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Re: [Phys-l] Zener physics



On 12/04/2006 01:50 AM, Michael Edmiston wrote:

....
[1] The course is
just trying to take away some of the black-box mystery of the
instruments they use.

[2] But this is one data point where students indeed
have some difficulty with the notion of using some diodes forward biased
and other diodes reverse biased.

Those two statements fit together.

It looks to me that the black-box issue is being "pushed down one
level", i.e. rather than treating the whole circuit as a black box,
the students are treating the "Zener thingy" as a black box, without
any model for what is inside or how it works.

If students don't have a model for how something works, many of them,
particularly the bright ones, will /make up/ a model. For example,
you could /make up/ a model for a 6.5V "reference diode thingy" by
putting ten forward-biased diodes in series. It even works. Such
a thing would be operated with forward bias, unlike a Zener.

The alternative is to take what I call the "physics" approach. Tell
the students:
-- Clarence Zener was a /physicist/, not a magician.
-- The Zener diode isn't magic, it's physics. It has valence
band physics, valence band physics, and tunneling physics.
-- Don't think of it as a "Zener thingy". It is a _diode_. A
heavily-doped diode. It is just like any other diode, with a
P-region, an N-region, and a depletion layer at the PN junction.
The depletion layer is very thin because of the high doping.
Electrons can tunnel through thin layers. Tunneling is physics,
not magic.
-- If you think of it as a diode, then you know darn good and well
the forward voltage drop will never be larger than one diode drop;
otherwise there would be some exponentially gimongous current.
Hence half a moment's thought should tell you that if you want to
see a 5.6V Zener effect you will have to reverse-bias the diode.
It's a diode!

You can estimate the Zener current using WKB. It's just physics.
The math is here
http://ece-www.colorado.edu/~bart/book/book/chapter4/ch4_5.htm
and you can get the corresponding picture of the band structure out
of any solid-state physics text.

A useful homework exercise is to sketch the IV characteristic of
back-to-back Zener diodes in (a) series and (b) parallel.


From the Car Talk book, I often quote the passage from the beginning of
the chapter on automotive electrical circuits. Tom and Ray say that
space is not the final frontier (as said in the opening of Star Trek
episodes), they say electricity is the final frontier because people
don't know diddly about it.

You can make the same argument about a lot of things. Feynman once
said he thought fluid dynamics was the last frontier. People spend
their lives surrounded by fluids, but they don't know diddly about
how fluids really behave; much of what they "know" is wrong.