Image Processing Reference
In-Depth Information
Complete transfer
100e
100e
Attention
100e
No charge
remains after transfer
Complete transfer = No increase or decrease of charge quantity
=
No new noise
No remaining charge = complete depletion
C
=1/(∂
V
/∂
Q
): No existence
kTC: No existence
FIGURE 5.7
The significance of complete charge transfer, CCDs' most important feature.
transferred from one MOS channel to another directly in CCDs, while in BBDs they are
transferred by way of impurity regions that lie under and between MOS gates and are not
depleted. This is related to the technology level at the time they were proposed, the essen-
tial difference of the presence of a nondepleted area in BBDs brings about kTC noise and
a limit to transfer efficiency.
5.1.1.1 Interline Transfer CCDs
Among image sensors, the only winner for a few decades, since it started to be produced in
the mid-1980s in Japan, was the interline transfer CCD (IT-CCD). The first CCD image sen-
sor proposed was the frame transfer CCD (FT-CCD),
4
which has a simpler pixel structure
than the IT-CCD. Although the IT-CCD was developed
5
2 years later than the FT-CCD,
it is
explained in detail at first regarding the significant role it played.
Figure 5.8 is a schematic diagram of the most common composition of an IT-CCD, hav-
ing three vertical and three horizontal pixels (3V × 3H).
Optical images are focused on an image area in which pixels are arrayed in a matrix
in a plane. The sensor part, consisting of a PD, especially a pinned PD, is equipped in
each pixel, as shown in the top right of Figure 5.8. It generates and integrates signal
charges by absorbing incident light arriving at the pixel as a portion of the optical image.
Each pixel is composed of a sensor part, a transfer electrode to read out signal charges
integrated in the sensor part, and a vertical scanner (vertical CCD [VCCD]) to transfer
signal charges to vertical direction. Signal charge packets read out to VCCD are trans-
ferred through a VCCD to a horizontal scanner (horizontal CCD [HCCD]). Each charge
packet arriving at the HCCD is transferred through it to the charge quantity measure-
ment part, consisting of a FDA, and be converted to signal voltage. Thus, scanners of
two directions such as vertical and horizontal are necessary in two-dimensional image
sensors. A sensor part, transfer electrode, and VCCD are arranged in a narrow area
of a pixel. To get higher sensitivity, a larger PD is desired, based on a narrower VCCD
area. Therefore, four-phase CCDs, which can treat a larger charge quantity with a nar-
rower area, are preferable for VCCDs, as shown in the middle right of Figure 5.8. Signal
charge packets of one line transferred to a HCCD are transferred to a FDA generally at
frequencies two or three orders higher than in VCCDs. In contrast to VCCDs, HCCDs
have few design limitations regarding width at variance, so it is easy for HCCDs to have
