Re: Testing a photodiode

From: Bradley1234 (someone_at_yahoo.com)
Date: 01/06/05 

Date: Thu, 06 Jan 2005 06:00:22 GMT

Okay, way back somebody said a diod is a diod, not a resistor, so the theme
became what is a diode, not what is a photodiode.

So thats why I mentioned the electrical properties of a diode being used for
a variable resistance. Of course its not a bidirectional resistance, but by
any other name becoming a variable resistance to current flow. (yes non
linear) I described the simple effect of recombination in the depletion
region at the PN junction that changes based on bias, to which you said this
was incorrect.

In photodiodes? If the photodiode is biased and light is applied the amount
of current flow should change. Net effect its resistance has changed.

The website mentioned shows a non standard photodiode, and yes I did the
search, interesting product line

> > > > > > > A diod is a diod, not a resistor.
> > > > > > > See it as a current source, and put it into a opamp to form a
> > current
> > > > to
> > > > > > > voltage converter...
> > > > > > >
> > > > > >
> > > > > > A diode is a variable resistor.
> > > > > >
> > > > > > the circuit reacts around the depletion region to increase or
> > decrease
> > > > it.
> > > > > > When the p type and n type silicon meet, the charges mix to form
a
> > > > neutral
> > > > > > region which is resistive. if the applied electricity is on one
> > side it
> > > > > > makes that neutral region expand out, if its the other way the
> > neutral
> > > > > > region is compressed
> > > > >
> > > > > Not really. :)
> > > >
> > > > Then please to explain me how silicon diodes work? All this time I
> > thought
> > > > it was the depletion region but its something else?
> > > >
> > > > If my simplified explanation of the solid state physics involved in
PN
> > > > junctions is incorrect Im grateful to learn where Im wrong.
> > >
> > > Please Google "Photodiode principles". There were 33,100 hits but
> > > the first one is adequate. Thanks.
> >
> > I did, and it still explains similarly to what I said. Ive worked on
the
> > design of semiconductors, lithography, processing, doping, circuits
design,
> > and enough circuit testing to be content with, and have trained and
shared,
> > possibly the wrong information with dozens and dozens of people. maybe
> > more.
> >
> > I enjoy studying, but to say just go do a google search? Id expect any
such
> > searches will simply verify and validate what I said.
>
> Did you do the search? Here is the first sentence of the first hit:
>
> "When these diodes are exposed to photons of energy greater than 1.12 eV
> (wavelength less than 1100 nm) electron-hole pairs (carriers) are
created.
> These photogenerated carriers are separated by the p-n junction electric
> field and a current proportional to the number of electron-hole pairs
> created flows through an external circuit."
>
> That is the most simple explanation of how a photodiode works. It's not
> a variable resistor but a a light to current converter.
>
> A device like a cadmium sulfide photocell is a variable resistor,
controlled
> by how much light falls on it. That's totally different than a
photodiode.
>
> I agree that a lot of Web search returns are useless - in fact this one
> goes on to talk about other things that aren't directly relevant, it is
> a start for many thing.
>
> I'll let others comment on whether your explanation is similar to this one
> but unless I missed something, you didn't even bother to mention anything
> about a photodiode, only a normal PN diode used in an electrical circuit.
>
> --- sam | Sci.Electronics.Repair FAQ Mirror:
http://repairfaq.ece.drexel.edu/
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