GaAs CMOS in Cray-3 ?

[Mark G. Johnson]

In article <7048 at microsoft.UUCP> alexm at microsoft.UUCP (Alexander Mulder) writes:
>The most striking feature of the Cray-3 for me is that it is built using
>GaAs CMOS. At least, that is what I've heard every time the Cray-3 was
>discussed. Now, I always thought that this just could not be true, because
>the electron mobility in GaAs may be much higher than that in Si, the
>hole mobility is MUCH less than Si. Still, the steady stream of information
>about the Cray-3 in GaAs CMOS continues. Something of that must be true ?
>Do they (Cray Computer, that is) use some magic compound for their
>P-devices ? 
>
>--- Alexander Mulder ------------------- uunet!microsoft!alexm --



I have before me a copy of U.S. Patent number 4,638,188.
The inventor is Seymour R. Cray of Chippewa Falls, Wisconsin.
The assignee is Cray Research Inc. of Minneapolis, Minnesota.


"PHASE MODULATED PULSE LOGIC FOR GALLIUM ARSENIDE"

    The patent describes a circuit family which is constructed of
    N-channel depletion-mode MESFETs, schottky diodes, and resistors.
    There are __no__ P-channel MESFETs.  To quote the final sentence
    of the Abstract section of the patent:

	"The logic is preferably implemented in gallium arsenide
	 metal oxide semiconductor technology, with the capacitors
	 formed from reverse-biased Schottky diodes, and all FET
	 switches capacitively coupled and self-biased".


Perhaps this is the source of the confusion; Cray Research Inc. and
Mr. Cray himself seem to refer to the technology as "Gallium Arsenide
MOS".   However, there is __no__ mention of complementary FETs in
the patent.  What *is* in the patent, however, is fascinating.  The basic
gate is a 16-input and-or-invert element: the NOR of 4 four-input ANDs.
Each gate, as a side effect, also functions as a latch.

What's really fascinating is that information is encoded as the phase
(0 degrees or 180 degrees) of a 50-50 square wave.  This guarantees
that every signal must transition exactly once per cycle, so power-supply
current transients are independent of what the logic is doing.  Followers
of Cray publications, dating way back to the early 60's and the CDC-6600,
will recognize power-supply-stability as one of Mr. Cray's pet projects.


          +----+    +----+    +----+    +----+    +----+    +----+
CLOCK     |    |    |    |    |    |    |    |    |    |    |    |
      ----+    +----+    +----+    +----+    +----+    +----+    +----
          .         .         .         .         .         .         .
          .         .         .         .         .         .         .
          .         .         .         .         .         .         .
          +----+    .    +----+    +---------+    +----+    .    +----+    
DATA      |    |    .    |    |    |    .    |    |    |    .    |    |    
      ----+    +---------+    +----+    .    +----+    +---------+    +
          .         .         .         .         .         .         .
          .    1    .    0    .    0    .    1    .   1     .    0    .
          .         .         .         .         .         .         .

(Incidentally, I didn't just stumble upon this patent while perusing
the Official Gazette :-).  It was published in the IEEE Journal of
Solid-State Circuits, which is where I found it.)

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

There have been papers published about complementary MESFET GaAs, but
_not_ by Cray Research.  The ones that I've seen use the really crappy
P-MESFETS for exactly one purpose: to construct very low-power load
elements inside static memory cells.  P-MESFETS were not used anywhere
else (presumably because the gm is so rotten due to poor hole mobility).
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	...!decwrl!mips!mark	(408) 991-0208