Re: Testing a photodiode

From: Bradley1234 (someone_at_yahoo.com)
Date: 01/07/05 

Date: Fri, 07 Jan 2005 03:04:55 GMT

"Sam Goldwasser" <sam@saul.cis.upenn.edu> wrote in message
news:6woeg2cdj6.fsf@saul.cis.upenn.edu...
> "Bradley1234" <someone@yahoo.com> writes:
>
> > "Sam Goldwasser" <sam@saul.cis.upenn.edu> wrote in message
> > news:6wfz1er8bk.fsf@saul.cis.upenn.edu...
>
> > > The current is proportional to optical power and is more or less
> > > independent of applied voltage. That's not what is normally
> > > considered to be a resistor.
> >
> > The current? so we agree that current changes. Im saying its because
the
> > resistance changes, not because power is created inside a diode that is
> > adding to the circuit. Are you saying optical power is transferred
right
> > out of the diode? It comes in as light and is converted to current?
>
> So how do you explain that a photodiode can operate in photovoltaic mode
> with no bias and generate power?

I dont deny this, its the effect silicon has which is "special" there is
current flow from the conversion, it doesnt need bias, but if there is bias,
the converted energy, as I understand it, is typically used to control the
size of the depletion region, which is the resistance

>
> > Im saying light affects the depletion region that causes a change in
> > resistance, its proportional to incident optical power but assuredly not
> > linearly proportional
>
> But the current in a photodiode is very linearly related to optical power
> until the device saturates.

Okay, here we are both describing a graph in general terms. If there is a
point where the device saturates, its no longer linear after that. I call
this nonlinear, if a typical photodiodes response contains a nearly linear
region? Then its sometimes very linearly related to optical power

>
> Each photon generates one or more electron-hole pairs. That is where the
> current comes from.
>

Well this is the ideal response, but not the actual response. To get to a
point to be able to get 1 to 1 reaction? This is the photon counter
problem. To be able count photons takes a very precise, temperature
controlled, and expensive photodiode, like APD (avalanche photodiode) which
isnt perfect. But in a general photodiode, its not necessarily efficient
where each photon gets converted.

The conversion causes current, but in typical sensor circuits Im guessing
the application is usually to control a bias current in a known linear
region, so it would be in the input to an opamp, with some bias and by the
photodiode varying resistance would provide varying current in response to
incident light, that output would feed an a/d converter and to be accurate,
it would require calibration

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