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DEMODULATION PIXELS IN CMOS/CCD 145
5.2.9 Summary
The demodulation contrast depends on several parameters: amplitude of gate
voltages, demodulation frequency, received optical power density and wavelength
of modulated light. Since the CCD shutter mechanism used never has 100%
efficiency, even under DC conditions, the demodulation contrast is never better
than about 60%.
The contrast decreases with higher modulation frequencies, since the
photoelectrons do not have enough time to reach the storage site (dump site
respectively). The gate voltages PGL and PGR should be chosen to be as high as
possible. This is in order to generate high electrical fields in the semiconductor.
Even for a voltage difference of 10 V between two adjacent photogates, the
saturation velocity of electrons is not yet reached. Therefore, the experimentally
measured optimum gate voltages of about 8 V are reasonable: an increase in
electrical field still causes an increased drift velocity and hence faster response of
charge carriers to the electrical field. For modulation voltages higher than 8 V the
maximum signal that can be handled decreases, since the charge handling
capability of the pixel is essentially influenced by the potential difference between
the modulation gate PGL and the integration gate IG (10.7 V).
Furthermore, the demodulation contrast decreases with increasing wavelength.
This is because the light penetrates deeper into the semiconductor before
photoelectrons are generated. These photoelectrons then need more time to reach
the depletion region, where they are separated to the dump- or integration site,
depending on their arrival. Also photoelectrons generated far from the surface
diffuse in random directions, so that they are equally likely to travel directly to the
integration or dump site, independently of the actual biasing conditions of the
modulated photogates PGL and PGR.
For the BCCD realization of the 1-tap demodulation pixel we have measured a
contrast of 40% using 20 MHz (more or less) sinusoidally modulated light of 630 nm
wavelength. The SCCD performance is only slightly worse, so that the availability of
a buried channel process option is not necessarily needed. For lower modulation
frequencies and blue light the contrast is better than 60%, quite close to the
prediction of the simple model introduced in Figure 5.14. The contrast decrease
towards higher modulation frequencies can at least partly be attributed to the more