Moore's law has no end in sight

September 15, 2014 // By R. Colin Johnson
Gallium arsenide (GaAs) will soon outpace silicon (Si) as a higher-performance choice for implementing integrated circuits, according to POET Technologies Inc. of Storrs-Mansfield, Conn.

The handwriting has been on the wall since the 1980s, according to former Bell Labs scientist, now co-founder and chief scientist at POET, Geoff Taylor.

According to Taylor, GaAs, as opposed to silicon, will boost electrical transistor performance while integrating optical circuitry capabilities. These qualities enable both higher performance and novel IC architectures, thereby extending Moore's Law indefinitely.

"Silicon digital logic hits the wall at 4 GHz, but we can produce small gallium arsenide [GaAs] analog circuits switching at 100 GHz today and 400 GHz in the not too distant future," he tells EE Times. "Plus POET fabricates optical emitters and detectors for on-chip optical interconnects."

Combining standard logic with optical components on the same chip also changes the design methodology, prompting a collaboration with Synopsys, Inc., of Hillsboro, Ore., to help design hybrid electro-optical devices. For instance, an optical loop achieves an ultra-low jitter oscillator with higher bandwidth than silicon, according to POET. By going to multi-wave lengths, POET also aims to build ultra-precise analog-to-digital converters by encoding voltages as wavelengths, resulting in higher resolution and bit rates with reduced power and fewer components.

POET's indium gallium arsenide ring oscillator is more accurate than silicon and can reach higher frequencies.<br />
(Source: POET)<br />
POET's indium gallium arsenide ring oscillator is more accurate than silicon and can reach higher frequencies. (Source: POET)

Other advantages of III-V over silicon is its lower operating voltage -- as low as 0.3 volts with electron mobilities as high as 12,000 cm2/ (V·s) achieved by strained quantum wells -- lowering the power required to operate III-V chips by 10 times or more, according to POET.

Of course, GaAs wafers are more expensive than Si, but the next generation of Si is already using silicon-on-insulator with fully depleted (SOI-FD) transistors, a technology that costs almost as much as GaAs, according to Taylor.

Most III-V elements, including indium (In), gallium (Ga), arsenide (As), and phosphorous (P) have much higher electron mobility than silicon, but have special fabrication problems that have prevented them from already taking over silicon -- namely, the lack of enhancement devices for digital circuits and of the p-channel transistor for complementary design. However, POET has found a way to grow successive layers of InGaAs on GaAs wafers, each with a little more indium, until they achieve a substrate on which both n-type and p-type transistors can be fabricated.

The p-types could ultimately achieve about a 1900 cm2/(V·s) hole mobility in the strained InGaAs quantum well, which is not as high a figure as the n-types, which achieve 8500 cm2/ (V·s). Both are higher than silicon, at 1200 cm2/ (V·s). POET has high hopes that it can eventually boost the n-types to greater than 12,000 in order to realize extremely high digital logic rates with complementary HFETs.


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