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Subject: Re: Is there a theoretical limit to the speed of a processor?

Author: Jesper Antonsson

Date: 11:10:39 06/07/02

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On June 06, 2002 at 20:12:36, Robert Hyatt wrote:

>On June 06, 2002 at 19:40:56, Roy Eassa wrote:

>>In any case, how many more years can we keep doubling speed every 1.5 - 2 years
>>before we run into the laws of physics?  I can't imagine it's more than a few
>>decades more.
>
>
>I find it amazing that we have not already run into this.  Clock frequencies
>above 2 gigahertz are way into the microwave range, and electrical properties
>of things change way up there...  How electrons propagate.  How atoms physically
>move around.  Etc.

I attended a university course in Modern Physics some years ago where one of the
professors argued that the physical limit using extensions of current technology
lie around 20 nm; less than that nothing could keep electrons from tunneling
around too much. Today, P4 processors are manufactured using a 130 nm process
(0.13 micron). If 20 nm holds, since the number of transistors are  proportional
to the inverse square of the line width, we could get about 42 (that is:
(130/20)^2) times the transistors of today in the same space. That means the
chip limit for RAM would lie somewhere around there; 256*42 Mbit is somewhat
more than 8 Gbit, so barring a technology breakthrough, don't expect much more
per chip. 42 is between 5 and 6 doublings, so given 2 years for each doubling,
we should hit the barrier in ten to twelve years.

Processor speed, then? Well, that depends on what you use the transistors for,
but clock speed alone has traditionally scaled *better* than the inverse square
of the line width, so 100 GHz for a processor should be a conservative estimate
of the limit. Then the extra transistors enable us to get even more speed, so
200 million nodes/s should be very possible before we hit the physical wall.
That's Deep Blue capacity on a chip, folks!

One problem is that the cost of plants seem to increase almost as fast as speed,
so that may stop development before the actual physical limit does. Lets hope it
won't, though. :-)



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