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

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

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.