Avalanche Photodetector
in [Design]
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From: Phil Hobbs on 8 Mar 2010 16:22 On 3/8/2010 1:54 AM, JosephKK wrote: > On Sat, 06 Mar 2010 17:36:31 GMT, Jan Panteltje<pNaonStpealmtje(a)yahoo.com> wrote: > >> Avalanche Photodetector, IBM, germanium, 1.5V, 40Gbps: >> http://domino.research.ibm.com/comm/research_projects.nsf/pages/photonics.apd.html > > Strange, APDs are very sensitive but not usually fast. T(on) may be quick and strong > but t(off) usually seems to be slow and sometimes troubled. Maybe there is some quench > circuitry involved that they are not talking about. Or maybe i, yet again, do not quite > know what i am about, Dr. Hobbs? 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 -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net |