Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 15

Tabl e 1. 2 APD types and their properties

APD type Wavelength range Gain Dark current Application

Silicon 300-1,100 nm 50-1,000 Low Fluorescence, microscopy

Germanium 800-1,600 nm 10-40 High Environment monitoring

InGaAs 900-1,700 nm 10-40 Low Fluorescence

a

Back Thinned
Deep Depletion

Front Back

Thinned

Silicon
Dioxide

Incident
Light

Incident
Light

Incident
Light

Poly-silicon
gate

Silicon Depletion

Layer

Silicon Depletion

Layer

Silicon

Depletion
Layer

b

100

90

Back Illuminated

80

70

Back Illuminated
Deep Depletion

60

50

40

30

Front Illuminated

20

10

0

250 500 750 1000

Wavelength (nm)

Fig. 1.8 Cross section of the three types of CCD ( a ) and their respective quantum efficiencies
( b ) (Reproduced with permission from Princeton Instruments)

a polysilicon gate, which causes significant loss. The maximum quantum efficiency
of a front-illuminated CCD is around 50%. Front-illuminated CCD is used for
stronger light signal detection, such as reflectance, transmission, and absorption
measurement.
For back-illuminated CCDs, the light enters the detector directly from the back-
thinned substrate side. As there is no light loss, the back-illuminated CCD has
higher quantum efficiency, up to 90%. Such kind of CCD is particularly useful