and they need to be correct in the circuit, they won't work very well backwards. If it has a metal case or tab, that is usually the collector.

A PNP (not shown), of course, has the two diodes reversed.

If it is really old (1950s - 1960s), it probably isn't a MOSFET.

simulate this circuit – Schematic created using CircuitLab