Ken wrote:
> 
> Dan Connelly wrote:
> 
> > P.S. There are also devices with electron-deficient islands formed in an
> > electron-rich region.... in this case the conduction is via "holes" rather
> > than "electrons".  Both devices are used in conjunction to form "CMOS".
> 
> Are these what they call depletion mode devices ?
> 
> P.S. Sorry for the off topic discussion folks and I promise to let it
> go after this. It's just that I love this stuff.
> 

Not necessarily -- they are PMOS ( as opposed to NMOS ).

Basically when one speaks of "electrons" in semiconductors one is
actually speaking of eigenstates of the macroscopic state of the
electron gas.... due to diffraction effects from the lattice,
quanta of electron charge manifest themselves as "quasiparticles" which
can have different energies at a given momentum.  Some of these states
are effectively all occupied, others effectively so high in energy they
are never occupied.  Two sets of states straddle the "Fermi level", which
is sort of a threshold energy required to achieve net charge neutrality
between electrons and protons.  Those states above the Fermi level by a
few fractions of an electron volt are electron-like -- they tend to be light
and fast.  Those just below the Fermi level are mostly occupied.... the states
which are NOT occupied act as positively charged quasiparticles called "holes".
It is the holes which act as the current carriers in PMOS devices.
They tend to be "heavier" and thus PMOS devices are slower than NMOS.

Basically, NMOS devices are used as pull-down switches (shorting things to ground)
while PMOS are pull-up devices (shorting things to the supply voltage).  Since
switches are faster when they support a larger voltage drop, this guarantees at
least one of the two switches supporting a circuit node will have a large voltage drop
across it at any given time, allowing transitions to be executed quickly.

Anyway, enough of that....  time to go to work :).

Dan

-- 
http://www.flash.net/~djconnel/

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