Optoelectronic Semiconductor Devices Principals and Characteristics

7.1.2 
OPTICAL ABSORPTION AND QUANTUM EFFICIENCY
 
The photo-induced carriers are created under light incidence by the intrinsic and extrinsic process 
(electrons are exited from a deep level to the conduction band and then contribute to photocurrent). The 
light having an energy lower than the band-gap energy can be converted into electrical power. (Example: 
photoconductors composed of CdS, PbS, An-doped Ge, Ga, Sb, etc., in the wavelength range over 5 µm 
but quite low responsivity and operating speed). The photoconductor detects light by monitoring the 
change in resistivity of the semiconductor. 
The commonly used photodetector is an intrinsic type and detects light at wavelengths close to and 
shorter then that corresponding to the band-gap energy of the semiconductor. 
the quantum efficiency, η
ph
, of a photodiode is defined by:
number of electron-hole pairs contributing to photo-induced current
number of incident photons
ph
η =
100
photo
ph
inc
I
q
P h
η
ν


=
×




(49) 
where
I
photo
- the photoinduced current,
P
inc
- the incident optical power. 
Quite often instead of the quantum efficiency, used the responsivity, S(A/W) 

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(
)
1.24
photo
ph
ph
inc
I
q
S
P
h
η
η λ µ
ν
=
=
=
m
(50) 
The quantum efficiency is influenced by the reflection of incident light at the surface of the photodiode, 
the recombination of photo-induced carriers at the surface and in the depletion layer, and the optical 
absorption outside of the depletion layer. Important to know that about 30% of the incident light is 
reflected at the surface because the refractive index of the semiconductor material is about 3.5. For this 
reason an antireflective dielectric film is deposited onto the surface to suppress the reflection, which can 
be reduced to less than 1%. 

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