[SI-LIST] Re: Doping effects.
- From: Ed Sayre III <esayre3@xxxxxxxx>
- To: rfengg@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 21 Mar 2003 10:45:26 -0500
Hi George,
A question right up my alley! When you dope a semiconductor you are
just providing more free electrons or holes, dependant on the dopant, to
reduce the resistivity. This is illustrated by the increases the drift
current through the material., At some point the doping becomes degenerate,
at which time the material is less silicon and more dopant. Basically
increasing the doping of a semiconductor makes it behave more like a metal,
to the point of degeneracy then it changes back.. This method of
degenerate doping can also be used to create an insolating layer in a
silicon substrate for isolation.
There are a very limited number of pure metals that are
superconductors, Niobium the most commonly used and the one with the
highest transition temperature, 4K. Others are Aluminum and Tin. The
transition temperature is the temperature at which a material becomes
superconducting. If you look at a resistivity versus temperature plot of
most materials and extrapolate to 0 K the resistivity does not go to zero
at 0 K, even superconductors above Tc. A superconductor will look like a
normal metal then transition, in a step wise fashion(the transition
region), to 0 resistance, at Tc, under DC conditions.
Since we use the interconnects of a chip, package, or system at
frequencies other than DC there is also another interesting
behavior. Superconductors actually have measurable loss at frequencies
above DC. These losses become significant and then pass those of cooled
Copper or Aluminum at about 100GHz for temperature of 77K (Liquid
Nitrogen). So the interesting thing is that cryo-cooled normal metals
actually perform better at high frequencies than high temperature or low
temperature superconductors. Ramo, Whinnery and VanDuzer, 3rd edition,
have an excellent plot of this on page 152 figure 3.16b.
BTW, this was one of the areas of my research. Hope this helps.
Regards
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NORTH EAST SYSTEMS ASSOCIATES, INC
-------------------------------------
"High Performance Engineering & Design"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Edward Sayre 3rd e-mail: esayre3@xxxxxxxx
NESA, Inc. http://www.nesa.com/
5 Lan Drive, Suite 200 Tel +1.978.392-8787 x 218
Westford, MA 01886 USA Fax +1.978.392-8686
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At 03:30 PM 3/21/2003 +1100, you wrote:
> Hi all,
> If doping increases the conductivity of Silicon , why cant we
> attain superconductivity with heavy doping of a material? Whats the
> phenomenon that limits the conductivity if we actually do increase
> carriers by doping?
>
>Thanks and regards,
>George.
>
>
>
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