IEEE Spectrum Gaas

Proof #4 COLOR 8/5/08 @ 5:15 pm BP BEYOND SILICON’S ELEMENTAL LOGICIN THE QUEST FOR SPEED, KEY PARTS OF MICRO- PROCESSORS MAY SOON BE MADE OF GALLIUM ARSENIDE OR OTHER “III-V” SEMICONDUCTORS BY PEIDE D. YE he first general-purpose lithography, billions of them are routinely microprocessor, the Intel 8080, constructed en masse on the surface of a sili- released in 1974, could execute con wafer. about half a million instructions As these transistors got smaller over the T per second. At the time, that years, more could fit on a chip without rais- seemed pretty zippy. ing its overall cost. They also gained the abil- Today the 8080’s most advanced descen- ity to turn on and off at increasingly rapid dant operates 100 000 times as fast. This phe- rates, allowing microprocessors to hum along nomenal progress is a direct result of the semi- at ever-higher speeds. ALL CHRISTIEARTWORK: BRYAN DESIGN conductor industry’s ability to reduce the size But shrinking MOSFETs much beyond their of a microprocessor’s fundamental building current size—a few tens of nanometers—will be blocks—its many metal-oxide-semiconductor a herculean challenge. Indeed, at some point in field-effect transistors (MOSFETs), which act the next several years, it may become impossi- as tiny switches. Through the magic of photo- ble to make them more minuscule, for reasons WWW.SPECTRUM.IEEE.ORG SEPTEMBER 2008 • IEEE SPECTRUM • INT 39 TOMIC-LAYER DEPOSITION provides one means for coating a semiconductor wafer with a high-k aluminum oxide insulator. The Abenefit of this technique is that it offers atomic-scale control of the coating thickness without requiring elaborate equipment. 1. Apply the gas trimethyl 2. Apply water vapor, which aluminum, which reacts with the reacts with the adhered hydroxyl groups attached to the trimethyl aluminum, forming a surface of the wafer, creating thin coating of aluminum oxide. a one-molecule-thick veneer. The repeated Water vapor application of trimethyl Trimethyl aluminum Methane by-product aluminum and water vapor in alternating steps serves to build up the insulator into a many-atom-thick layer (shown schematically here as just a thin vertical slice). of fundamental physics rather along with other researchers it as a III-V semiconductor. There than nuts-and-bolts engineering. in industry and academia, have are more than a dozen such com- So people like me have been look- recently made some advances that pounds, including gallium nitride ing at other ways to boost their might soon allow transistors built and indium phosphide, but gal- speed. In particular, we’ve been with gallium arsenide or a related lium arsenide is the most com- laboring to build them using com- compound to be used for large- mon example and therefore the pound semiconductors like gal- scale digital ICs. That capability best studied. It currently accounts lium arsenide, which would allow would go a long way toward bring- for about 2 percent of the semi- such transistors to switch on and ing us microprocessors that can conductor market. off much faster than their silicon blaze along at triple or even qua- Gallium arsenide devices cousins can. druple the speed of today’s best. cost a lot more than ones built of This strategy is by no means Achieving that goal will no doubt silicon—the raw materials are new. Practically ever since the sili- require other improvements in about 10 times as expensive—but con MOSFET was invented in 1960, semiconductor technology to take they serve well for certain special- engineers have been attempting to place in parallel, but gallium arse- ized applications, including high- come up with a gallium arsenide nide or something close to it could efficiency solar cells, laser diodes, version suitable for large-scale be key. No wonder some of us have and one very special kind of field- integrated circuits. No one has yet been unwilling to give up on this effect transistor: the high-electron- succeeded. Those repeated fail- remarkable material. mobility transistor, or HEMT, ures have led to the oldest joke in which is used in cellphones, com- Silicon Valley: gallium arsenide— allium arsenide’s munication systems, and radars, it’s the technology of the future, two main compo- among other things. and it always will be. nents come from HEMTs are remarkable devices But that perennial skepticism the third (gallium) because they overcome a funda- may be about to vanish. My col- G and fifth (arsenic) mental problem of solid-state phys- leagues and I at Purdue Uni- columns in the right-hand portion ics. Semiconductors, as their name versity’s Birck Nanotechnology of the periodic table of elements, implies, normally don’t conduct Center, in West Lafayette, Ind., which is why cognoscenti refer to electricity all that well. Usually, 40 INT • iEEE SPECTRUM • SEPTEMBER 2008 WWW.SPECTRUM.IEEE.ORG WWW.SPECTRUM.IEEE.ORG SEPTEMBER 2008 • IEEE SPECTRUM • INT 41 they must be doped with other at the interface fails to bond with with higher dielectric constants, kinds of atoms to become electri- the adjacent silicon dioxide, leav- which could be made physically cally conductive. But those impu- ing what’s called a dangling bond. thicker without compromising rities tend to interfere with the These defects disrupt the flow of the electrical functioning of the movement of electrons through electrons in the channel, but they transistor. Eventually, suitable the semiconductor’s crystal lat- are rare enough that they don’t compounds were found. Intel, for tice, limiting the conductivity that materially degrade the perfor- instance, is now using a hafnium- can be obtained. mance of a transistor. based gate insulator on some of its In HEMTs, electrons are intro- Gallium arsenide is a different most advanced microprocessors duced into a III-V semiconductor story. When it oxidizes, it forms a [see “The High-k Solution,” IEEE not by doping but by placing the complex mixture of Ga2O3, As2O3, Spectrum, October 2007]. material in contact with another and As2O5. Beginning in the 1960s, To control the thickness of III-V compound that is doped. some researchers tried using these “high-k” dielectrics (a des- In essence, electrons fall a short these native oxides for a gate ignation that refers to the symbol distance into the undoped mate- insulator, but that tactic proved used for a material’s dielectric rial, allowing a thin layer of it— worse than useless because the constant, the Greek letter kappa), the channel—to conduct electric- native oxides create all kinds of manufacturers apply them to the ity extremely well whenever the defects at the interface with the silicon substrate using a tech- transistor is switched on. gallium arsenide, which destroy nique called atomic-layer depo- HEMTs can be used singly or the electrical conductivity of the sition. It’s quite ingenious, really. in integrated circuits with, say, adjacent channel. Clearly, a bet- The trick is that you employ a 100 or even 1000 of them clustered ter material needed to be found if chemical carrier molecule that together, but they can’t yet work there was to be any hope of mak- sticks to the target surface but for microprocessors. The problem ing gallium arsenide MOSFETs not to itself. Such a chemical thus is that too many of the electrons for digital ICs. deposits a one-molecule-thick that are supposed to flow through veneer. A second treatment with the channel from the transis- esearchers con- another carrier molecule removes tor’s source electrode to its drain tinued the decades- the first carrier, leaving a two- instead seep out the controlling long quest, testing atom-thick layer of the desired input electrode—the gate—creat- silicon dioxide, sil- material behind. Repeated appli- ing heat. With millions of leaky R icon nitride, silicon cation of the two carrier gases, one transistors crowded together oxynitride, and aluminum oxide, alternating with the other, allows on the same chip, things would among other candidates. They also chip makers to deposit various quickly get hot enough to melt. tried adding a third material, such high-k gate insulators on silicon In a silicon MOSFET, a layer of as sulfur, silicon, or germanium, with atomic-level precision. intervening insulation (traditionally between the substrate and the insu- In 2001, Glen Wilk, then my col- silicon dioxide) prevents electrons lator to negate the pernicious effects league at Bell Labs, and I decided from slipping out of the channel of dangling bonds. Yet the results to try to put a high-k gate insula- into the gate. In a HEMT, the chan- always proved disappointing, and tor—in this case, aluminum oxide nel is separated from the gate by a by the early 1990s most investi- (Al2O3)—on top of gallium arse- semiconductor, which, as you might gators had simply given up. Two nide using atomic-layer deposi- expect, is somewhat conductive. notable exceptions were Minghwei tion, which was all the rage at the What’s needed here, of course, is an Hong and Matthias Passlack at Bell insulator, but for decades there have Labs, who had developed a way of Al2O3 gate insulator p-type In0.65Ga0.35As been no good gate insulators avail- depositing a combination gallium able for gallium arsenide. From time oxide–gadolinium oxide insulator Gate to time over the years, researchers on a III-V substrate, a strategy that Source Drain seem to uncover a promising mate- Passlack later refined at Motorola rial, but nothing ever really panned and at a company Motorola spun off Channel out—until recently. in 2004, Freescale Semiconductor. It’s easy enough to understand, At that time, the engineers n-type regions at least in general terms, why find- making silicon MOSFETs were ing a gate insulator for silicon was beginning to experience trou- p-type In0.53Ga0.47As easier than it has been for gal- ble with their gate insulator too.

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