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

132 CHAPTER 5
Figure 5.17 Control signal shape for modulated photogates (8 V, 20 MHz).
It is significant that, for the BCCD realization, the difference of the PGM-voltage and
the voltage amplitude of the modulated photogates does not have a great influence
on the demodulation contrast. For a good contrast, above all the voltage of the
middle photogate is important. As illustrated in Figure 5.18, this voltage, (to be
more precise, the voltage difference between PGM and the zero-biased modulated
photogate, PGL or PGR), determines the position of the potential minimum in the
horizontal direction and hence the junction, which decides whether the
photoelectrons travel to the left or to the right. The larger this potential difference,
the more this potential minimum moves to the borders of the light sensitive area
and the larger is the resulting shutter efficiency and demodulation contrast. This
effect is most significant for the buried channel structure since the potential shapes
in the buried channel (away from the surface) are smoother than directly at the
surface, but it can also be observed for the surface channel device.
In contrast to the BCCD, where the depletion width depends on the biasing of the
buried channel, in SCCDs the voltage amplitude of the CCD gates also influences
the depletion width. Since a deep depletion width results in a faster capturing
process of the photoelectrons, we observe an increase in demodulation contrast for
increasing modulation amplitudes. In addition, the size of the fringing fields grows
for larger potential differences between two adjacent gates. In BCCDs a relatively
small potential difference between the single CCD gates already leads to a
sufficient fringing field.