avalanche photodiodes vs normal photodiodes

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.

Cheers

Phil Hobbs

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

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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

No, sorry. If you can use a lens to focus the light down onto a smaller PD, that's almost always a win.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

It all depends on the application. With avalanche you can detect a single edge of a Gaussian pulse and it is most useful in LIDAR. Normal diodes can be much faster but you need the well defined frequency burst or train of pulses to detect 10 ns worth of equivalent energy.

Mathew Orman

On a sunny day (Wed, 16 Nov 2011 17:35:45 -0800) it happened Jamie M wrote in :

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?

d

There are the Bell inequality measurements, where you get two NIR photons from hitting some x-tal with blue laser light and then do photon coincidence counting with two APD's. (But that's a pretty small niche also.)

George H.

t -

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

On a sunny day (Fri, 18 Nov 2011 00:23:36 -0800) it happened Jamie M wrote in :

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.

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?

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

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

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

Hi,

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

cheers, Jamie

time.

allow 50 MHz don't you think?

No, you don't have to quench PMTs because they don't avalanche--one primary photoelectron gets you a nice 5- to 20-ns pulse of 10**5 to

10**7 electrons at the anode, which is easily enough to detect in 50 ohms. There are some other things you have to worry about, such as internal light emission, afterpulses, and ion events, but that's show biz. The main thing is that PMTs have 10**6 times less dark current per unit area than APDs. They're a lot cheaper per unit area, too.

There's a place in the world for APDs, but in well designed instruments it isn't a very big place--mostly lower count rates and very small areas, or when mechanical ruggedness and magnetic field immunity are key attributes.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net