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Posted: 06:42:29 PM, 14/08/2015

A look at SiGe, nanometer ICs and multivalued logic

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In recent weeks, much attention has been devoted to silicon-germanium (SiGe) heterojunction technology as the next step in building silicon-based ICs down below 10 nanometers. It is not the first time that the semiconductor industry has turned to this compound heterojunction semiconductor for help. However, this time most of the attention has centered around IBM's continued support for SiGe in its foundries.

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In June IBM ramped up its foundry business with new SiGe and silicon-on-insulator processes targeted mostly at wireless radio-frequency (RF) circuits. Traditionally fabricated with more exotic heterojunction technologies such as gallium arsenide, RF circuits in mobile devices are being fabricated instead with SiGe because of its speed advantages, performance/power tradeoffs, and its compatibility with silicon.

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And this month (July 5), IBM announced the first 7nm (nanometer) node test silicon ICs with functioning transistors, developed in collaboration with GlobalFoundries and Samsung at Suny Polytechnic Institute. This breakthrough may change the way companies build integrated circuits, especially as scaling gets close to 1 nm and below.

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Michael Liehr of the SUNY College of Nanoscale Science and Engineering, left, and Bala Haranand of IBM examine a wafer comprised of the new SiGe 7nm-based integrated circuits. 
(Source/Credit: Darryl Bautista/IBM)
Michael Liehr of the SUNY College of Nanoscale Science and Engineering, left, and Bala Haranand of IBM examine a wafer comprised of the new SiGe 7nm-based integrated circuits.
(Source/Credit: Darryl Bautista/IBM)
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The team took advantage of the fact that SiGe increases the frequency of a transistors short-circuit current gain, thus creating the conditions that accelerate the movement of electrons across a transistor structure. In a conventional silicon transistor, higher doping would lower the current gain and allow leakage back to the collector. In a SiGe transistor, the bandgap potentials maximize the current gain and minimize leakage, making possible much higher gate frequencies at lower power.

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In addition, germanium makes the silicon act more like a conductive metal structure and less like an insulator. The base is more conductive and so the more the base resistance of a transistor structure decreases, less noise it produces versus pure-silicon devices, especially important in the sub-10 nanometer range where noise is an ongoing problem.

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Germanium, from which the first solid state semiconductor transistor was created, is now integral to the latest generation of nanometer-sized transistors used in silicon-based ICs.
(Source:  Bell Labs)
Germanium, from which the first solid state semiconductor transistor was created, is now integral to the latest generation of nanometer-sized transistors used in silicon-based ICs.
(Source: Bell Labs)
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SiGe has always been the back-burner IC technology the semiconductor industry turned to for very specific problems. For example, analog IC designers have used variations of bipolar SiGe (silicon germanium) and BiCMOS (bipolar-CMOS) SiGe to get the performance they needed. And in some special cases germanium-strained-silicon CMOS creates fast PMOS (positive channel MOS) transistors that allow for fast, complementary low-leakage digital design in mission critical designs.

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But beyond these specialized niches, the semiconductor industry has several times turned to SiGe as a way to increase the amount of digital information a logic circuit can process or a memory device can hold without necessarily reducing transistor size. Because SiGe is a heterojunction structure, theoretically it can generate more than one signal threshold level reliably: not just on or off, but several other intermediate signal levels as well.

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Unlike homojunction-based digital circuits, which have only two threshold voltages that can be detected reliably (on or off), SiGe transistors can generate three or four such signals naturally. Several times since the early 1990s, companies and organizations such as Bell Labs, IBM, Intel, Signetics, National Semiconductor, and even Motorola were looking at SiGe and its multithreshold capabilities to create circuits that operated on other forms of logic other than binary. In one paper I remember reading that Motorola's Research Labs had come up with a SiGe CMOS transistor design that could reliably produce three threshold levels. They had also borrowed some techniques from analog circuit design that allowed them to detect and manipulate those signals and create some primitive trinary logic functions as well as a trinary logic memory device.

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Theoretically, because SiGe could be used to build devices that move beyond simple 0/1, on-off based binary logic, ICs built using SiGe can reliably generate multiple signal levels that are easily discriminated. These ICs could be used to build 3-base, 4-base, and higher logic functions, effectively increasing a device's information density without further shrinking the transistor structure. Also, the interconnects could be reduced if signals in the circuit assume four or more levels rather than only two. In random access memory, storing two or three values instead of one doubles or triples the information density of the memory without increasing die size.

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Each of these forays into multi-threshold, multi-valued logic came at a time when the semiconductor industry was up against the wall on how much further they could push lithography technologies to product affordable integrated circuits. Unfortunately, most of those walls were economic, while advances in photolithography allowed the industry to push transistor sizes down to the nanometer level.

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But now the semiconductor industry is up against the wall of physical limits, as well as horrendously expensive financial ones. At IBM's 7nm node, we are not that far away from sub-nanometer devices in the picometer range.

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Whether the multi-threshold capabilities of SiGe are ever used, SiGe has a number of other features that may extend silicon-based circuits lifetime a bit further. This fact gives the history of semiconductors in today's electronic devices a certain symmetry. If you remember, it was germanium, not silicon that started the semiconductor revolution in 1947. That was when researchers at Bell Laboratories used it to create the first solid-state device, a point-contact transistor. And the first mass consumer electronics devices, transistor-based portable radios, ran on germanium diodes.

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