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

74 CHAPTER 3
amount of the charge packet that remains after the transport losses δ resulting from
the transport from one pixel to the next. One pixel mostly consists of three or four
CCD gates (three- / four-phase CCDs), however, also two-phase or even onephase
CCDs have been reported (TEU).
( − δ)
CTE = 1
Equation 3.21
After transportation of the charge packet by N pixel positions, the following share of
the original charge remains:
( ) N
1−
N
CTE = δ
Equation 3.22
Typical CCD processes offer CTE values between 0.9999 and 0.99999. The
following table shows the influence of both CTE and number of charge transfers
(number of pixels) on the remaining charge packet.
CTE=0.99 CTE=0.999 CTE=0.9999 CTE=0.99999
N=10 90% 99% 99.9% 99.99%
N=100 37% 90% 99% 99.9%
N=1000 0% 37% 90% 99%
Remaining charge after N transfers for different CTE values.
The CCDs as we have described them so far are called surface channel CCDs
(SCCD). As can be seen in Figure 3.7 (b) the potential minimum in the vertical
direction for the classical MOS diode in deep depletion or strong inversion mode is
located directly at the junction between semiconductor and gate oxide. Since the
photoelectrons tend to stay at the potential minimum, the charge transport, which is
due to a superimposed potential difference in the horizontal direction, takes place at
the junction between semiconductor and oxide surfaces. Unfortunately, this junction
region is characterized by charge traps (caused by lattice mismatch, surface states
and impurities), which typically capture free electrons and randomly release them at
later times. This is one main technical limitation for a good CTE in SCCDs.