# pnp/npn transistors

Discussion in 'Electronic Repair' started by wonderer, May 15, 2006.

1. ### wondererGuest

A high school teacher tells the class that pnp stands for positive,
negative, positive and that npn stands for negative, positive,negative.

Is he right?

2. ### Guest

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.

3. ### Pooh BearGuest

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

Graham

4. ### Pooh BearGuest

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 ).

http://en.wikipedia.org/wiki/Semiconductors#N-type_doping

Graham

5. ### mcGuest

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

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,

6. ### mcGuest

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."

7. ### Guest

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