How do you tell different semiconductors (mainly transistors, sometimes even voltage regulator ICs, etc.) apart?

Do they all basically look the same or are there visual clues to indicate a certain type?

I mean, it's easy to tell a FET from a BJT when reading symbols from a schematic diagram, but when it comes to dismantling, troubleshooting, and repairing consumer electronics, I'm always getting stuck because I can't tell them apart. They always seem to have the same generic TO-92, TO-220, TO-263 appearance.




Failing that, are there any tests I can do (for example, with a multimeter) to determine exactly what I'm dealing with?

  • \$\begingroup\$ Are you planning on testing them after removal, first? Or are you trying to test them in-circuit? \$\endgroup\$
    – jonk
    Oct 19, 2017 at 19:35
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    \$\begingroup\$ The visual clue is the part number, printed or engraved in the housing. If there is none you must consult the documentation of the device. If it has none, you can try to play detective. \$\endgroup\$ Oct 19, 2017 at 19:36
  • \$\begingroup\$ Part number, trace the traces around it to figure out how it sits in the schematic, diode test with multimeter to find out type and pinout and general experience. \$\endgroup\$
    – winny
    Oct 19, 2017 at 20:28
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    \$\begingroup\$ Yup markings is mostly it. You CAN glean a guess by doing some diode type measurements, but even then you can be way off. BTW it used to be easier before surface mount when devices had the space to print big numbers on or around them. \$\endgroup\$
    – Trevor_G
    Oct 19, 2017 at 21:50
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    \$\begingroup\$ @tjt263 I don't have a lot (read: very little) of experience to apply here. So others will need to respond, I think. In-circuit, and covering JFET and MOSFET as well as PNP and NPN, would be 'tricky' I imagine. NPN and PNP isn't so hard, if all you need to test is the base-emitter or base-collector junction. Beyond that, I'd need to think a lot more to come up with design ideas. \$\endgroup\$
    – jonk
    Oct 19, 2017 at 22:11

3 Answers 3


In the days of larger packages there was enough room to print some form of part ID on the device. As packages have become smaller and part types more varied, this becomes increasingly difficult.

For example, in the past a TO-92 could be an NPN or a PNP bipolar, which you could tell apart by a voltmeter. As time went on, it could also be a JFET, MOSFET, a voltage reference, voltage regulator, current source, voltage monitor/reset, and so on. Good luck figuring that out with a voltmeter. But there’s enough room on the flat to print some kind of ID so you could tell a 2N3904 from an MPSA13 from a 2N7002 from a DS1233.

Now, shrink down to an SOT-23. Same devices, but less real estate to print. If you’re lucky you get a 2- or 3-letter code that the manufacturer uses buried at the end of the data sheet.

And now there are parts way smaller than that. So small in fact that you need a microscope to see the marking. Example: World’s smallest discrete FET.

What to do? You need a schematic and a BOM (Bill Of Material) to figure it out.

Another fun development in miniaturization: boards with no reference designators. Saves space yanno. So besides the schematic and BOM, you need an annotated layout too.

Ain’t progress wonderful?


Basic PNP and NPN transistors can be differentiated by using an ohmmeter.

The PN junctions are essentially diodes when the leads are tested separately. They will show open circuit in one direction and some measurable resistance in the other.

  • \$\begingroup\$ Yes, that sounds right. But I'm asking more about, say: Telling the difference between an N-Channel MOSFET & NPN Bipolar Transistor. Or a P-Channel MOSFET & PNP Bipolar Transistor. Hope that makes sense. It's hard to explain with so many different types. \$\endgroup\$
    – voices
    Oct 22, 2017 at 8:33
  • \$\begingroup\$ You pretty much have to go by the part no. printed on the device. Also, beware: the pinouts aren't consistent even for the same type of device like a NPN bipolar transistor in a TO-92. \$\endgroup\$
    – Rob Lewis
    Feb 9, 2020 at 21:37

The packages are just containers for various bits of silicon, sized based on how large the silicon is, and how much heat it needs to dissipate,

There will be a part number and manufacturers logo printed on 1 face of the device, generally its not too hard to work out what it is by searching for that number, e.g. 7805, or LM741,

But there are some harder ones, especially when it gets to smaller SMD packages, which at a point they sometimes change to 3 or 4 digit code that is not related to the part number, e.g. atmel eeproms, these codes are documented at the end of the chips datasheet, but not all search engines can find that. so it sometimes takes working out its general purpose and looking against that specific manufacturer.


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