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From: Phil Hobbs on 23 Mar 2010 11:43
On 3/23/2010 9:59 AM, George Herold 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.

The conductivity comes from photogenerated carriers, so there's one copy
of the shot noise there. Another copy comes from the stochastic
recombination, so a photoconductor with unity gain (i.e. where the
transit time equals the carrier lifetime) has exactly twice the shot
noise.

If there's a lot of gain, i.e. you get to reuse the same carriers lots
of times before they recombine, the shot noise gets amplified too--just
like an APD or PMT.

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: Phil Hobbs on 23 Mar 2010 11:55
On 3/23/2010 11:40 AM, John Larkin wrote:
> 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
>

People have been doing that sort of sampling for 25-30 years--first
using coplanar transmission line on an electrooptic crystal like LiNbO3,
where you look at the polarization shift in the light, and somewhat
later using the plasma-optical effect, which can be either an
optical-readout sampler or a switch. Dan Grischkowsky and Jean-Marc
Halbout were doing it when I joined IBM, and before that, Dave Bloom was
doing it at Stanford (he might have started it at Bell, I forget).

Using a femtosecond laser (a Hall-Haensch comb generator) will get you
unmatched stability, but of course the experiment has to be synched to
the laser rather than the other way round.

Telecom modulators are getting up above 50 GHz bandwidth, but only in
the 1.5 um band, which is too long for GaAs.


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: George Herold on 23 Mar 2010 13:39
On Mar 23, 11:33 am, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold
>
>
>
>
>
> <ggher...(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.comandsearchfor "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- Hide quoted text -
>
> - Show quoted text -

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

Hmm, Yeah this is a bit confusing. The photons make the charge
carriers and reduce the resistance... and as Phil H. says there is
generation and recombination noise. But this should only depend on
the light level and not on how the device is biased. For a given
light level if I double the bias voltage (and thus double the current)
does the noise go up?

I'm using 100M and 1 G resistors from Ohmite to make a 10nA current
source. I'll look at the noise next chance I get.

George H.


From: Robert Macy on 23 Mar 2010 13:42
On Mar 23, 10:39 am, George Herold <ggher...(a)gmail.com> wrote:
> On Mar 23, 11:33 am, John Larkin
>
>
>
>
>
> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> > On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold
>
> > <ggher...(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.comandsearchfor"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-Hidequoted 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- Hide quoted text -
>
> > - Show quoted text -
>
> "> Since photons make current, probably so. But LDRs are so messy it's
>
> > likely buried in other gunk."
>
> Hmm, Yeah this is a bit confusing.  The photons make the charge
> carriers and reduce the resistance... and as Phil H. says there is
> generation and recombination noise.  But this should only depend on
> the light level and not on how the device is biased.  For a given
> light level if I double the bias voltage (and thus double the current)
> does the noise go up?
>
> I'm using 100M and 1 G resistors from Ohmite to make a 10nA current
> source.  I'll look at the noise next chance I get.
>
> George H.

sorry for the VERY naive question, but would some form of junction
leakage have lower noise? Or are junction leakages at and above
johnson?
From: George Herold on 23 Mar 2010 13:59
On Mar 23, 11:43 am, Phil Hobbs
<pcdhSpamMeSensel...(a)electrooptical.net> wrote:
> On 3/23/2010 9:59 AM, George Herold 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.comandsearchfor "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.
>
> The conductivity comes from photogenerated carriers, so there's one copy
> of the shot noise there.  Another copy comes from the stochastic
> recombination, so a photoconductor with unity gain (i.e. where the
> transit time equals the carrier lifetime) has exactly twice the shot
> noise.
>
> If there's a lot of gain, i.e. you get to reuse the same carriers lots
> of times before they recombine, the shot noise gets amplified too--just
> like an APD or PMT.
>
> 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 nethttp://electrooptical.net- Hide quoted text -
>
> - Show quoted text -

Thanks Phil, I must admit I don't quite 'see' how that works out. If
I 'reuse' a photo generated charge carrier say ten times I get ten
times the shot noise? (OK twenty times since I get twice the shot
noise to begin with.) I'll have to think about that. The bias
current divided by the photo current is something like the gain of the
LDR.

Say (speaking of noise) I think I remember you mumbling sometime ago
that you had a nice derivation of the Johnson noise formula.
Something that didn't involve the counting of modes in an infinite
transmission line as Nyquist first derived it. Have you written this
up anywhere?

George H.


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