Re: localization: insulators, white, etc.

["John S. Denker" <jsd@MONMOUTH.COM>]

On 04/11/2003 04:42 PM, Chuck Britton wrote:
> Isn't diamond a large-gap semiconductor

Indeed it is.

> and also a pretty good
> (practical except for expense) insulator at 'normal' temperatures????

Well, assuming it is pure and crystalline,
calling it a semiconductor is the starting
point and ending point.

Physically, it's like a vacuum.  If you inject
some charge into a vacuum, it looks like an
excellent conductor.  In the absence of injected
charge, it looks like an insulator.

Note that I avoided saying "is" a conductor or
"is" an insulator.  It _is_ a semiconductor.

The size of the gap (relative to kT) is relevant
if you're trying to build a minority-carrier
device, such as a bipolar junction transistor,
but it is quite irrelevant for majority-carrier
devices such as field-effect transistors.  I've
built FET circuits that work just fine at
cryogenic temperatures.  The fact that the gap
was huuuge compared to kT did not turn the device
into an insulator.

Whether you have an efficient means for injecting
charge depends on details.  A corona point is
known to be good for injecting charge into a
vacuum with very modest driving voltage.  Every
VLSI chip has multi-millions of ohmic contacts.
Making them is a bit of a black art, but I
conjecture that the physics is roughly analogous
to a corona point.

Practical insulators contain _traps_ which
immobilize injected charge.