[SI-LIST] Re: si-list Digest V5 #210

From the Handbook of Physics

Semiconductors..."At absolute zero the ideal structure is an insulator.  At
elevated temperatures, electrons can be thermally excited from the valence
band to the conduction band, giving intrinsic conductivity from equal number
of conduction electrons and holes....."

Tom Dagostino
Teraspeed Labs
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Beaverton, OR 97008
503-430-1065
http://www.teraspeed.com
tom@xxxxxxxxxxxxx

Teraspeed Consulting Group LLC
121 North River Drive
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401-284-1827

Teraspeed is the registered service mark of
Teraspeed Consulting Group LLC

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Muranyi, Arpad
Sent: Monday, May 16, 2005 4:02 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: si-list Digest V5 #210


Thanks for the two responses to my question from Lynne, and Chris.

Let me recap the thread, and restate my question, because I don't
feel that it was completely answered yet.

The thread started with the question on what happens with silicon
semiconductor devices below -55 C.  A response came back to say
that at 77 K silicon will act as an insulator.  Someone else
responded that this was not the case.  Then the previous writer
corrected themselves saying that they were referring to pure
silicon being an insulator at such low temperatures.

This is when I asked my question.  I may have read too much into
the thread, but it seemed to imply that PURE silicon is an insulator
at those low temperatures, but less of an insulator at higher
temperatures, such as room temperature.  This surprised me.

I am certainly not an expert in this area, but I thought that PURE
silicon was an insulator because the way its crystal structure is
built.  There are no free electrons in it.  I don't see how temperature
can effect its conductance, unless something drastic starts happening
in the crystal structure.  Chris' response to my question pretty
much confirmed this, although I would be curious to hear more about
what those mechanisms are which can do that.

Lynne's response seems to apply to doped silicon.  I agree the mobility
of those carriers can be influenced by temperature a great deal, but
again, I was curious about PURE silicon, since that's what my first
response (question) was triggered by.

So is my recollection correct that at normal room temperatures and
thereabout, PURE silicon would still be a perfect insulator, or does
it have to be cooled way down (say to 77 K) to become an insulator?

Thanks,

Arpad
-----------------------------------------------------------------------
-----Original Message-----
From: lgreen [mailto:lgreen22@xxxxxxxxxxxxxx]=20
Sent: Monday, May 16, 2005 3:28 PM
To: Christopher.Jakubiec@xxxxxxxxxxxx; Muranyi, Arpad; =
si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: si-list Digest V5 #210

Hi, Arpad,

Silicon bulk resistivity (and conductivity) is strongly temperature
dependent. At room temperature, the resistivity is higher than metals, =
but
too high to use it as a good insulator (hence the name semi-conductor).
Resistivity is also very sensitive to impurities, with the usual =
impurities
leading to a slightly n-type material in silicon.

At "low" doping (about 10-100x the impurity level), the resistivity =
becomes
both lower and less sensitive to temperature.  It also becomes possible =
to
predictably control the built-in junction potential.

At very high temperatures, of course, the thermally generated carriers =
can
dominate over the doping.  But that is normally outside the operating =
range,
unless you get a local high current density.

- Lynne


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On
Behalf Of Christopher.Jakubiec@xxxxxxxxxxxx
Sent: Monday, May 16, 2005 2:17 PM
To: arpad.muranyi@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: si-list Digest V5 #210

Arpad,

I agree with your statement to an extent.  Current flow in silicon is
related to the amount of free carriers (either holes or electrons) that =
are
available to participate.  Free carriers can be induced both by means of
impurity doping in the silicon, and electron/hole pairs can be thermally
generated even in pure silicon (although not normally for a practical
purpose).

Regards,

Chris
Infineon Technologies
=3D20

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Muranyi, Arpad
Sent: Monday, May 16, 2005 2:15 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: si-list Digest V5 #210


Jut to clarify things I would like to ask a question:

My understanding was that pure silicon is not conductive, not even at =
room
temperature(?).  The doping is what freezes up some electrons to make it
conductive.  How does this relate to temperature?

Thanks,

Arpad
-------------------------------------------------------------------------=
--
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
=3D3D On Behalf Of Daniel Chow
Sent: Monday, May 16, 2005 10:44 AM
To: John Zasio
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: si-list Digest V5 #210

My bad.

My statement was true for pure silicon.

Heavy doping significantly changes the properties of silicon.



-----Original Message-----
From: John Zasio [mailto:zasio@xxxxxxxxxxxxxxxxxxx]=3D3D3D20
Sent: Monday, May 16, 2005 10:25 AM
To: Daniel Chow
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: si-list Digest V5 #210

Daniel,

Silicon devices do work and work well at liquid nitrogen =3D3D
temperatures.=3D3D3D20 CMOS devices will run at two to three times =
faster than
at room temperature.

In the mid 80s ETA Systems shipped a CMOS Supercomputer cooled by liquid

nitrogen. Although the product was not a commercial success, =
the=3D3D3D20
technology worked very well.

John Zasio

Daniel Chow wrote:

>Jon,
>
>No silicon part will work at liquid nitrogen temperatures (77 Kelvin).
>
>Silicon is an insulator at that temperature.  The charge carriers are
>"frozen out" at low temperatures.
>
>Please refer to Chapter 8 of "Solid State Physics" by Charles Kittel
for more details.
>
>Thanks!

>Daniel Chow, Ph.D.
>Sr. Product Engineer
>ALTERA
>Office: (408) 544-8100
>Fax: (408) 544-7602
>Email: dchow@xxxxxxxxxx=3D20
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