Active pixel sensor with shared readout structure

An active pixel sensor with a shared readout structure controls the switching of its transistors in a time-divided manner in conjunction with the appropriate switching of the voltage of a variable voltage source, whereby two pixels in a sensor can share a common readout structure and a selecting transistor commonly used in conventional art is not required. The present invention comprises: a first photodiode and a first NMOS transistor, wherein the anode and cathode of the first photodiode are coupled to a ground and the source of the first NMOS transistor, respectively, and a first selecting signal is coupled to the gate of the first NMOS transistor; a second photodiode and a second NMOS transistor, wherein the anode and cathode of the second photodiode are coupled to the ground and the source of the second NMOS transistor, respectively, and a second selecting signal is coupled to the gate of the second NMOS transistor; and a third NMOS transistor and a fourth NMOS transistor, wherein the drains of the first and second NMOS transistors are coupled to the source of the third NMOS transistor and the gate of the fourth NMOS transistor, and a reset signal is coupled to the gate of the third NMOS transistor, and the drains of the third and fourth NMOS transistors are coupled to a variable voltage source.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates in general to an active pixel sensor. More 
specifically, it relates to an active pixel sensor with a shared readout 
structure. 
2. Description of the Related Art 
Charge coupled devices are well-known image sensing devices utilized in 
various applications. In addition to charge coupled devices, active pixel 
sensors are also applied as image sensors. Active pixel sensors work by 
using photodiodes in conjunction with NMOS transistors (fabricated by the 
standard CMOS process) to sense images (light intensity). 
FIG. 1 illustrates a circuit diagram of an active pixel sensor. In FIG. 1, 
the drain and source of a NMOS transistor T1 are coupled to a constant 
voltage source VB and the cathode of a photodiode Dp, respectively, and 
the anode of the photodiode Dp is coupled to a ground. The drain and 
source of a NMOS transistor T2 are coupled to the constant voltage source 
VB and the drain of a NMOS transistor T3, respectively, and the gate of 
the NMOS transistor T2 is coupled to the cathode of the photodiode Dp. 
The active pixel sensor depicted in FIG. 1 utilizes the photodiode Dp to 
sense light intensity (images) and transforms it into electric signals, 
and then the electric signal is outputted from the source of the NMOS 
transistor T3. The operation of the active pixel sensor is described in 
detail as follows in reference to the timing chart of FIG. 2. For brevity, 
the waveforms in FIG. 2 are not plotted in actual voltage amplitudes and 
time lengths. 
In the horizontal blanking interval, the NMOS transistor T3 is turned on by 
a selecting signal SL. In Co time interval (1), the NMOS transistor T1 is 
not turned on, and the voltage (VIN) at node A is amplified by the NMOS 
transistors T2 and T3, thereby obtaining a voltage V1 from terminal 
read.sub.-- out (the source of the NMOS transistor T3). In time interval 
(2), the NMOS transistor T1 is turned on by a reset signal RST, and a 
light-induced current generated by the photodiode Dp flows from the 
constant voltage source VB through the NMOS transistor T1 and the 
photodiode Dp to the ground, thereby charging the voltage at the node A to 
VIN'. In time interval (3), the NMOS transistor T1 is turned off, and 
voltage VIN' at node A is amplified by the NMOS transistors T2 and T3, 
thereby obtaining a voltage V2 from terminal read.sub.-- out. The 
difference between the voltages V2 and V1 corresponds the light intensity 
sensed by the photodiode Dp. 
In general, an image-sensing device is implemented by using a plurality of 
active pixel sensors arranged in a two-dimension array of columns and 
rows. The IC layout of the image sensing device is schematically depicted 
in FIG. 3, wherein the regions enclosed by dashed-lines represent the 
active pixel sensors. For brevity, only a part of the conductive lines are 
shown in this figure, such as the selecting signal line SL, reset signal 
line RST, voltage source line VB, and output line read.sub.-- out. Based 
on the CMOS process, in every active pixel sensor, the selecting signal 
line SL and the reset signal line RST are fabricated by polysilicon lines, 
and the voltage source line VB and output line read.sub.-- out are 
fabricated by metal lines. Referring to FIG. 1 and FIG. 3, the voltage 
source line VB and output line read.sub.-- out are connected to the 
drain/source regions (n-type diffusion regions), and the source of the 
NMOS transistor T1 is connected to the gate (polysilicon gate) of the NMOS 
transistor T2. Consequently, fabricating an active pixel sensor requires 
three contact regions, three NMOS transistors, and a photodiode. 
In view of the active pixel sensor described, the light intensity sensed by 
every photodiode is outputted via a readout circuit formed by transistors 
T1, T2, and T3. In an image sensing device, if two photodiodes disposed at 
two adjacent active pixel sensors can share the same readout circuit, then 
the chip (circuit) area required for fabricating the image sensing device 
and the process complexity can be reduced. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide an active pixel 
sensor with a shared readout structure allowing two photodiodes disposed 
at two adjacent active pixel sensors to share the same readout circuit. 
The present invention controls the switching of the transistors in a 
time-divided manner in conjunction with the appropriate switching of the 
voltage of a variable voltage source, whereby two pixels in a sensor can 
share a common readout structure and a selecting transistor commonly used 
in conventional art is not required. 
The present invention achieves the above-indicated objects by providing an 
active pixel sensor with a shared readout structure which comprises: a 
first photodiode and a first NMOS transistor, wherein the anode and 
cathode of the first photodiode are coupled to a ground and the source of 
the first NMOS transistor, respectively, and a first selecting signal is 
coupled to the gate of the first NMOS transistor; a second photodiode and 
a second NMOS transistor, wherein the anode and cathode of the second 
photodiode are coupled to the ground and the source of the second NMOS 
transistor, respectively, and a second selecting signal is coupled to the 
gate of the second NMOS transistor; and a third NMOS transistor and a 
fourth NMOS transistor, wherein the drains of the first and second NMOS 
transistors are coupled to the source of the third NMOS transistor and the 
gate of the fourth NMOS transistor, and a reset signal is coupled to the 
gate of the third NMOS transistor, and the drains of the third and fourth 
NMOS transistors are coupled to a variable voltage source. 
The variable voltage source mentioned above is switched between a high 
voltage and a low voltage, and the operation of the active pixel sensor 
with a shared readout structure is described as follows. 
When the variable voltage is switched to the high voltage and the first (or 
second) NMOS transistor is turned on by the first (or second) signal, a 
first response voltage in response to a first voltage at the gate of the 
fourth NMOS transistor is outputted by the fourth transistor. 
When the third NMOS transistor is turned on by the third signal, a second 
response voltage in response to a second voltage at the gate of the fourth 
NMOS transistor is outputted by the fourth NMOS transistor, and the 
difference between the first and second response voltages corresponds to 
the light intensity sensed by the first (or second) photodiode. 
When the variable voltage is switched to the low voltage and the first (or 
second) NMOS transistor is not turned on by the first (or second) signal 
and the third transistor is turned on by the third signal, the voltage at 
the gate of the fourth NMOS transistor is reset to the first voltage. 
Then above operations are carried out again to read the light intensity 
sensed by the other photodiode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 4, the circuit connection of an active pixel sensor with 
a shared readout structure is described as follows. 
The anode and cathode of a first photodiode D1 are coupled to a ground and 
the source of a first NMOS transistor M1, respectively, wherein a first 
selecting signal S1 is coupled to the gate of the NMOS first transistor 
M1. 
The anode and cathode of a second photodiode D2 are coupled to a ground and 
the source of a second NMOS transistor M2, respectively, wherein a second 
selecting signal S2 is coupled to the gate of the second NOMS transistor 
M2. 
The source of a third NMOS transistor M3 is coupled to the drains of the 
first and second NMOS transistors (M1, M2) and the gate of a fourth NMOS 
transistor M4. The drains of the third and fourth NMOS transistors (M3, 
M4) are coupled to a variable voltage source VC, and a reset signal RST is 
coupled to the gate of the third NMOS transistor M3. 
The images (light intensity) sensed by the first and second photodiodes 
(D1, D2) are outputted from the terminal (read.sub.-- out) at the source 
of the fourth NMOS transistor M4 by appropriate switching of the first and 
second signal (S1, S2) and the reset signal RST in conjunction with the 
switching of the voltage of the variable voltage source VC. The variable 
voltage source VC is switched between a high voltage (3 V) and a low 
voltage (0 V). 
FIG. 5 schematically illustrates an image sensing device comprising the 
active pixel sensors according to the present invention, wherein for 
brevity, only the photodiodes (D1, D2) arranged in a two-dimensional array 
of rows and columns are depicted. When the image sensing device outputs 
data, the image data (light intensity) sensed by photodiodes is read out 
along the horizontal direction, from the first row to the last (n-th) row. 
Along the vertical direction, the photodiodes (D1, D2) at two adjacent 
rows are conjoined into a single pixel sensor unit. Therefore, every two 
adjacent photodiodes (D1, D2) share the same readout structure, under 
appropriate switching the first and second signal (S1, S2) and the reset 
signal RST in a time-divided manner in conjunction with switching the 
voltage of the variable voltage source VC. 
The operation of the active pixel sensor with a shared readout structure is 
described in detail as follows in reference with the timing chart of FIG. 
6. For brevity, the waveforms in FIG. 6 are not plotted in actual voltage 
amplitudes and time lengths. 
In time interval (a), the variable voltage is switched to a high voltage (3 
V), and the first transistor M1 is turned on by the first selecting signal 
S1 (5V), and the voltage V.sub.R at node B is outputted from the 
transistor M4, thereby obtaining a first response voltage Vout1 at 
terminal read.sub.-- out. 
In time interval (b), the third transistor M3 is turned on by the reset 
signal RST (5 V), and the light-induced current is generated by the 
photodiode D1, which flows form the variable voltage source VC through the 
third and first transistors (M3, M1) and the photodiode D1 to the ground. 
The variable voltage VC (3 V) charges the voltage V.sub.R at node B. 
In time interval (c), the third transistor M3 is turned off by the reset 
signal RST (0 V), and the variable voltage source stops charging the 
voltage V.sub.R at node B. The voltage V.sub.R at node B is outputted from 
the transistor M4, thereby obtaining a second response voltage Vout2 at 
terminal read.sub.-- out. The difference between the second response 
voltage Vout2 and the first response voltage Vout1, that is (Vout2-Vout1), 
represents the light intensity sensed by the first photodiode D1. Then, 
the variable voltage source VC is switched to a low voltage (0 V), and the 
first transistor M1 is turned off by the first selecting signal S1 (0 V). 
In time interval (d), the third transistor M3 is turned on by the reset 
signal RST (5 V) so that the voltage V.sub.R at node B is discharged to 
the voltage level of the variable voltage source VC (0 V), thereby 
resetting the voltage V.sub.R at node B and completing the reading 
operation to the photodiode D1. 
Similarly, the light intensity sensed by the other photodiode (the second 
photodiode D2) is read out by carrying out the above operation in the case 
of replacing the first transistor M1 and first selecting signal S1 with 
the second transistor M2 and the second selecting signal S2, respectively. 
The first and second selecting signals are not switched to 5 V at the same 
time so that the first and second transistors will not be turned on at the 
same time. 
In view of the above descriptions, to fabricate an active pixel sensor with 
two sensing pixels requires four NMOS transistors, three contact regions, 
and two photodiodes. To fabricate a conventional active pixel sensor with 
a single sensing pixel requires three NMOS transistors, three contact 
regions, and one photodiode. 
Therefore, when fabricating an image sensing device with 640.times.480 
pixel resolutions, 640.times.480.times.3 NMOS transistors, 
640.times.480.times.3 contact regions, and 640.times.480 photodiodes are 
required by using the conventional active pixel sensor. However, when 
fabricating an image sensing device with 640.times.480 pixel resolutions 
by using the active pixel sensor with a shared structure, only 
640.times.480.times.2 NMOS transistors, 640.times.480.times.1.5 contact 
regions, and 640.times.480 photodiodes are required. It is obvious that 
fabricating an image sensing device with by using the present invention 
can significantly reduce the required number of NMOS transistors and 
contact regions. Consequently, the chip area needed for fabricating the IC 
is reduced, and the throughput is enhanced. Moreover, the reliability of 
operation is also improved, because of the simplified circuit structure 
and control. 
While the invention has been described by way of example and in terms of 
the preferred embodiment, it is to be understood that the invention is not 
limited to the disclosed embodiments. On the contrary, it is intended to 
cover various modifications and similar arrangements as would be apparent 
to those skilled in the art. Therefore, the scope of the appended claims 
should be accorded the broadest interpretation so as to encompass all such 
modifications and similar arrangements.