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From: wonderer on 15 May 2006 01:52
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?


From: simo.kaltiainen on 15 May 2006 03:16
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.

From: Pooh Bear on 15 May 2006 08:15


simo.kaltiainen(a)sunpoint.net wrote:

> 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


From: Pooh Bear on 15 May 2006 08:20


wonderer wrote:

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

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

From: mc on 15 May 2006 08:59
> 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,


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