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February 8th 05, 10:31 PM

On Mon, 07 Feb 2005 16:55:42 -0000, (David DiGiacomo)
wrote:

In article ,
Roy Lewallen wrote:
As I recall, only transistors designed as saturated
switches (2N918 comes to mind, but it's been a long time, so don't
quote
me) are gold doped. Ones which aren't, and I'm quite sure the 2N3904
is
in that category, will have long reverse recovery times so will make
poor RF rectifiers. Circuits became too fast for saturated switches
long, long ago, so I'd be surprised if gold doping is done any more
except for replacement transistors in very old equipment.

Don't people still use 2N2369As, or at least the plastic version?
If not, what do they use instead?

(Does gold doping work for PNP transistors? I don't see why it
wouldn't,
but I've never seen a specific reference to a gold-doped PNP.)
Without going into the details of Quantum chemistry and electronic
configurations (which is certainly more challenging and exciting) you
can easily see it from a simplified perspective of the energy bond
model of semiconductors.
For a npn transistor doped with Gold the equilibrium density (ed) for
impurity electrons in Silicon doped with gold is given by the well
known Fermi-Dirac statistics equation.
ed = B*Nd*exp(-Edonors/kT) E donors is the energy required for Golds
electron to freely dwell in the Silicon lattice ... a value much
smaller then the E gap energy value for Si to create an electron hole
pair.
Smaller values of Edonor imply larger values free donor electrons.

If a pnp transistor was to be formed from Si doped with Gold the
equilibrium density (ph) for impurity holes in Silicon doped with gold
is
ph = A*Na*exp(-Eacceptor/kT) Eacceptor is the energy required for a
electron to be stripped from the Silicon atom by the Gold atom.

As Eacceptor is greater then Edonor only a npn transistor can be formed
from Si doped with Gold.

Its even more exciting to look at the Physics of Gold plated
transistors. Very intriguing .. just like the MOSFET transistor!

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