248
6 Photodetection Devices
If the optical power level is 10 μW, then from Eq. (6.6) the photocurrent generated
is.
i p = R P in = (0.65 A/W)(10 μW) = 6.5 μA
In most photodiodes the quantum efficiency is independent of the power level
falling on the detector at a given photon energy. Thus the responsivity is a linear
function of the optical power. That is, the photocurrent i p is directly proportional
to the optical power P in incident upon the photodetector, so that the responsivity R
is constant at a given wavelength (at a given value of hν). Note, however, that the
quantum efficiency is not a constant at all wavelengths because it varies according
to the photon energy. Consequently, the responsivity is a function of the wavelength
and of the photodiode material (because different materials have different bandgap
energies). For a given material, as the wavelength of the incident photon becomes
longer, the photon energy becomes less than that required to excite an electron from
the valence band to the conduction band. The responsivity thus falls off rapidly
beyond the cutoff wavelength, as shown in Fig. 6.4.
Example 6.6 As shown in Fig. 6.4, for the wavelength range 1300 nm < λ < 1600 nm,
the quantum efficiency for InGaAs is around 90%. Thus in this wavelength range the
responsivity is.
R =
ηq
hν
=
ηqλ
hc
=
(0.90)
1.6 × 10
−19 C
λ
6.625 × 10 −34 J-s
3 × 10 8 m/s
= 7.25 × 10
5
λ
For example, at 1300 nm.
R = [7.25 × 10
5
(A/W)/m] (1.30 × 10
−6 m) = 0.92 A/W
At wavelengths higher than 1600 nm, the photon energy is not sufficient to excite
an electron from the valence band to the conduction band. For example, In 0.53 Ga 0.47 As
has an energy gap E g = 0.73 eV, so that from Eq. (6.2) the cutoff wavelength is.
λ c =
1.2406
E g (eV)
=
1.2406
0.73
= 1.7 μm
At wavelengths less than 1100 nm for InGaAs, the photons are absorbed very close
to the photodetector surface, where the recombination rate of the generated electron–
hole pairs is very short. The responsivity in this material and in other photodetectors thus decreases rapidly for smaller wavelengths, because many of the generated
carriers do not contribute to the photocurrent.
6 Photodetection Devices
If the optical power level is 10 μW, then from Eq. (6.6) the photocurrent generated
is.
i p = R P in = (0.65 A/W)(10 μW) = 6.5 μA
In most photodiodes the quantum efficiency is independent of the power level
falling on the detector at a given photon energy. Thus the responsivity is a linear
function of the optical power. That is, the photocurrent i p is directly proportional
to the optical power P in incident upon the photodetector, so that the responsivity R
is constant at a given wavelength (at a given value of hν). Note, however, that the
quantum efficiency is not a constant at all wavelengths because it varies according
to the photon energy. Consequently, the responsivity is a function of the wavelength
and of the photodiode material (because different materials have different bandgap
energies). For a given material, as the wavelength of the incident photon becomes
longer, the photon energy becomes less than that required to excite an electron from
the valence band to the conduction band. The responsivity thus falls off rapidly
beyond the cutoff wavelength, as shown in Fig. 6.4.
Example 6.6 As shown in Fig. 6.4, for the wavelength range 1300 nm < λ < 1600 nm,
the quantum efficiency for InGaAs is around 90%. Thus in this wavelength range the
responsivity is.
R =
ηq
hν
=
ηqλ
hc
=
(0.90)
1.6 × 10
−19 C
λ
6.625 × 10 −34 J-s
3 × 10 8 m/s
= 7.25 × 10
5
λ
For example, at 1300 nm.
R = [7.25 × 10
5
(A/W)/m] (1.30 × 10
−6 m) = 0.92 A/W
At wavelengths higher than 1600 nm, the photon energy is not sufficient to excite
an electron from the valence band to the conduction band. For example, In 0.53 Ga 0.47 As
has an energy gap E g = 0.73 eV, so that from Eq. (6.2) the cutoff wavelength is.
λ c =
1.2406
E g (eV)
=
1.2406
0.73
= 1.7 μm
At wavelengths less than 1100 nm for InGaAs, the photons are absorbed very close
to the photodetector surface, where the recombination rate of the generated electron–
hole pairs is very short. The responsivity in this material and in other photodetectors thus decreases rapidly for smaller wavelengths, because many of the generated
carriers do not contribute to the photocurrent.