# All NPN transistors work the same way?

I have a doubt and I cannot find a certain answer on the internet.

All NPN transistors should work in the same way?, I mean, the only difference in all the models are just the rating currents and voltages?

My question is driven as I want to control the speed of a DC motor with an Arduino, and the suggestion is to use an NPN transistor, and on the example they use the BUF654, but on my country there's no model like that available.

So the BUF654 will work the same as a 2N5551 transistor?, the difference will just be the ratings?

• If somebody has a "doubt" about something there must be a reason behind this doubt. Why do you think that there could be a basic difference in the working principle of npn transistors? – LvW Jul 28 '18 at 9:03
• The pinout can also be different. – Dampmaskin Jul 28 '18 at 10:45
• @LvW I've concluded that the word "doubt" is often used by non-native English speakers when they mean "question". I don't know if they are using some on-line translation or they are taught that way in school, but I see it very often. It is annoying because the meanings are different, but there it is. – Elliot Alderson Jul 28 '18 at 10:56
• @Elliot Alderson certainly, I'm not a native English speaker, and a doubt for me is something that I'm not sure about, I'm an electrical engineer so I have some knowledge on electronics, and in my understanding the answer to my question is "yes", however I wasn't 100% sure as this is a very basic question, that why I called a doubt, because I wasn't totally sure about it, I will search if it's wrong to say it in that way, so thanks for the comment – Ronald Petit Jul 28 '18 at 14:25
• In English, the word "doubt" often implies that you believe that something you have been told is untrue or unlikely, as in "I doubt that you can eat the whole pizza." At the very least it means that you already have some information about something and you're not sure it is true. A "question" may mean that you have no existing knowledge of something or that you don't have an opinion about whether something is true or not. It's a subtle difference, I know, but native English speakers will hear it. – Elliot Alderson Jul 28 '18 at 15:09

There are two ways to see whether one transistor will substitute for another in a specific application.

1) Compare the transistor ratings
2) Compare the new proposed transistor with the application requirements

Even if the second transistor is inferior to the first, if it meets the requirements of the application, it will still be OK.

In your specific case, let's compare those two transistors 2N5551 and BUF654

parameter    BUF654    2N5551

power 25C    80W       625mW


there's a big difference before we look at anything else!

max VCEO     400V      160V
max IC       12A       600mA


I don't think we need to compare any more. You're probably going to need a bigger boat. The 2N5551 is definitely inferior to the BUF654.

However, it may still be big enough for the application. It may be a really tiny motor, drawing only a few hundred mA, and the circuit switching PWM, rather than burning power to control it. In which case, the 2N5551 may be OK.

• Then again, I've seen God knows how many low-voltage, low-power circuits on this site which use 1N4007s as general purpose diodes, so it's not actually certain that a 2N551 won't work in this (unspecified) application. – WhatRoughBeast Jul 28 '18 at 12:50

Bipolar transistors can have various base-regions width and various distances from the base-bond-wire to the useful base located between emitter and collector; this distance, and the aspect ratio of the path, and the resistance per cube of the path material, determine the rbb' which is a huge factor in setting the Boltzmann/Johnson/Nyquist random noise floor.

That same base region sets the transit time between emitter and collector, which sets Ftau (also called Falpha, and scaled by 1/beta becomes Fbeta); thus the maximum useful frequency for untuned amplifiers is set by the base region; oscillators may work a bit faster than Ftau because of energy resonances and thus more optimal energy movement.

Current specs and Voltage specs have already been mentioned.

Thermal resistance will vary dramatically, determined by die size and how heat can exit the bipolar die; if only bond wires to remove heat, expect 500 degree Centigrade per watt, or more. If a heavy copper slab is under the die, and your package provides access to the underside of that copper slab in a heat removal scheme, you may have 1 or 2 degree Centigrate per watt.

Then there are the high frequency junction capacitances.

Also the huge logarithmic range of operation, from picoAmps to milliAmps, may have a slope of 1 or a slope of 2, or some slope between 1 and 2, determined by various nuances of bipolar construction I have never needed to explore. Just be aware the 0.058 volts of delta_Vbase per 10:1 change in current ... may not be an accurate number.

The details of the exponential behavior (0.058 volts per 10:1) is crucial in defining the Taylor Series model of bipolar distortion, useful in IP2 and IP3 predictions. As briefly mentioned in the prior paragraph, different part numbers may have different logarithmic values, which will change the IP2 and IP3 predictions (and the actual measured distortion properties).