Re: Efficiency

Ric Locke wrote:
On Tue, 18 Sep 2007 19:11:35 +0100, Gerry Quinn wrote:

In article <1190130075snz@xxxxxxxxxxxxxxxxxxx>, ames@xxxxxxxxxxxxxxxxxxx says...

One thing puzzles me a bit: the same Group containing C, Si &
Ge also contains Tin and Lead; and we do not often hear about
semiconductors made from those last two. <g> Would that be a
consequence of the larger atoms? Or would Sn and Pb circuits
have to operate at temperatures below what we humans consider
convenient?
It's because of the larger atoms, essentially: larger atoms have more possible electron orbitals closer together in energy. In the solid state this leads to an unfilled continuum of available energy states for electrons with (essentially) no added energy, so the outer (valence) electrons can move freely under the influence of an electric field. That makes tin and lead metals rather than semiconductors.

- Gerry Quinn

In fact, tin's conduction electrons are so mobile that they survive
oxidation. The ubiquitous tin oxide, used as the transparent electrode
in most types of flat panel displays, is almost as conductive as the
base metal. Compare aluminum, more conductive in the metallic state
while its oxide is one of the best insulators available.

And of course carbon is on the other side -- electrons not mobile enough, whacking great gap between a fully filled energy band and the next available empty band.

This is a general pattern -- non-metals with small atoms in the top right of the periodic table, metals with large atoms at the bottom and left (well, most of it, to tell the truth) and a thin band of semi-metals in between.

Tim
.

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