🚀 go-pugleaf

In article <5300@decwrl.DEC.COM>, williams@kirk.dec.com (John Williams) writes:
> GaAs is 10-100 times faster than Silicon.

Most GaAs and silicon devices work by field effect (MESFET, JFET, MOSFET)
wherein a charge on the gate induces an equal & opposite charge in the
channel, and this induced charge is what carries the output current.
Switching speed is determined by how fast you can move this induced
charge, i.e. how much output current you can get for a given charge
applied on the input.

To move the charge faster, you can use a higher-mobility material,
such as GaAs, whose (electron) mobility is 5 to 6 times that of
silicon.  You can also apply a stronger electric field (usually
the same voltage over a shorter distance).  But the charges will
only move just so fast (velocity saturation).  Most modern devices
run at that limit.  This limiting velocity is approximately 50 km/s
for Ge, 100 for Si, 220 for GaAs, and 250 for InP.

Mobility is a measure of how far the electrons can fly between
collisions.  For very short gates in GaAs or InP, some electrons
will make it all the way across without a collision ("ballistic
transport") which allows much higher velocities.  This is still
a research topic.

The way GaAs currently gets most of its speed is the short gate
length.  But silicon is catching up in that respect.

Silicon is so successful primarily because it is so easy to make
superb silicon dioxide gate insulation.  (Any freshman can do it).
GaAs has no such natural insulator, so the gate is isolated with
a Schottky diode.  The diode only insulates in one direction; the
inputs have to stay below +0.7 volts or the diode will conduct.
This means the inputs either can't swing very far, or they swing
negative.  Swinging negative requires level-shifting, which eats
chip area and power; small swings require tolerances tighter than
can be readily manufactured.

GaAs is also brittle and poisonous.

Reference:  "Physics of Semiconductor Devices" (2nd ed), S. M. Sze,
	Wiley & Sons 1981.

Don Speck	speck@vlsi.caltech.edu	seismo!cit-vax!speck