Image Processing Reference
In-Depth Information
Row select MOSFET
Row select line
Vertical
signal line
Pixel
Light
Pixel
amplifier
Photodiode
Column select MOSFET
Horizontal signal line
Column select line
On-chip
output amplifier
Horizontal access circuit
FIGURE 5.38
Basic configuration of CMOS image sensor.
It is also important that CMOS sensors are manufactured based on the CMOS process.*
This means various functional circuitries constructed of CMOS transistors can be arranged
on a sensor chip. Additionally, CMOS sensors can share in the bounty of progress in CMOS
process technology. These features are the origin of evolvability of CMOS sensors.
From the viewpoint of the functional elements discussed in Figure 1.11, the path that sig-
nal charges follow at each pixel is in the order of (1) sensor part, (3) signal charge quantity
measuring part, and (2) scanning part, while in CCD and MOS sensors it is in the order of
(1), (2), and (3).
Image sensors with the capability to amplify function in each pixel are called
active pixel
sensors
(APS). Hence, image sensors without this pixel amplifying function, such as CCD
and MOS sensors, are called
passive pixel sensors
(PPS).
5.3.2 Pixel Technology of CMOS Sensors
Since CMOS sensors have an amplifier in each pixel, some elements are necessary in a
pixel. Examples of specific pixel configuration are shown below.
5.3.2.1 Three-Transistor Pixel Configuration
A basic pixel configuration is shown in Figure 5.39. Because of the three transistors in a
pixel, it is called a three-transistor (3-Tr) pixel configuration. The pixel corresponds to the
pixel in Figure 5.38. The
n
-type region of the PD is directly connected to the gate input of
the drive transistor of the SFA, and to voltage source
V
dd
by way of reset transistor RST.
This node has effective input capacitance
C
in
. The source of the drive transistor, which is
the output of the SFA, is connected to the TSL by way of row select transistor RS. The load
transistor is arranged as a common one to form the SFA with drive transistors belonging to
the vertical signal line at one end of the signal line. A vertical signal line is connected to a
number of vertical pixels that have output capacitance
C
sl
of some or some tens of picofar-
ads (pF). Row select transistor RS and reset transistor RST work as switching transistors,
while the drive transistor works as an analog transistor in the SFA. This configuration can
*
This does not mean that CMOS sensors manufactured by the low-cost vanilla CMOS process can compete
against CCDs on performance, as will be discussed later.