Gallium Arsenide Semiconductors on the Horizon

Masem writes: "According to this Chicago Tribune article, Motorola has developed a cheaper solution for putting gallium arsenide on top of silicon in order to allow for better chip designs with speeds nearly 40 times what silicon only chips would allow. While it was well known that gallium arsenide addition was favorable, it was also very expensive; Motorola's new process (covered by 200+ patents) should keep the chip prices low when these new designs are released in 2 years." The AP says they've applied for 270 patents.

Comments

Kudoes, but

[4of12]

Am I the only one that finds it just a little bit of stretch to talk about about fantastic technology that helps to make GaAs cheaper for real life applications on the one hand -- and then mention 200+ patents on the other hand?

I know, I know, that the hope of financial gain provides the dollars for this kind of research, but let's be real: it won't be that cheap.

Sorry Folks.

[clark625]

Nothing to see here. Move along, please.

Okay, this just happens to be the research area I work in--and I know full well the problems associated with getting high quality GaAs on Si. It's not nearly as simple as it sounds. So, it appears that Motorola found a "magical" insulating layer to put between the Si substrate and the GaAs layer. Wonderful. But it won't ever be anything but a novelty.

Here's why: In industry, everything is driven by economic margins. Plus, the pure Si industry is now very mature and they will not simply add new machinery to their processes that screw up their entire production line. That makes sense, really. Why on earth ruin a perfectly great production line just to toy around?

The other great point is final production cost. There is no way the pure Si industry will adopt a single step that is far costlier than the rest of their production line combined. Then add to the fact that those industries are adverse to any step that may slow down their production runs or cause unnecessary problems.

Sorry, people. If you want GaAs on Si, there is only one way that it can be made which will result in something the Si industry is not too adverse to. That means epitaxial growth of any buffering layers followed by high quality GaAs growth. The biggest problem that still hasn't been worked out is how does one go about making proper interconnections? Also, the buffering layer can be very conductive--and that is sometimes very hard to control. Motorola has got their heads up where it doesn't belong if they think the world is going to go crazy over this.

Re:"40 times faster"

[SysKoll]

Excellent point. The propagation delays are now about 50-70% of the clock cycle of a modern digital chip at the current speeds of several hundred MHz.

So any improvement of the semiconductor commutation speed is just a "nice to have" technology these days. Think of it. Assume that your chip spends 70% of its time waiting for signal propagation. Even if you suddenly get your transistors to switch instantly (that is, infinitely fast), you'll only increase the speed of a cycle by 100/70 = 1.43, or 43%. And then no more improvements.

That's why the biggest performance increases will now come from breakthrough in signal propagation speed: Copper wires, low-K dielectric, and more layers for denser circuits.

-- SysKoll

Re:"40 times faster"

[SysKoll]

It's a good question, and the answer is "roughly c/3". But it's not the whole picture.

Signal propagation in chips is not limited just by the speed of light. You have leaks due to line capacitance, which also induces coupling (crosstalk) between adjacent lines. If you send a nice square pulse on a 3-mm long straight metal line crossing half a chip (I've seen it!) you'll get an ugly, slow-rising pulse full of parasites picked by crosstalk on its way. And, oh, it will also bounce so badly that you better be prepared to sustain NEGATIVE voltages.

Want more fun? Get a 500-MHz signal on a metal line, and have the line do a sharp 90-degree turn. Everything then happens as if most electrons you send miss the turn and keep moving on their trajectory as bullets from a railgun. Not only will your signal be badly attenuated, it will also induce a crazy crosstalk in anything near that 90-degree corner.

See why chip designers become crazy? Sometimes you wonder how something as simple as an electron can be such a devious little bastard. :-)

GaAs problems.. Si ain't so bad

[crgrace]

GaAs has been used for chips for years. Cost is of course a problem but there are others that make it very unlikely this will be used in general purpose microprocessors. The first problem is GaAs has a much higher defect density than silicon because it is a superlattice of gallium and arsenic and not a single crystal like silicon. For this reason GaAs chips have MUCH less yield than silicon chips so the number of transistors that can be integrated in GaAs in much less, even if it is put on a silicon substrate.

The second problem is the lack of a good thermal oxide in the GaAs material system. Silicon uses SiO2 which is an excellent insulator and more importantly has an extremely clean interface with silicon, so there are very few traps at the oxide-si interface. Because GaAs doesn't have a good oxide, MOS field-effect transistors (MOSFETS) are impossible and so digital GaAs chips use MESFETS, which are FETs without the oxide. It turns out the good oxide in silicon makes a lot of things possible that are impossible in GaAs. For example, the si oxide makes for a very high input impedance for Si transistors so they can be used to make dense RAM and very simple registers that rely on a high impedenence node. This structures are not possible in GaAs so more complicated, higher power circuits are required in GaAs to achieve the same functionality.