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'[OT] What is a Schottky ?'
1999\05\11@083457 by Benjamin Petersen

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Sorry about my lag of analogic electronics, but what is a Schottky Diode ?

Regards
Benjamin Petersen

1999\05\11@130245 by Roland Andrag

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>Sorry about my lag of analogic electronics, but what is a Schottky Diode ?


Also called a 'hot-carrier' diode. Basically it has a very low forward drop
voltage - say around .25 V instead of the normal silicon 0.6 to 0.7 V.  A
very good book is The art of electronics, by Horowitz and Hill, if you plan
to learn anything but basic electronics, especially analogue. It contains
just as much digital electronics knowledge, but no PICs. Still a must have.

Cheers
Roland

1999\05\11@133513 by Mark Willis

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Roland Andrag wrote:
>
> >Sorry about my lag of analogic electronics, but what is a Schottky Diode ?
>
> Also called a 'hot-carrier' diode. Basically it has a very low forward drop
> voltage - say around .25 V instead of the normal silicon 0.6 to 0.7 V.  A
> very good book is The art of electronics, by Horowitz and Hill, if you plan
> to learn anything but basic electronics, especially analogue. It contains
> just as much digital electronics knowledge, but no PICs. Still a must have.
>
> Cheers
> Roland

 They're a little more expensive than 1N4148's et al, they're great
where you want to drive a circuit off a Parallel Port, or anywhere else
that those diode drops add up too fast and you need them not to.
They're also pretty fast diodes, so protection networks are great when
done with Schottky's.

 The forward voltage drop does come up to more like 0.4V at higher
currents, still about 1/2-1/3 the drop can mean your circuit works
without a "Wall Wart".

 Mark

1999\05\11@143456 by Bob Drzyzgula

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This is the problem I too had for a long time with
Schottkys. You'd see them in circuits, you'd see them
referred to in messages, you'd see descriptionss -- as in
AoE -- of when you'd want to use one, but despite however
common they were, for some reason no one ever explains
*what* they are. Like, descriptions of how a FET works are
a dime a dozen, but it seems as if it is assumed that by
the time you care about Schottkys you already know what
they are.

As a total coincidence, just last night I went looking
for a real description of them (AoE doesn't help here,
as far as I could tell). After two hours (!) of digging
through vendor websites and general-purpose search engines,
(endless product bulletins, EE course descriptions,
how-they-are-used notes, etc.), I finally came up with
this reference:

http://www.microsemi.com/micnotes/400ser/401.htm

Which was more like what I was looking for. The document is
also available as a pdf at:

http://www.microsemi.com/micnotes/401.pdf

Undoubtedly I just wasn't doing a good job of
Web searching, but nevertheless I wrote to
Microsemi to say thanks for having that document
up... :-)

--Bob

On Tue, May 11, 1999 at 10:32:49AM -0700, Mark Willis wrote:
{Quote hidden}

--
============================================================
Bob Drzyzgula                             It's not a problem
spam_OUTbobTakeThisOuTspamdrzyzgula.org                until something bad happens
============================================================

1999\05\11@162728 by Lynx {Glenn Jones}

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So if schottkies are so fast, why dont they make transistors out of them?
or do they?

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A member of the PI-100 Club:
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058209749445923078164062862089986280348253421170679

1999\05\11@184450 by Barry King

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A Schottky diode's junction is some kind of metal to silicon
junction, which conducts differently, with a smaller forward voltage
drop.  (I always get the spelling wrong, you got it right, its
"Schottky" and capitalized, its the inventor's name.  I wonder which
company got THAT patent for a dollar?)

> So if schottkies are so fast, why dont they make transistors out of them?
> or do they?
Schottky diodes are not necessarily all that fast.  The 1N5817,
for example, is a Schottky rectifier, but its rated for use at 60Hz,
which by switching regulator standards is SLOOOW.

And there are diodes that are ultra-fast that are not Schottky
diodes.

However, the diodes used in switching power supplies must be BOTH
very fast and low drop for best efficiency.  The huge quantities
of switchmode power supplies being built has created a huge demand
for fast Schottkys, that makes them common and therefore cheaper.

I think the best way to build a transistor using metal / silicon
junctions is called a MOSFET.  They DO make those :)

Semiconductor Physics was a while ago, so I don't remember if the
Schottky junction trick is applicable to bipolar transistors.
Usually, low forward drop is not an issue in bipolar transistors,
since the controlled current (Collector current) is not flowing
across a regular forward-biased PN junction.

------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
.....barryKILLspamspam@spam@nrgsystems.com
"The witty saying has been deleted due to limited EPROM space"

1999\05\11@190742 by Vincent Deno

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Barry King wrote:

> > So if schottkies are so fast, why dont they make transistors out of them?
> > or do they?
> Schottky diodes are not necessarily all that fast.  The 1N5817,
> for example, is a Schottky rectifier, but its rated for use at 60Hz,
> which by switching regulator standards is SLOOOW.
>

Schottky barriers ARE made of a metal and semiconductor junction.  They are
intrisically faster than normal p-n junctions because of charge storage.  Like
any other junction, they can be physically laid out to optimize different
performance issue (power, switching speed, etc...)  But the Fermi level of the
metal make the barrier much easier to overcome in the forward biased mode.


{Quote hidden}

They do in fact use Schottky junctions in switching transistors.  In cuttoff
mode the collector-base junction is reversed-biased.  To switch to saturation,
you have to close the depletion region first.  By putting a Schottky barrier
diode from base to collector, you can dramatically improve this switching
operation.

>
> ------------
> Barry King, KA1NLH
> Engineering Manager
> NRG Systems "Measuring the Wind's Energy"
> Hinesburg, Vermont, USA
> barryspamKILLspamnrgsystems.com
> "The witty saying has been deleted due to limited EPROM space"

1999\05\11@190749 by William Chops Westfield

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Yes, of course there are Schottky transistors.  These were used to make
the 74Sxx series logic - the fastest available in its day.

BillW

1999\05\11@213611 by Russell McMahon

picon face
There is no such thing as a Schottky transistor per se BUT they are
used to speed up switching of conventional transistors by stopping
them going into deep saturation - have a look at the internal circuit
of any 74LSxx (low power Schottky) or 74Sxx (High power :-) Schottky)
logic IC. A bit old tech now :-).

Subject: Re: [OT] What is a Schottky ?

>So if schottkies are so fast, why dont they make transistors out of
them?
>or do they?
>

1999\05\11@224711 by Lynx {Glenn Jones}

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Oh of course, now i remember that the S stands for Schottky, and LS is low
power schottky. Thanks for reminding me.

------------------------------------------------------------------------------
A member of the PI-100 Club:
3.1415926535897932384626433832795028841971693993751
058209749445923078164062862089986280348253421170679

On Tue, 11 May 1999, William Chops Westfield wrote:

> Yes, of course there are Schottky transistors.  These were used to make
> the 74Sxx series logic - the fastest available in its day.
>
> BillW
>

1999\05\20@122434 by John Payson

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|Semiconductor Physics was a while ago, so I don't remember if the
|Schottky junction trick is applicable to bipolar transistors.
|Usually, low forward drop is not an issue in bipolar transistors,
|since the controlled current (Collector current) is not flowing
|across a regular forward-biased PN junction.

That reminds me... one of the design features of a BJT is that,
unlike a MOSFET, it will only allow current to flow in one
direction while it is switched on.  So I was wondering...

If you have a supply of base current, what are the limitations
of using the BJT as a diode [between collector and emitter].
Obviously the transistor will only be on if the base voltage is
0.7 volts above (NPN) or below (PNP) the emitter voltage, but
in many applications this would not pose a problem if the drop
between collector and emitter is small.

For example, what would be the effects of replacing a blocking
diode on the positive leg of the power-supply input to a device
with a PNP transistor whose base was tied to the negative sup-
ply through a suitable resistor?  Would high-voltage transients
from the device be able to go backward through the transistor,
or would it not allow such things?



Attachment converted: wonderland:WINMAIL.DAT 3 (????/----) (000070E2)

1999\05\21@091532 by paulb

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John Payson wrote:

> That reminds me... one of the design features of a BJT is that,
> unlike a MOSFET, it will only allow current to flow in one
> direction while it is switched on.  So I was wondering...

 That's a extraordinarily broad statement John!

> If you have a supply of base current, what are the limitations
> of using the BJT as a diode [between collector and emitter].

 The prime limitation as I see it, is that the first statement is
entirely wrong!  In fact, I seem to recall deliberate use of transistors
with collector and emitter transposed specifically to obtain a reduced
Vec(sat) (sic.)

 And a transistor with its collector capacitively coupled to a signal
line (with no other collector bias components) as a gate certainly
pre-dates the use of a MOSFET in the same configuration.
--
 Cheers,
       Paul B.

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