PI0512WS Page 3 of 8 – July 26, 2001

During normal operation, the impinging photons in or near the PN photodiode junction

generate free charges that are collected and stored on the junction's depletion capacitance.

Since on average a certain fraction of charge is generated and collected for each impinging

photon, the number of collected charges will proportionally increase with light exposures.

Figure 3 shows the stored signal charge as function of light exposure at a wavelength of

575 nm. The exposure is the product of the light intensity in nW/cm

seconds. The charge accumulates linearly until reaching the saturation charge where the

corresponding exposure is the saturation exposure.

0

10

20

30

40

50

60

70

0

50

100

150

200

Exposure (nJ/cm^2)

Saturation

Charge

Saturation

Exposure

Figure 3. Stored signal charge as function of exposure at a wavelength

of 575 nm.

The responsivity is defined as ratio of saturation charge divided by saturation exposure. The

measured typical responsivity of a photodiode is 3.3

the predicted responsivity of the photodiodes as a function of wavelength.

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

3.0E-04

3.5E-04

4.0E-04

4.5E-04

5.0E-04

100

300

500

700

900

Wavelength (nm)

QE=20%

QE=40%

QE=60%

QE=80%

Figure 4. Predicted spectral response.