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avalanche photodiodes vs normal photodiodes

J

Jamie M

Jan 1, 1970
0
Hi,

Here is a $1000 avalanche photodiode (APD), it has a max 2000VDC
reverse bias voltage:

http://www.advancedphotonix.com/ap_products/pdfs/SD197-70-72-591.pdf

Its specs are responsivity of 95 A/watt, 10nA dark current,
1.5pA/sqrt(Hz), 10ns response time.

How close can one get to these specs with a normal (cheap!) photodiode
that is reverse biased to right before the diode breakdown voltage? If
operating in geiger mode (biased past breakdown voltage) what kind of
response time could be expected with an active geiger circuit, and could
the responsitivity of a normal photodiode compare with an avalanche
photodiode?

cheers,
Jamie
 
J

Jamie M

Jan 1, 1970
0
Not close. APDs have a doping profile that produces an isolated
multiplication region way down inside the die, so that you have some
thickness to absorb all the light and then amplify it. In a normal PIN
diode, the doping density in the I region is reasonably constant, so
photon absorption and multiplication would occur throughout. That means
that if you have any decent amount of gain, photons absorbed near the
surface see the whole gain, whereas those absorbed further in see a
lower gain (roughly exponentially declining with depth).

There is a corner of the (speed, photocurrent, area) space in which APDs
are better than PIN diodes and photomultipliers, but it's very small,
and doesn't involve photon counting.

Hi,

Hmm, so if you angle a large area PIN diode 10 degrees away from the
incoming light, will this increase the responsivity since the light will
have more effective thickness to be absorbed in the P region? Also the
incoming light would have to be refocused/magnified to match the reduced
cross section of the angled PIN photodiode. I'm just thinking of a way
to use a cheap diode as a high responsivity APD :)

cheers,
Jamie
 
J

Jamie M

Jan 1, 1970
0
I dunno, but for< 50$ you can buy a PMT on ebay that equals that.
So the issue would then be size and vibration resistance.
Size relative to rest of equipment.
Maybe lifetime too?

Hi,

I think PMT's are better in all specs than semiconductors except for
frequency bandwidth, ie. an APD can do 10 MHz+ bandwidth afaik.

cheers,
Jamie
 
J

Jamie M

Jan 1, 1970
0
PMTs are faster than most APDs, especially at higher gain, where APDs
slow down. The best PMTs (microchannel plates) can do 100 ps edges.

Hi,

wow so why are APD's used when they are so expensive? I guess it is to
do with the cross section area being lower than an equivalent PMT?

cheers,
Jamie
 
J

Jamie M

Jan 1, 1970
0
Not sure what you mean.
In the sixties and seventies we used PMTs in the flying spot film scanners,
and that output was at least 5 MHz wide to start with, 10 should be very easy.
The bandwidth limiting part was the persistence (afterglow) of the CRT
the PMT looked at.
There was a multi-stage RC tail compensation network, very difficult
to adjust.

Hi,

I thought the PMT's were triggered avalanche devices like geiger mode,
with a recharge time for each trigger.

cheers,
Jamie

CRT
with
blank
screen film

] | (======= PMT --- video 5MHz wide
|
|
\ /
. motion of film, pneumatically advanced one frame during TV flyback time.
Called the 'fast pull down scanner' most noisy thing you ever heard...

Anyways, something that has better than 10ns rise and fall times *should* allow 50 MHz don't you think?
 

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