I read somewhere that PNP transistors have lower noise than NPN transistors. The majority carriers in NPN are electrons where as in PNP they are holes, so I thought the noise would be much be less in NPN transistors.
Which is correct?
Electronic – Do PNP transistors have low noise
npnpnptransistors
Related Solutions
This question is far far far too well answered on the web to be sensibly answered here. This really is a "Google can tell you better than we can" type question.
Excellent examples include.
Wikipedia - very good discussion
Electronics tutorials- transistors.
and 6 other transistor related pages.
Much more ...
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.
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Best Answer
I am very sure that the noise generated by holes and electrons is quite the same. If there are differences in the NPN and PNP transistor, they must be an indirect cause and not caused by the type of carriers.
Essentially there are only two dominant types of (white) noise sources
1) shot noise (this is due to the discreteness of carrier flow when passing a potential barrier such as a PN junction). This is independent on the device structure, doping levels, material compostition, carrier type and so on. It is fundamental and the same for holes and electrons (independent on temperature, only depends on current!).
2) thermal or resisitive noise (due to brownian motion of carriers in a resisitive layers) This is also fundamental and only depends on temperature and overall resistance.
Hence transisitors, having e.g. different layer resistances will also generate different amount of noise. This may be the cause for different behavior of PNP and NPN transisors (or more precisely, between any pair of transistors, not just different polarity). Shot noise is quite the same in all bipolar transistors.
BTW: The noise generated by surface states (as mentioned by rawbrawb) is always some kind of 1/f noise and vanishes at a certain frequency (in contrast to the above two white noise sources). Contrary to white noise (which is quite easily explained) 1/f noise is a vrey complicated phenomenon with many different effects that can cause it. It strongly depends on material quality, device strucure and so on. Strong differences between devices are expected for 1/f noise. So always look for the frequency the noise is specified at!