3D Time-of-flight distance measurement with custom - Universität ...

DEMODULATION PIXELS IN CMOS/CCD 129
To clarify the point: “A red-light-electron might diffuse directly into the integration
gate (IG) although the photogates (PGL/ PGR) are biased in ‘dump-direction’. The
blue-light electron, however, will be captured by the photogate’s electrical field
before diffusing to the IG and will then be forced to travel to the dump site.”
Therefore, we expect a better demodulation efficiency for short-wavelength-light
than for long-wavelength light. This expectation will be confirmed in Section 5.2.3.
potential
incident light
3/10 7/10
optical generation of
charge carriers
Figure 5.15 Simple model of shutter inefficiency (II), now considering a larger
penetration depth of the light.
In Figure 5.15 we have added these considerations to the previous shutter nonefficiency
model: A certain proportion of the optically generated charge carriers also
diffuses directly into the storage sites (and probably into gates even further away).
For the dimensions chosen in Figure 5.15 this corresponds to a demodulation
contrast of only (7/10-3/10) / (7/10+3/10) = 4/10 = 40%.
Additionally, photoelectrons generated far from the electrically active zone need
time to reach the demodulating electrical field. This temporal delay in arrival time
will lead to an additional wavelength-dependent decrease in demodulation contrast
for high frequencies.