[SI-LIST] Re: si-list Digest V5 #210
- From: Ed Sayre III <esayre3@xxxxxxxx>
- To: venkat@xxxxxxx
- Date: Mon, 23 May 2005 14:23:42 -0400
Venkat and others,
The reason a MOS device is faster at low temperature is principally due
to the increased electron mobility with decreasing temperature. Most
material resistivity is directly related to temperature. This is exactly
why a device edge rate decreases (increasing rise time) with increasing
temperature. If the edge is slowing down, then there is not as much charge
transfer because the resistance of the materials is retarding current flow.
(very simply put)
If you are looking for a more detailed explanation please refer to the paper.
"Cryogenically Cooled CMOS,"K. Rose, R. Mangaser, C. Mark, and E. Sayre 3rd
, Critical Reviews in Solid State and Materials Sciences, 24, (1), 63-99,
(1999).
Regarding Undoped or a intrinsic, silicon;
Intrinsic Silicon has a very high resistivity at most temperatures since
the Silicon atoms are held in the lattice via covalent bonds and thus no
holes or electrons are free. This assumes a perfect crystalline structure
without defects. The addition of impurity atoms add or remove electrons
dependant on the location in the periodic table (3 or 5 columns) (n-type or
p-type material). Doping concentration and Temperature effect the number
of free electrons which is represented by the Fermi level of the material.
I recommend the modular series on Solid state devices for all who are
looking for a review..
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/
235 Littleton Rd, Unit 2 Tel +1.978.392-8787
Westford, MA 01886 USA Fax +1.978.392-8686
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At 12:25 PM 5/23/2005 -0500, Satagopan, Venkat Raghavan (UMR-Student) wrote:
>I guess I am missing something about this thread.
>My understanding from few of the threads is that=20
>Increase temperature leads to more number of electrons in conduction =
>band
>Increased temperature leads to reduction in threshold voltage Vt of say =
>MOSFET.
>Reduction of Vt would increase the switching speed of the device making =
>it faster.
>=20
>So how would reducing the temperature to liquid nitrogen levels make the =
>device work two to three times faster.
>Kindly help me clear my confusion.
>Thanks
>Venkat
>=20
>
>________________________________
>
>From: si-list-bounce@xxxxxxxxxxxxx on behalf of Tom Dagostino
>Sent: Mon 5/16/2005 6:51 PM
>To: arpad.muranyi@xxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [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
>13610 SW Harness Lane
>Beaverton, OR 97008
>503-430-1065
>http://www.teraspeed.com
>tom@xxxxxxxxxxxxx
>
>Teraspeed Consulting Group LLC
>121 North River Drive
>Narragansett, RI 02882
>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]=3D20
>Sent: Monday, May 16, 2005 3:28 PM
>To: Christopher.Jakubiec@xxxxxxxxxxxx; Muranyi, Arpad; =3D
>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, =
>=3D
>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 =3D
>impurities
>leading to a slightly n-type material in silicon.
>
>At "low" doping (about 10-100x the impurity level), the resistivity =3D
>becomes
>both lower and less sensitive to temperature. It also becomes possible =
>=3D
>to
>predictably control the built-in junction potential.
>
>At very high temperatures, of course, the thermally generated carriers =
>=3D
>can
>dominate over the doping. But that is normally outside the operating =
>=3D
>range,
>unless you get a local high current density.
>
>- Lynne
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
>=3D
>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 =
>=3D
>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
>=3D3D20
>
>-----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 =
>=3D
>room
>temperature(?). The doping is what freezes up some electrons to make it
>conductive. How does this relate to temperature?
>
>Thanks,
>
>Arpad
>-------------------------------------------------------------------------=
>=3D
>--
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>=3D3D3D 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]=3D3D3D3D20
>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 =3D3D3D
>temperatures.=3D3D3D3D20 CMOS devices will run at two to three times =3D
>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, =3D
>the=3D3D3D3D20
>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=3D3D20
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>=20
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