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Ieeespectrum2016germ 12.Germanium.INT -12.Germanium.NA [P]{NA}.indd 40 11/15/16 3:49PM GUTTER CREDIT GOES HERE GERMANIUM RETURNS: Germanium was an early transistor material. Now its charge-carrying abilities and advanced fabrication technology make it an attractive material for future chips. As a proof of concept, the author and his team used germanium-on-insulator wafers to construct inverters containing first planar transistors [bottom left] and then FinFETs [top left]. The two transistors in the latter inverter contain finlike channels, which stand out from the plane of the wafer [top right, one set of fins (in pink) from a bird’s-eye view; bottom right, an oblique view of another set]. The distance between each fin in the top right image is in the tens of nanometers. SWITCHING CHANNELS REPLACING SILICON WITH GERMANIUM IN A KEY TRANSISTOR COMPONENT COULD GIVE CHIPS A SIGNIFICANT SPEED BOOST—AND GIVE MOORE’S LAW A VITAL REPRIEVE BY PEIDE D. YE SPECTRUM.IEEE.ORG INTERNATIONAL DEC 2016 | 41 OES HERE OES G GUTTER CREDIT CREDIT GUTTER 12.Germanium.INT - 12.Germanium.NA [P]{NA}.indd 41 11/15/16 3:49 PM early 70 years ago, two physicists at Bell a channel made of germanium. SPEEDY CIRCUITS: This Telephone Laboratories—John Bardeen and Since then, we’ve demonstrated nine-stage CMOS ring oscillator, presented in 2015, Walter Brattain—pressed two thin gold con- the first complementary-metal- was built on a germanium- tacts into a slab of germanium and made oxide- semiconductor (CMOS) on-insulator wafer. a third contact on the bottom of the slab. circuits—the kind of logic inside The flow of current through this configu- today’s computers—made with germanium grown on ordinary sili- N ration could be used to turn a small signal con wafers. We have also constructed a range of different transistor into a larger one. The result was the first transistor—the architectures using the material. These include nanowire devices, amplifier and switch that was, arguably, the greatest which may be next in line when the present state-of-the-art transis- invention of the 20th century. Thanks to Moore’s Law, tor design, known as the FinFET, can’t be miniaturized any longer. the transistor has delivered computers far beyond any- Best of all, it turns out that putting germanium back into the mix thing thought possible in the 1950s. isn’t as big a challenge as it might seem. Transistors that use a com- Despite germanium’s starring role in the transistor’s bination of silicon and germanium in the channel can reportedly early history, it was soon supplanted by silicon. But be found in some recent chips, and they made an appearance in a now, remarkably, the material is poised for a comeback. 2015 demonstration of future chip-manufacturing technology by The world’s leading-edge chipmakers are contemplat- IBM and partners. These developments could be the first steps in ing a change to the component at the very heart of the an industry trend to adopt the use of higher and higher propor- transistor—the current-carrying channel. The idea is to tions of germanium in the channel. In a few years’ time, we may replace the silicon there with a material that can move find that the material that brought us the transistor has helped current at greater rates. Building transistors with such usher it into a new age of remarkable performance. channels could help engineers continue to make faster and more energy-efficient circuits, which would mean GERMANIUM WAS FIRST ISOLATED AND IDENTIFIED by the German chemist better computers, smartphones, and countless other Clemens Winkler in the late 19th century. Named in honor of gadgets for years to come. Winkler’s homeland, the material was long considered a poor For a long time, the excitement over alternative chan- conducting metal. That changed during World War II, when ger- nels revolved around III-V materials, such as gallium manium’s semiconducting properties—that is, its ability to switch arsenide, which are made from atoms that lie in the between permitting and blocking the flow of current—were dis- columns just to the left and right of silicon in the peri- covered. Solid-state devices based on germanium boomed in the NIVERSITY odic table of elements. I was active in that research. postwar years; U.S. production grew from a few hundred pounds U In fact, eight years ago, I wrote a feature for this mag- in 1946 to meet a demand for over 45 metric tons of the stuff by azine heralding the progress that had been made in 1960. But silicon ultimately won out; it became the material of WU/PURDUE WU/PURDUE G constructing transistors with III-V channels. choice for logic and memory chips. EN H But as we eventually discovered, the III-V approach has There are some good reasons why silicon dominated. For one some fundamental physical limitations. It’s also likely to thing, silicon is far more abundant and thus a lot cheaper. Sili- be too expensive and difficult to integrate with existing con also has a wider bandgap, the energy hurdle that must be ES AND ABOVE: ABOVE: AND ES silicon technology. So a few years ago, my team at Pur- overcome in order for a transistor to carry current. The larger G due University, in West Lafayette, Ind., began experi- the bandgap, the harder it is for current to leak across the device menting with a different kind of device: a transistor with when it’s supposed to be off, draining power. As an added ben- PA PREVIOUS 42 | DEC 2016 | NORTH AMERICAN | SPECTRUM.IEEE.ORG 12.Germanium.INT - 12.Germanium.NA [P]{NA}.indd 42 11/15/16 3:49 PM efit, silicon also has better thermal conductivity, making HOW GERMANIUM STACKS UP it easier to draw away heat so that circuits don’t overheat. Given all those advantages, it’s natural to wonder why Gallium Indium Silicon Germanium arsenide arsenide we’d ever consider introducing germanium back into the Property (Si) (Ge) (GaAs) (InAs) Unit channel. The answer is mobility. Electrons move nearly Bandgap 1.12 0.66 1.42 0.35 eV three times as readily in germanium as they do in silicon Electron when these materials are close to room temperature. And cm2 mobility 1,350 3,900 8,500 40,000 V·s holes—the electron voids in a material that are treated like at 300 kelvins positive charges—move about four times as easily. Hole mobility 2 The fact that both electrons and holes are so mobile in 450 1,900 400 500 cm at 300 K V·s germanium makes the material a convenient candidate for constructing CMOS circuits. CMOS employs two differ- Maximum possible 1 0.6 2 3.5 × 107 cm ent kinds of transistors: the p-channel FET (pFET), whose electron s channel contains an excess of free-moving holes, and the velocity n-channel FET (nFET), which has a similar excess of elec- Critical 6 V 0.25 0.1 0.004 0.002 × 10 cm trons. The faster these electrons and holes can move, the electric field faster the resulting circuits can be. And because less volt- Thermal 1.5 0.58 0.5 0.27 W age must be applied to draw those charge carriers along, conductivity cm·K circuits can also consume considerably less energy. Of course, germanium isn’t the only such high- mobility material. The III-V compounds mentioned earlier, materials such To make germanium—or any alTERNATIVE channel material— as indium arsenide and gallium arsenide, also boast excellent elec- work in mass manufacturing, we must find a way to tron mobility. In fact, electrons in indium arsenide are nearly 30 incorporate the material on the dinner-plate-size sili- times as mobile as they are in silicon. So far, so good. The prob- con wafers that are used to make today’s chips. Fortu- lem is that this amazing property does not extend to the holes in nately, there are multiple ways to build a germanium indium arsenide, which are not much more mobile than holes in layer on a silicon wafer that can then be fashioned into silicon. That limitation makes it almost impossible to make a high- channels. Using a thin layer of the stuff significantly performance pFET, and the lack of a fast pFET rules out speedy mitigates two key problems with germanium—the fact CMOS circuitry, which is not designed to tolerate a very large dif- that the material is costlier than silicon, and that it is ference in the speed between nFETs and pFETs. a relatively poor conductor of heat. One potential fix is to take the best of each material. Research- But replacing silicon in a transistor channel isn’t just ers at various institutions, such as the European semiconductor a matter of slotting in a thin, high-quality layer of ger- research organization Imec and IBM’s Zurich laboratory, have manium. The channel has to work seamlessly with the demonstrated ways to make circuits that build nFET channels other components of the transistor. from a III-V material and pFET channels from germanium. This The transistor in today’s ubiquitous CMOS chips is technique could lead to very fast circuits, but it also complicates the metal-oxide-semiconductor field-effect transis- the manufacturing process. For this and other reasons, we favor a straight-germanium ALTERNATIVE CHANNEL PATHS: There are multiple ways to approach. The germanium channels should significantly boost create transistors with high-mobility, nonsilicon channels.
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