Avalanche Photodetector

Very cool, but made comical by the PR pictures showing snow avalanches.

On a sunny day (Sat, 06 Mar 2010 21:52:19 -0800) it happened Kevin McMurtrie wrote in :

Yes the reality in this world is slowly being replaced by 'artist impresions' and 'simulations'.

Nobody notices, the next mars rovers will actually be just some simulation running on google's computers. And NASA will still get 100 billion for that mission.

This sort of thing is standard in press-release science. The application sounds silly to me.

John

s.apd.html

Strange, APDs are very sensitive but not usually fast. T(on) may be = quick and strong=20 but t(off) usually seems to be slow and sometimes troubled. Maybe there = is some quench=20 circuitry involved that they are not talking about. Or maybe i, yet = again, do not quite=20 know what i am about, Dr. Hobbs?

strong

some quench

not quite

APDs are quick if the multiplication ratio is low and the ratio of the avalanche cross sections of electrons and holes is small.

If only electrons cause avalanche ionization (Si is close to this condition), the APD can be fast and fairly quiet--the initial electron and all the multiplied ones arrive at once, while the holes trickle in in 1 transit time. No bad speed tradeoff until the multiplication ratio is over 50.

If electrons and holes have equal probability of inducing avalanches, then the avalanche bounces back and forth randomly and can last much longer and be much noisier. InGaAs is like that, which is why InGaAs APDs have to be run at gains below about 10.

The better modern APDs use heterostructures, so the multiplication occurs in a different semiconductor than the photodetection.

Cheers

Phil Hobbs