pnp/npn transistors

Yes, he is. The letters mean the order of the types of semiconductor materials in the structure of the component. N-type (negative) semiconductor has extra electrons which it wants to get rid of, P-type (positive) lacks some electrons and it wants to get ones.

Please explain how any material has extra ( or lacks ) any electrons !

Graham

N actually stands for 'n material', p for 'p material'. The silicon ( typically ) is 'doped' with other elements to make the n and p material which have different electrical properties. The difference is that in one conduction through the material takes place using electrons ( n-type ) and in the other by so-called 'holes' ( p-type ).

Graham

Well, for starters, it is *not* electrically charged. The electrons *do* match the protons, making the whole thing neutral, just as with most of the other matter in the world.

What differs is whether there's *room* for the electrons in the crystal structure.

Around every silicon atom, there's room for 4 electrons in the crystal structure of silicon. These are the 4 valence electrons (outermost shell electrons) of silicon.

Put in an atom with only 3 electrons in its outer shell, and there's *still* room for 4. There is no net electrical charge, but there is a "hole" which an electron could move into. (Put in a lot of these and you get P-type silicon.)

Put in an atom with 5 electrons in its outer shell, and there's an electron left over. In terms of electrical charge, it needs to stay there (to balance the charge of the protons in the nucleus), but the crystal structure doesn't accommodate it. It hangs around without fitting tightly into a specific position. That is, it becomes a conduction electron. (Put in a lot of these and you get N-type silicon.)

Conduction electrons make it easy for matter to conduct electricity because they're free to move. Holes also make it easy for matter to conduct electricity because a hole can move; that is, an electron from the next atom can fall into it, leaving a hole at the next atom instead of where it started out.

Now then. In a diode, you apply a voltage to pull the holes (in the P-type material) toward the conduction electrons (in the N-type material). In the middle of the diode, they meet and join, and conduction occurs. If you apply the voltage the opposite way,

Reposting in order to finish the message...

you apply a voltage to pull the holes (in the P-type material) toward the conduction electrons (in the N-type material). In the middle of the diode, they meet and join, and conduction occurs. If you apply the voltage the opposite way, the holes and conduction electrons never meet, and no current flows.

Transistors are more complicated. One key concept is that if you can somehow bring a lot of electrons into a piece of P-type material, it starts to act like N-type. That is, it has conduction electrons because you brought them in by conduction, not because they came in with the atoms.

Well... Consider a three-layer stack, N, P, N. Use the first two (N, P) as a diode and make it conduct. That brings plenty of electrons into the P layer.

While this is going on, also apply a positive voltage to the third electrode (N). That ought not to make it conduct, because the second P N junction would have voltage on it in the reverse direction. But what actually happens is that when you make the first (N P) junction conduct, it's like opening a floodgate. Once those electrons get into the central P region, they go right on through. (The second P N junction doesn't stop them because the P layer is full of electrons and no longer acting like P material, no longer relying on holes to do its conducting.) You get many more electrons that you ask for; that is, the small current through the first junction is amplified. That's how an NPN transistor works.

I predict that someone will jump on me and say, "No, it's not that way at all, it can only be described mathematically." To which I reply: "Just because *you* can only quote formulas doesn't mean the physical phenomenon isn't real."

Pooh Bear kirjoitti:

I should use "quotation" marks :-) or something like that. It's true that in this case the material hasn't extra electrons to get rid of (or lack, for getting them), but there are "extra" electrons to transport the charge in the material, and in the p-type, there is "lack of them", meaning those "holes" to transport charge (electron gets to a hole).

But, there is actually such thing as material having extra, or lack of, electrons (meaning that the atom is out of octet, and it "wants" to get electrons, or get rid of them, to get the octet), but then it would be a case of chemistry (off-topic).

Yes, but the P and N, in the material types, stand for the positive and negative