Physics and Engineering of Radiation Detection

6.5. Photodetectors 411

C.3 QuantumEfficiencyandResponsivity

The quantum efficiency of a photodiode detector is used to characterize its efficiency
of creating charges that contribute to the output signal. It is generally defined by
the ratio

ξ=

rate of electron generation

intensity of incident photons

. (6.5.63)

Note that here therate of electron generationrefers to the electrons that contribute
to the output signal. Hence, ifIγis the current in amperes that generates the output
signal and is generated by the incident photons, then we can write

Rate of electron generation =

Iγ

e

s−^1 , (6.5.64)

withebeing the electronic charge. Iγ is sometimes also referred to as thepho-
tocurrent. Since the photocurrent in an APD actually gets amplified by a factorG,
therefore we can also write the above expression as

Rate of electron generation =

Iout

〈G〉e

s−^1 , (6.5.65)

whereIout=〈G〉Iγ is the current measured at the APD output and〈G〉is the
mean APD gain. The second term in the definition of the quantum efficiency is the
intensity of incident photons. It can be calculated from the incident powerPand
the mean wavelengthλof the incident photons through the relation

Intensity of incident photons =

Pλ

hc

s−^1. (6.5.66)

Hence the expression for the quantum efficiency can be written as

ξ =

Iout/〈G〉e

Pλ/hc

=

Iout

P〈G〉

hc

eλ

=

R

〈G〉

hc

eλ

, (6.5.67)

whereR=Iout/P is known as theresponsivityof the photodetector. Responsivity
is a standard parameter that is often quoted by the APD manufacturers and is used
to compare the characteristics of different APDs. Its generally quoted units are
amperes per watt. The expression for responsivity in terms of the efficiency can be
deduced from the above expression as

R=

〈G〉eξλ

hc

, (6.5.68)

or in Standard units it can be written as

R=8. 07 × 105 〈G〉eξλ AW−^1. (6.5.69)