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Extending Moore's Law: Progress underscores SiGE, nm-ICs

Posted: 29 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:IBM? SiGE? silicon? IC? silicon-on-insulator?

Considerable focus has been dedicated to silicon-germanium (SiGe) heterojunction technology as the next step in building silicon-based ICs down below 10nm. However, it is not the first time that the semiconductor industry has turned to this compound heterojunction semiconductor for help. This time most of the attention has spotlighted on IBM's continued support for SiGe in its foundries.

In June, IBM ramped up its foundry business with novel SiGe and silicon-on-insulator processes targeted mostly at wireless 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 trade-offs, and its compatibility with silicon.

And this month (July 5), IBM announced the first 7nm 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 1nm and below.

Michael Liehr of the SUNY College of Nanoscale Science and Engineering, left, and Bala Haranand of IBM

Michael Liehr of the SUNY College of Nanoscale Science and Engineering, left, and Bala Haranand of IBM examine a wafer comprised of the SiGe 7nm-based ICs. (Source/Credit: Darryl Bautista/IBM)

The team took advantage of the fact that SiGe increases the frequency of a transistor's 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 maximise the current gain and minimise leakage, making possible much higher gate frequencies at lower power.

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-10nm range where noise is an ongoing problem.

Germanium

Germanium, from which the first solid state semiconductor transistor was created, is now integral to the latest generation of nanometre-sized transistors used in silicon-based ICs. (Source: Bell Labs)

SiGe has always been the back-burner IC technology the semiconductor industry turned to for very specific problems. For example, analogue IC designers have used variations of bipolar SiGe and BiCMOS SiGe to get the performance they needed. And in some special cases germanium-strained-silicon CMOS creates fast positive channel MOS (PMOS) transistors that allow for fast, complementary low-leakage digital design in mission critical designs.

But beyond these specialised 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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