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From: Phil Hobbs on 23 Mar 2010 11:11
On 3/22/2010 11:33 PM, Tim Williams wrote:
> "Phil Hobbs"<pcdhSpamMeSenseless(a)electrooptical.net> wrote in message
> news:4BA80B04.2000500(a)electrooptical.net...
>> I don't know the origin of the photoresponse hysteresis of CdS and CdSSe,
>> but a SWAG would be that there are long-lived trap states that get
>> saturated in very bright light, and that this effect makes the carrier
>> lifetime longer.
>
> Oh, ya know, for a physics lab we exposed CdS photocells to a strobe light
> and watched the "exponential" decay. Now, theory claims recombination is
> proportional to concentration, so the exponential feeds back into itself
> effectively as a varying tau. We were supposed to measure this 1/R
> correspondence (where R is inversely proportional to carrier concentration,
> hence the measurement), but when I ran through all the plots and such, it
> looked stupid. On a whim, I took sqrt(tau) instead and that was linear. I
> pretty well stumped the professor with that one. ;-)
>
> Weird? Ya...
>
> Oh, and don't forget you can use CdS photocells as MOSFETs.
> http://sparkbangbuzz.com/cds-fet/cds-fet.htm
>
> Tim
>

Free-free recombination is proportional to carrier density squared, so
dG/dt = -kG**2 for some k. There are solutions proportional to 1/t.

On the other hand, if the recombination happens mostly at the ends of
the elements, it'll be proportional to the current. Did your experiment
run at constant current or constant voltage?

Cheers

Phil Hobbs



--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: Tim Williams on 23 Mar 2010 11:21
"Phil Hobbs" <pcdhSpamMeSenseless(a)electrooptical.net> wrote in message
news:4BA8DA38.60006(a)electrooptical.net...
> Free-free recombination is proportional to carrier density squared, so
> dG/dt = -kG**2 for some k. There are solutions proportional to 1/t.
>
> On the other hand, if the recombination happens mostly at the ends of the
> elements, it'll be proportional to the current. Did your experiment run
> at constant current or constant voltage?

Not really much of either, it was a voltage divider.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: Phil Hobbs on 23 Mar 2010 11:31
On 3/23/2010 11:21 AM, Tim Williams wrote:
> "Phil Hobbs"<pcdhSpamMeSenseless(a)electrooptical.net> wrote in message
> news:4BA8DA38.60006(a)electrooptical.net...
>> Free-free recombination is proportional to carrier density squared, so
>> dG/dt = -kG**2 for some k. There are solutions proportional to 1/t.
>>
>> On the other hand, if the recombination happens mostly at the ends of the
>> elements, it'll be proportional to the current. Did your experiment run
>> at constant current or constant voltage?
>
> Not really much of either, it was a voltage divider.
>
> Tim
>

Interesting. I've never really understood the finer details of the
physics. For instance, the books say that photoconductive gain still
goes as the carrier lifetime even when that's longer than the transit
time--IOW when the a carrier gets to the bond pads, apparently another
carrier is magically injected to compensate.

Some charge neutrality incantation is apparently responsible, but that's
pretty hard to believe when the element is a long skinny serpentine
thing like a Vactrol, and is made of nasty polycrystalline stuff.
Also, any such process would have to be stochastic, which would give
rise to additional noise over and above the 2x shot noise from
stochastic generation and stochastic recombination. (It would be an
interesting measurement, if I had any reason to care about photoconductors.)

Cheers

Phil Hobbs



--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: John Larkin on 23 Mar 2010 11:33
On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold
<ggherold(a)gmail.com> wrote:

>On Mar 23, 8:32�am, Wimpie <wimabc...(a)tetech.nl> wrote:
>> On 23 mar, 12:57, Wimpie <wimabc...(a)tetech.nl> wrote:
>>
>>
>>
>>
>>
>> > On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal
>>
>> > Murray) wrote:
>> > > In article <4BA7797C.7020...(a)electrooptical.net>,
>> > > �Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> writes:
>>
>> > > > �Night lights use CdS or CdSSe photoconductors, but they don't have to
>> > > >have any kind of stability or repeatability, and they don't. �(CdSSe's
>> > > >resistance can vary 5X due entirely to previous illumination history.)
>>
>> > > What is the mechanism for that history quirk?
>>
>> > > What is the time scale? �How long does it remember the history?
>>
>> > > --
>> > > These are my opinions, not necessarily my employer's. �I hate spam.
>>
>> > Hello,
>>
>> > Go towww.perkinelmer.comandsearch for "photocell", or follow the
>> > links. When at the photocell page, on the right side you have an
>> > application note link. They have a very nice document on the LDRs that
>> > includes the memory effect also.
>>
>> > Best regards,
>>
>> > Wim
>> > PA3DJSwww.tetech.nl
>> > without abc, PM will reach me
>>
>> The document that I have is no longer present on the Perkinelmer
>> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf
>> you can find the document about LDR. I will leave it there for about
>> 10 days.
>>
>> Best regards,
>>
>> Wim
>> PA3DJSwww.tetech.nl- Hide quoted text -
>>
>> - Show quoted text -
>
>Thanks for the document, page 35 talks about shot noise in LDR's. Are
>they just throwing equations around or is there shot noise in LDR's?
>
>George H.

Since photons make current, probably so. But LDRs are so messy it's
likely buried in other gunk.

One of my guys is building a test setup to measure shot/excess noise
in resistors. We need a ~~ 100M resistor to create a small (50 nA
maybe) bias current with below shot-level noise. It's not clear if
high-value cermet resistors have shot noise or not, so we plan to
measure a bunch.

Metal film resistors don't go to very high values; we can probably get
10M and likely 22M, and maybe even 50M, so we may have to do a series
string. Even then I want to measure them to make sure they behave. I'm
guessing that axials are better than surface mount, because of the
bigger available surface for depositing metal.

It's an interesting problem, trying to generate a nA-range DC current
with low noise. Low voltage across a low-value metal-film resistor
doesn't work because of Johnson noise. High value resistors are noisy
in their own right.

John


From: John Larkin on 23 Mar 2010 11:40
On Tue, 23 Mar 2010 11:31:21 -0400, Phil Hobbs
<pcdhSpamMeSenseless(a)electrooptical.net> wrote:

>On 3/23/2010 11:21 AM, Tim Williams wrote:
>> "Phil Hobbs"<pcdhSpamMeSenseless(a)electrooptical.net> wrote in message
>> news:4BA8DA38.60006(a)electrooptical.net...
>>> Free-free recombination is proportional to carrier density squared, so
>>> dG/dt = -kG**2 for some k. There are solutions proportional to 1/t.
>>>
>>> On the other hand, if the recombination happens mostly at the ends of the
>>> elements, it'll be proportional to the current. Did your experiment run
>>> at constant current or constant voltage?
>>
>> Not really much of either, it was a voltage divider.
>>
>> Tim
>>
>
>Interesting. I've never really understood the finer details of the
>physics. For instance, the books say that photoconductive gain still
>goes as the carrier lifetime even when that's longer than the transit
>time--IOW when the a carrier gets to the bond pads, apparently another
>carrier is magically injected to compensate.
>
>Some charge neutrality incantation is apparently responsible, but that's
>pretty hard to believe when the element is a long skinny serpentine
>thing like a Vactrol, and is made of nasty polycrystalline stuff.
>Also, any such process would have to be stochastic, which would give
>rise to additional noise over and above the 2x shot noise from
>stochastic generation and stochastic recombination. (It would be an
>interesting measurement, if I had any reason to care about photoconductors.)
>
>Cheers
>
>Phil Hobbs

There are also photoresistors made from radiation-damaged GaAs. You
can make a ps-speed electrical signal sampler from a GaAs
photoresistor banged by a fs-range laser. That would make a nice
sampling scope if the laser weren't so big and expensive, and if you
could trigger it without huge amounts of jitter.

John

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