THE FIRST microchips to reliably combine the performance of gallium arsenide
with the cheapness of silicon are being put through their paces by a US chip
maker. The technology could help realise the dream of cheap, superfast circuits
that communicate via light rather than electric currents.
Chip makers would like to move to gallium arsenide (GaAs) because it can be
used to build circuits that switch on and off much faster than those on silicon
chips. It’s this switching speed that dictates how fast computers run. And
unlike standard silicon, GaAs allows electrons to jump its “energy gap” and
produce visible light, so it can be used to make light-emitting diodes and
lasers.
But this performance comes at a price. A wafer of GaAs 15 centimetres across
cost $400, compared with a mere $30 for a 20-centimetre silicon
wafer. GaAs is also brittle, and combining it with silicon is tough: atoms in a
crystal of GaAs lie 4.1 per cent further apart than atoms in a silicon crystal.
When GaAs crystals are grown onto silicon, the material is strained, and the
overall structure contains defects.
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Now Motorola has found a better way to join the two materials. The trick is
to place an ultra-thin layer of a third material between them. Starting with a
silicon substrate, Motorola deposits a couple of micrometres of a ceramic oxide
called strontium titanate before adding the GaAs.
Motorola researchers were initially interested in strontium titanate as an
insulator to replace oxide layers in silicon chips. But chip designer Jamal
Ramdani found that the titanate lattice could accommodate the structure of both
semiconductors without causing strain. Motorola research director Jim
Prendergast says the mainly silicon chips’ electrical characteristics are
virtually identical to those made on a pure GaAs substrate—and much
cheaper.
Cellphone circuits using the new technology have been working reliably for
months, according to Prendergast. The company has reportedly filed some 270
patents covering the technology.
“The biggest application that we see is in the optical domain”, integrating
low-cost silicon electronics with gallium arsenide light emitters and receivers,
says Prendergast. But Motorola has yet to build a combination chip that does
this. And it will be no easy task, according to Joachim Piprek from the
University of California at Santa Barbara. Getting light and electrons to move
across the strontium titanate interface will the biggest problem, he predicts.