Roy Lewallen wrote:

[...]

The second is that the ratio of reverse to forward current increases as
the signal gets smaller and smaller, reaching one at the limit. This can
be observed by looking at the I-V curve of a diode. At the origin, the
curve is a straight line - the diode behaves just like a resistor.

[...]

Roy Lewallen, W7EL

Excellent description - thanks.

Only one small problem - as Win pointed out, Bob Pease feels a
diode-connected 2N3904 has lower leakage at low voltage than a 1N4148:

"What's All This Comparator Stuff, Anyhow?"

http://www.elecdesign.com/Articles/A...9517/9517.html

Does this mean a 2N3904 has a shallower slope than a 1N4148 through zero, or
perhaps one or the other has an offset, such as the Agilent Zero Bias
Schottky Detector Diodes shown in AN969?

http://www.spelektroniikka.fi/kuvat/schot8.pdf

Regards,

Mike Monett

Mike Monett wrote...

Roy Lewallen wrote:

The second is that the ratio of reverse to forward current increases as
the signal gets smaller and smaller, reaching one at the limit. This can
be observed by looking at the I-V curve of a diode. At the origin, the
curve is a straight line - the diode behaves just like a resistor. ...

Excellent description - thanks.

Only one small problem - as Win pointed out, Bob Pease feels a
diode-connected 2N3904 has lower leakage at low voltage than a 1N4148:
"What's All This Comparator Stuff, Anyhow?"
http://www.elecdesign.com/Articles/A...9517/9517.html

Does this mean a 2N3904 has a shallower slope than a 1N4148 through zero,
or perhaps one or the other has an offset, such as the Agilent Zero Bias
Schottky Detector Diodes shown in AN969?

No, it means its a better diode at low currents. See my curves again,
http://www.picovolt.com/win/elec/com...de-curves.html Note the
1n458 and the JFET diodes, which follow the theoretical 60mV/decade rule
down to very low currents. As for Roy Lewallen's "ratio of reverse to
forward current" argument, there is no reverse current for these fine
fellows, at least for DC and reasonably low frequencies. It's the very
crummy gold-doped 1n4148 that falls over. Awwkk!


--
Thanks,
- Win

Winfield Hill wrote:

No, it means its a better diode at low currents. See my curves again,
http://www.picovolt.com/win/elec/com...de-curves.html Note the
1n458 and the JFET diodes, which follow the theoretical 60mV/decade rule
down to very low currents. As for Roy Lewallen's "ratio of reverse to
forward current" argument, there is no reverse current for these fine
fellows, at least for DC and reasonably low frequencies.

Sure there is. All diodes have reverse current.

It's the very
crummy gold-doped 1n4148 that falls over. Awwkk!

The gold doping is done to dramatically reduce charge storage time.
Without it, the voltage across a diode continues to be in the forward
direction for some time after you reverse the current through it. While
a non-gold-doped diode might look good in DC tests, it makes a lousy
rectifier of RF. In the extreme case, it acts like a PIN diode (which is
simply a diode designed intentionally to have a long charge storage, or
reverse recovery, time).

Alas, life is full of tradeoffs.

Roy Lewallen, W7EL

I need to clarify this. My comments apply only to junction diodes, which
virtually all silicon diodes are. Schottky diodes don't exhibit this
charge storage effect. That's one reason they're often used in high
frequency switching supplies. Their leakage current is, however, much
greater than silicon diodes.

Roy Lewallen, W7EL

Roy Lewallen wrote:

The gold doping is done to dramatically reduce charge storage time.
Without it, the voltage across a diode continues to be in the forward
direction for some time after you reverse the current through it. While
a non-gold-doped diode might look good in DC tests, it makes a lousy
rectifier of RF. In the extreme case, it acts like a PIN diode (which is
simply a diode designed intentionally to have a long charge storage, or
reverse recovery, time).

Alas, life is full of tradeoffs.

Roy Lewallen, W7EL

Mike Monett wrote:

Excellent description - thanks.

Only one small problem - as Win pointed out, Bob Pease feels a
diode-connected 2N3904 has lower leakage at low voltage than a 1N4148:

"What's All This Comparator Stuff, Anyhow?"

http://www.elecdesign.com/Articles/A...9517/9517.html

Does this mean a 2N3904 has a shallower slope than a 1N4148 through zero, or
perhaps one or the other has an offset, such as the Agilent Zero Bias
Schottky Detector Diodes shown in AN969?

http://www.spelektroniikka.fi/kuvat/schot8.pdf

Regards,

Mike Monett

I'm not sure what you mean by an "offset" -- all diodes cross through
the origin of the I-V curve, when excited by DC, anyway -- unless they
contain a battery. In the reverse direction, the current pretty much
levels off beyond a small reverse voltage. The current of this level
part is the saturation current.

Again, don't think that good DC characteristics make for a good RF
detector. A number of other factors, which have been discussed here, are
very important. 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.

You can learn a lot with a very simple setup consisting of nothing more
than a variable amplitude signal generator, a diode, load resistor and
capacitor, and a meter or scope. SPICE should also show these effects
provided you use good models.

Roy Lewallen, W7EL

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.)

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?

Yes.

If not, what do they use instead?

CMOS has no storage.


(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.)

No. Doping "sex" (of gold) is wrong for the N-type base, which is
where you're trying to induce recombination.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

I read in sci.electronics.design that Jim Thompson
wrote (in k58f015dte50otru2ams8qavq858c3sl8b@
4ax.com) about 'gold doping (was Diode and very small amplitude
high frequencies signals)', on Mon, 7 Feb 2005:

No. Doping "sex" (of gold) is wrong for the N-type base, which is where
you're trying to induce recombination.

What to use, then? Ion-implanted krypton?
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

On Mon, 7 Feb 2005 17:26:30 +0000, John Woodgate
wrote:

I read in sci.electronics.design that Jim Thompson
wrote (in k58f015dte50otru2ams8qavq858c3sl8b@
4ax.com) about 'gold doping (was Diode and very small amplitude
high frequencies signals)', on Mon, 7 Feb 2005:

No. Doping "sex" (of gold) is wrong for the N-type base, which is where
you're trying to induce recombination.

What to use, then? Ion-implanted krypton?

Sounds good to me, and it'll glow in the dark ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

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!