Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!tut.cis.ohio-state.edu!pt.cs.cmu.edu!MATHOM.GANDALF.CS.CMU.EDU!lindsay
From: lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay)
Newsgroups: comp.arch
Subject: Re: Clock Rates
Keywords: fast references quantam GaAs heterostructure ballistic
Message-ID: <7294@pt.cs.cmu.edu>
Date: 10 Dec 89 19:47:38 GMT
References: <1106@sunquest.UUCP>
Organization: Carnegie-Mellon University, CS/RI
Lines: 52


In article <1106@sunquest.UUCP> terry@sunquest.UUCP (Terry Friedrichsen) 
	writes:
>Could you do me (and perhaps many others in comp.arch) a favor, if it's
>convenient, and post some references to these "proposed exotic devices"?
>Sounds like interesing reading (I GOTTA find out what a "quantum dot" is!).

Sure. The one-color diagrams are in
	IEEE Journal of Quantam Electronics vol. QE-22, #9, Sep86.
	(Special issue: hundreds of pages: have a recent physics degree.)
	+ keep an eye on Applied Physics Letters.

Two-color diagrams are hard to find, the last one I saw was:
	Electronics Oct88 p.143 "Will Quantam-Effect Technology
	Represent a Quantam Jump in ICs?"
	
Three-color diagrams are likewise scarce:
	R.T.Bate, Scientific American vol. 256, #3, Mar88, p. 96.
	Mark Reed, Byte, May89, p.275 "The Quantam Transistor"
	Both gentlemen are physicists at Texas Instruments.

Working devices are even scarcer, i.e. not yet.  Also, products are 6
to infinity years away, so don't get _too_ excited. The basic insight
is very simple: there is a limit below which transistors will not
work: this is about the 0.2 - 0.35 micron level. Below that, quantam
effects will be unavoidable. So, the dream is to make quantam effects
into a feature rather than a bug.

Ballistic transistors are nearer-term. The insight here is that
electrons, moving through a crystal because of an applied voltage, do
_not_ travel at their "drift velocity". In fact, they accelerate,
then bump into the lattice ("emit phonons"). Then they accelerate
again, and so on. So, imagine a channel region shorter than the mean
free path. Electrons can cross "ballistically".

A GaAs ballistic transistor would have a channel of about 0.4
microns.  This is doable. A silicon ballistic transistor would have
to be smaller - bad news for silicon. Diamond devices would be the
biggest (if only we could make them). Drift velocities are higher at
lower temperatures, so liquid nitrogen cooling would allow larger
devices. However, the article I'm stealing all this from claims that
the biggest benefit occurs when the electrons are injected at high
speed, probably by a heterostructure. So far, heterostructures have
been GaAlAs on top of GaAs, or GaInAs on InP, or the like: bleah:
all hard to work with. There are recent reports of silicon hetero-
structures using germanium, or silicon carbide.  Hmmm: the set of
possible futures keeps growing.

Disclaimer: I don't do this stuff: I collect. Please correct any
mistakes, and information donations are welcome.
-- 
Don		D.C.Lindsay 	Carnegie Mellon Computer Science