Biophotonics_Concepts_to_Applications

(d) At wavelengths less than 1100 nm, the photons are absorbed very close

to the photodetector surface, where the recombination rate of the gen-

erated electron–hole pairs is very short. The responsivity thus decreases

rapidly for smaller wavelengths, since many of the generated carriers

recombine rapidly and thus do not contribute to the photocurrent.

5.2 Avalanche Photodiodes.

Avalanche photodiodes(APDs) internally multiply the primary signal photocurrent
before it enters the input circuitry of the following amplifier. This action increases
receiver sensitivity because the photocurrent is multiplied before encountering the
thermal noise associated with the receiver circuit. As shown in Fig.5.5,pho-
tocurrent multiplicationtakes place when the photon-generated carriers traverse a
region where a very high electricfield is present. In this high-field region, a
photon-generated electron or hole can gain enough energy so that it ionizes bound
electrons in the valence band upon colliding with them. This carrier multiplication
mechanism is known asimpact ionization. The newly created carriers are also
accelerated by the high electricfield, thus gaining enough energy to cause further
impact ionization. This phenomenon is the avalanche effect.

Electron-hole

pair created by

an absorbed photon

Hole

eh pairs

created

p region

Multiplication

region

Intrinsic (i) region

n region

Fig. 5.5photocurrent multiplication inConcept of External circuit +

an APD

5.1 ThepinPhotodetector 127