Quadibloc wrote:
Of course, lead is objected to due to its toxicity - and tellurium is
a very rare element, even if its price has, so far, been depressed due
to limited applications. Silicon Carbide can function at high
temperatures, but so far it can only be used for small chips due to a
high prevalence of defects, and perhaps there are other problems with
Lead Telluride of which I'm not aware.
Where to start? There are two generic errors in such proposals. The first is the misconception that mobility translates into transistor drive current and thus into switching speed. In direct band-gap materials, the mobilities are high, but the saturated drift velocities are no better than silicon,
and this is reflected in the magnitude of IDsat. We went through all this
25 years ago with Fujitsu's HEMT, which never appeared in the promised supercomputers, but makes an excellent low-noise microwave amplifier,
since the low-field region of the transistor is the source of most of the noise. (Low field regions are the only place you ever see the high mobility.)
The other problem is the technology of narrow-gap materials. Don't imagine that silicon processing is remotely applicable. On a good day, you might equal the sophistication of the Ge alloyed-junction technology (but, after all, that was good enough for Kilby's first IC). Remember that the valence bands are the bonding states and the conduction bands are the anti-bonding states. Narrow-gap materials are just barely holding themselves together.
(A bit of oversimplification, but a good guiding principle.) Things like selective doping by ion implantation are unlikely to be adequately controllable,
since at the temperature required for annealing, the crystal is likely to produce defects that work to nullify your efforts.
Finally, suppose that you can make a MOSFET structure. Can it be turned
off? The bandgap is 0.29 eV, which means that interband tunneling will render the off-state energy barrier pretty much transparent.
Unlike Lead Telluride, another material with a greater hole mobility than silicon has successfully been made into CMOS digital circuitry:
http://phys.org/news/2014-12-germanium-silicon-cmos-devices.html
Unlike Lead Telluride, another material with a greater hole mobility than silicon has successfully been made into CMOS digital circuitry:
http://phys.org/news/2014-12-germanium-silicon-cmos-devices.html
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