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PIN Photodiode: needs biasing?

R

Robert Scott

Jan 1, 1970
0
I have been doing some experimenting with a transimpedance amp (LF353) and a PIN
photodiode. So far I have been using only a simple 1M feedback resistor with
zero bias on the diode. Then I noticed in the datasheet for the PIN photodiode
(NEC PH302) it shows a graph of photodiode current as a function of reverse
bias. According to that graph, the photodiode current should be about double
with 2 volts of reverse bias on the diode as compared with zero bias. When I
tried it, the gain of the system remained the same (I have a squarewave light
signal generated by a LED). It did improve the transient response quite a bit,
especially when I rasied the bias to 7 volts (due to the reduced diode
capacitance, I guess) but I saw no evidence of increased photodiode current. Is
this bogus? I have not seen this stated anywhere else besides the NEC PH302
datasheet, but Graeme in his classic book on Photodiode Amplifiers does say that
PIN diodes are usually used with bias. Why is that? Why PINs and not all
photodiodes?


Robert Scott
Ypsilanti, Michigan
 
H

Helmut Sennewald

Jan 1, 1970
0
Robert Scott said:
I have been doing some experimenting with a transimpedance amp (LF353) and
a PIN
photodiode. So far I have been using only a simple 1M feedback resistor
with
zero bias on the diode. Then I noticed in the datasheet for the PIN
photodiode
(NEC PH302) it shows a graph of photodiode current as a function of
reverse
bias. According to that graph, the photodiode current should be about
double
with 2 volts of reverse bias on the diode as compared with zero bias.
When I
tried it, the gain of the system remained the same (I have a squarewave
light
signal generated by a LED). It did improve the transient response quite a
bit,
especially when I rasied the bias to 7 volts (due to the reduced diode
capacitance, I guess) but I saw no evidence of increased photodiode
current. Is
this bogus? I have not seen this stated anywhere else besides the NEC
PH302
datasheet, but Graeme in his classic book on Photodiode Amplifiers does
say that
PIN diodes are usually used with bias. Why is that? Why PINs and not all
photodiodes?


Robert Scott
Ypsilanti, Michigan

Hello Robert,

The photo current is independent of the reverse voltage at such low
currents.
I guess they measured the photo current with an ampere-meter with 100kOhm
input resistance.

See figure 3.3 of this application-note.
http://sales.hamamatsu.com/assets/applications/SSD/photodiode_technical_information.pdf

Best regards,
Helmut
 
R

redbelly

Jan 1, 1970
0
I have been doing some experimenting with a transimpedance amp (LF353) and a PIN
photodiode. So far I have been using only a simple 1M feedback resistor with
zero bias on the diode. Then I noticed in the datasheet for the PIN photodiode
(NEC PH302) it shows a graph of photodiode current as a function of reverse
bias. According to that graph, the photodiode current should be about double
with 2 volts of reverse bias on the diode as compared with zero bias. When I
tried it, the gain of the system remained the same (I have a squarewave light
signal generated by a LED). It did improve the transient response quite a bit,
especially when I rasied the bias to 7 volts (due to the reduced diode
capacitance, I guess) but I saw no evidence of increased photodiode current. Is
this bogus? I have not seen this stated anywhere else besides the NEC PH302
datasheet, but Graeme in his classic book on Photodiode Amplifiers does say that
PIN diodes are usually used with bias. Why is that? Why PINs and not all
photodiodes?

Robert Scott
Ypsilanti, Michigan

Robert,

What I have noticed with unbiased silicon photodiodes in general is
that they are pretty linear up to about 0.2 or 0.25 V output, no
matter what current + load resistor combination is used. By the time
the output gets up to 0.3 or 0.4 V, the response (output vs. incident
photo power) is pretty well flattening out. You don't mention what
output current or voltage you are getting, but hopefully this info
helps you determine if you should need biasing for linearity.

That being said, it has been 12+ years since I worked with a PIN
photodiode, and I'm not sure if they behave vastly different, in terms
of linearity, vs. a regular PN type. If speed is an issue, then as
you noticed biasing will help you with that.

Regards,

Mark
 
J

Joerg

Jan 1, 1970
0
Robert said:
I have been doing some experimenting with a transimpedance amp (LF353) and a PIN
photodiode. So far I have been using only a simple 1M feedback resistor with
zero bias on the diode. Then I noticed in the datasheet for the PIN photodiode
(NEC PH302) it shows a graph of photodiode current as a function of reverse
bias. According to that graph, the photodiode current should be about double
with 2 volts of reverse bias on the diode as compared with zero bias. When I
tried it, the gain of the system remained the same (I have a squarewave light
signal generated by a LED). It did improve the transient response quite a bit,
especially when I rasied the bias to 7 volts (due to the reduced diode
capacitance, I guess) but I saw no evidence of increased photodiode current. Is
this bogus? I have not seen this stated anywhere else besides the NEC PH302
datasheet, but Graeme in his classic book on Photodiode Amplifiers does say that
PIN diodes are usually used with bias. Why is that? Why PINs and not all
photodiodes?

Provided you built a good TIA where the voltage at the photodiode node
does not move the reverse bias is primarily intended to reduce the diode
capacitance and thus improve speed. It also allows you to reduce the
compensation cap Cfb for the TIA, making it a bit more hot. But that
really only matters in the 100MHz+ speed ranges, not so much for an LF
series amp.

I finished a photodiode design about a month ago and this one went to
100MHz. I moved the bias around a bit but even there it didn't matter. I
just kept it in case the client wants to go higher one day.
 
J

John Larkin

Jan 1, 1970
0
I have been doing some experimenting with a transimpedance amp (LF353) and a PIN
photodiode. So far I have been using only a simple 1M feedback resistor with
zero bias on the diode. Then I noticed in the datasheet for the PIN photodiode
(NEC PH302) it shows a graph of photodiode current as a function of reverse
bias. According to that graph, the photodiode current should be about double
with 2 volts of reverse bias on the diode as compared with zero bias. When I
tried it, the gain of the system remained the same (I have a squarewave light
signal generated by a LED). It did improve the transient response quite a bit,
especially when I rasied the bias to 7 volts (due to the reduced diode
capacitance, I guess) but I saw no evidence of increased photodiode current. Is
this bogus? I have not seen this stated anywhere else besides the NEC PH302
datasheet, but Graeme in his classic book on Photodiode Amplifiers does say that
PIN diodes are usually used with bias. Why is that? Why PINs and not all
photodiodes?


Robert Scott
Ypsilanti, Michigan

I think current can be lower at zero bias because some photon-smacked
hole-electron pairs recombine and are lost. With a reverse-bias field,
they are swept up and collected quickly.

For the first few volts of reverse bias, junction capacitance drops
rapidly, so the *circuit* gets faster. That effect flattens out
quickly, but higher bias sweeps carriers out faster and continues to
improve *diode* speed.

At very high voltages (50? 100?) you can get avalanche multiplication
that greatly increases gain (and noise.) Some diodes are designed to
do this well.

Something like that.

John
 
J

Joerg

Jan 1, 1970
0
John said:
I think current can be lower at zero bias because some photon-smacked
hole-electron pairs recombine and are lost. With a reverse-bias field,
they are swept up and collected quickly.

For the first few volts of reverse bias, junction capacitance drops
rapidly, so the *circuit* gets faster. That effect flattens out
quickly, but higher bias sweeps carriers out faster and continues to
improve *diode* speed.

At very high voltages (50? 100?) you can get avalanche multiplication
that greatly increases gain (and noise.) Some diodes are designed to
do this well.

Just be aware that some of the communications diodes cannot tolerate
more than 5V. At least per abs max in the datasheet.
 
I have been doing some experimenting with a transimpedance amp (LF353) and a PIN
photodiode. So far I have been using only a simple 1M feedback resistor with
zero bias on the diode. Then I noticed in the datasheet for the PIN photodiode
(NEC PH302) it shows a graph of photodiode current as a function of reverse
bias. According to that graph, the photodiode current should be about double
with 2 volts of reverse bias on the diode as compared with zero bias. When I
tried it, the gain of the system remained the same (I have a squarewave light
signal generated by a LED). It did improve the transient response quite a bit,
especially when I rasied the bias to 7 volts (due to the reduced diode
capacitance, I guess) but I saw no evidence of increased photodiode current. Is
this bogus? I have not seen this stated anywhere else besides the NEC PH302
datasheet, but Graeme in his classic book on Photodiode Amplifiers does say that
PIN diodes are usually used with bias. Why is that? Why PINs and not all
photodiodes?

Robert Scott
Ypsilanti, Michigan

The datasheet I found doesn't start at zero. Do you have a link?
 
I have been doing some experimenting with a transimpedance amp (LF353) and a PIN
photodiode. So far I have been using only a simple 1M feedback resistor with
zero bias on the diode. Then I noticed in the datasheet for the PIN photodiode
(NEC PH302) it shows a graph of photodiode current as a function of reverse
bias. According to that graph, the photodiode current should be about double
with 2 volts of reverse bias on the diode as compared with zero bias. When I
tried it, the gain of the system remained the same (I have a squarewave light
signal generated by a LED). It did improve the transient response quite a bit,
especially when I rasied the bias to 7 volts (due to the reduced diode
capacitance, I guess) but I saw no evidence of increased photodiode current. Is
this bogus? I have not seen this stated anywhere else besides the NEC PH302
datasheet, but Graeme in his classic book on Photodiode Amplifiers does say that
PIN diodes are usually used with bias. Why is that? Why PINs and not all
photodiodes?

Robert Scott
Ypsilanti, Michigan

The datasheet I found doesn't start at zero. Do you have a link?
 
H

Helmut Sennewald

Jan 1, 1970
0
That is the same one I found. Since the graph doesn't go to zero, how
did you conclude the shift for 2v back bias?

Hello miso,

They obviously measured with an ampere-meter with 100kOhm input resistance.
This is nonsense of course in a photo-diode application when you measure
over a
wide range of light(photo current). As soon as there is a voltage drop of a
very few 100mV on the load resistor, the measured current isn't correct
because
the photo diode gets internally forward biased by this value. Either a
negative bias
voltage will help to prevent that or a short circuit current measurement
(transimpedance amplifier).

Best regards,
Helmut
 
R

redbelly

Jan 1, 1970
0
That is the same one I found. Since the graph doesn't go to zero, how
did you conclude the shift for 2v back bias?

Robert is talking about the photocurrent (bottom of p. 3 in the .pdf
file), whereas you seem to be talking about the graph of leak current
(top of p. 3).

Mark
 
Robert is talking about the photocurrent (bottom of p. 3 in the .pdf
file), whereas you seem to be talking about the graph of leak current
(top of p. 3).

Mark

Does this mean the efficiency of the detector increases with bias? Or
are they lumping leakage in with photocurrent?
 
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