Photodiode problem: higher currents in photoconductive than in photovoltaic mode

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usefulfacts@gmail.com - 29 Nov 2006 22:12 GMT

Hi,

I have a silicon photodiode and I am trying to characterize it. When I
measured the currents generated in photovoltaic mode (PV) of operation
(without applying a bias) and in photoconductive (PC) mode of operation
(applying a 5V reverse bias voltage), I noticed that the Signal to
noise (dark current) was much better for the photoconductive mode of
operation.

Now, everywhere that I have read suggests that in the Photovoltaic mode
of operation the circuit should be much more sensitive.

For examlple, in photovoltaic mode at some optical power, I get 0.044
nA of current being generated with a dark current of about 0.003 nA.
In the photoconductive mode of operation, I get 3.98 nA of current
generated when biased at 5V with a dark current (light off) of about
the same 0.003 nA.

Also, shouldn't the dark current in photoconductive mode of operation
increase with the applied reverse bias voltage? However, I noticed a
constant dark current while varying my reverse bias voltage.

Also, I am using a picoammeter for my measurements.

Any help or suggestions would be greatly appreciated.

Thanks.

Reply to this Message

Sjouke Burry - 30 Nov 2006 02:01 GMT

> Hi,
>
[quoted text clipped - 23 lines]
>
> Thanks.

You might try to increase the voltage to the avalance
voltage, at which it starts to behave like a photo-
multiplier. However, it might also break your cell.
To avoid that use a series resistor to limit the max.
current.(And dont use an expensive cell).

Reply to this Message

redbelly - 30 Nov 2006 12:43 GMT

> Hi,
>
[quoted text clipped - 7 lines]
> Now, everywhere that I have read suggests that in the Photovoltaic mode
> of operation the circuit should be much more sensitive.

The sensitivity should be the same, so long as it is not saturating in
photovoltaic mode.

> For examlple, in photovoltaic mode at some optical power, I get 0.044
> nA of current being generated with a dark current of about 0.003 nA.
> In the photoconductive mode of operation, I get 3.98 nA of current
> generated when biased at 5V with a dark current (light off) of about
> the same 0.003 nA.

What resistance is on the diode in the photovoltaic measurement? My
first thought was that it is saturating (too high a resistance). That
generally means the diode voltage is at least 0.2 or 0.3 volts, which
for 0.044 nA would mean a whopping several-Gigohm resistance.

> Also, shouldn't the dark current in photoconductive mode of operation
> increase with the applied reverse bias voltage? However, I noticed a
> constant dark current while varying my reverse bias voltage.

A dark photodiode behaves pretty much like a regular diode. The
reverse current (dark current) is fairly constant, increasing only
slightly, so long as you're below the reverse breakdown point.

> Also, I am using a picoammeter for my measurements.
>
> Any help or suggestions would be greatly appreciated.
>
> Thanks.

Reply to this Message

usefulfacts@gmail.com - 30 Nov 2006 21:13 GMT

I am still not clear why there is such a discreptancy between
photovoltaic mode and photoconductive mode. By applying a reverse bias
voltage, can the current generated increase 100 times?

> usefulfa...@gmail.com wrote:
> > Hi,
[quoted text clipped - 30 lines]
>
> > Thanks.- Hide quoted text -- Show quoted text -

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redbelly - 30 Nov 2006 23:26 GMT

> I am still not clear why there is such a discreptancy between
> photovoltaic mode and photoconductive mode. By applying a reverse bias
> voltage, can the current generated increase 100 times?

Yes, most definately. It depends very strongly upon what resistance
value the diode is driving. That particular information would enable
me to help you better.

Regards,

Mark

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Electronics Forum / Basics / November 2006

Photodiode problem: higher currents in photoconductive than in photovoltaic mode