Graphene Won't Replace Silicon In CPUs, Says IBM 81
arcticstoat writes "IBM has revealed that graphene can't fully replace silicon inside CPUs, as a graphene transistor can't actually be completely switched off. In an interview, Yu-Ming Lin from IBM Research (Nanometer Scale Science and Technology) explained that 'graphene as it is will not replace the role of silicon in the digital computing regime.' Last year, IBM demonstrated a graphene transistor running at 100GHz, while researchers at UCLA produced a graphene transistor with a cut-off frequency of 300GHz, prompting predictions of silicon marching towards its demise, making way for a graphene-based future with 1THz CPUs. However, Lin says, 'there is an important distinction between the graphene transistors that we demonstrated and the transistors used in a CPU. Unlike silicon, graphene does not have an energy gap, and therefore, graphene cannot be "switched off," resulting in a small on/off ratio.' That said, Lin also pointed out that graphene 'may complement silicon in the form of a hybrid circuit to enrich the functionality of computer chips.' He gives the example of RF circuits, which aren't dependent on a large on/off ratio."
IBM is both right and wrong (Score:5, Interesting)
Why they're right:
Graphene is a metal (or semimetal, whatever). Capacitive effects cause your current gain (ratio between input current and output current) to drop with frequency. The highest practical frequency of operation is where the gain is 1. IBM demonstrated a year ago a transistor whose current gain reached 1 at 100GHz (also known as fmax or unity gain frequency). However, that's just current gain. Digital circuits require a voltage gain as well. You can have high current gain but not have a high voltage gain by having a low output resistance.
Why they could be wrong
Without giving a crash course in electronics, computer (not all) transistors these days operate by raising/lowering a potential that either "gates" electrons and keeps them from passing through or allows them to go through freely. How big the gate is depends upon something called the band gap size, i.e, the energy required to move an electron from an atom (valence band) into participating in macro-scale conduction (conduction band). Graphene does not have a band gap.
However, you can artificially create a band gap by excluding the low energy, long wavelength electron states that exist at the bottom of the conduction and top of the valence bands. You do this by patterning your graphene into strips below about 30nm in width. In this way, no electron state with a wavelength longer than the width of the strip can exist. In a few years (right when graphene starts to hit its stride outside academia), such patterning will be possible (Intel is at 32nm now, if you recall).
diamond (Score:5, Interesting)
Diamond, on the other hand, has a band gap of 5.5 eV, and has _excellent_ thermal properties.
Re:Congratulations (Score:3, Interesting)
The problem is that bilayer graphene doesn't have the same linear dispersion that a single layer has, so it might not be possible to reach such high clock speeds with bilayers. The point with single layer graphene is that the conduction electrons behave as though they are massless and so have a very high mobility. This feature is diminished somewhat in a bilayer.
Re:IBM is both right and wrong (Score:5, Interesting)
And this is what exactly I'm doing right now. Believe me, its darn hard to go below 10 nm even with ebeam lithography. But I achieved 100x on/off ratio on room temperature (10^6 on 4K). I saw IBM results on bilayer graphene with 300x on/off 1.5 years ago. There RF graphene research (on epitaxial) and DC device research on exfoliate graphene has quite different goal. You don't want to combine those into one device, because you won't achive any of those goals in time. In a long term: yes, you can achieve both. There will be fast, high on/off ratio graphene transistors, I promise.
Re:diamond (Score:4, Interesting)
Unfortunately, it's a pain in the ass to get other materials to interface with diamond. I admit I haven't looked into it in a few years, but last I heard, diamond transistors looked like a dead end.
Re:diamond (Score:5, Interesting)
Yep, with the 5.5eV band gap you surely can turn a diamond device off. And it does have really great (the best, actually) thermal properties. But...it's really hard to get a useful n-type material (gotta boron-dope it first - which is easily done and makes a diamond p-type semiconductor - then expose it to a deuterium - yes, deuterium, not hydrogen - plasma, to passivate the boron acceptors and form some shallow donors); and there's no convenient native oxide like Si has; and etc....Diamond will take as much R&D to be useful in common electronics as will graphene. But diamond is sexier than graphene, that's for sure.