Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1999-03-31
2004-12-14
Garber, Wendy R. (Department: 2712)
Television
Camera, system and detail
Solid-state image sensor
C348S302000
Reexamination Certificate
active
06831691
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a solid-state image pickup device and, more particularly, to a solid-state image pickup device comprising pixels arranged in two dimensions.
BACKGROUND OF THE INVENTION
A two-dimensional solid-state image pickup device, where the pixels are arranged to form a matrix (with rows and columns), and in which each pixel includes a photoelectric converting element, such as a photodiode, and means for drawing out photoelectric charges generated in the photoelectric converting element onto an output signal line, have had a wide variety of applications. Such solid-state image pickup devices can be subdivided into a CCD type and a MOS type depending on the means for reading put (drawing out) the photoelectric charges generated in the photoelectric converting element. In a CCD image pickup device, the photoelectric charges are transferred while being accumulated in potential wells. In a MOS image pickup device, on the other hand, charges accumulated in the pn junctions capacitance of a photodiode are read out by means of a MOS transistor.
The structure of each pixel in a conventional MOS solid-state image pickup device will be described next in reference to FIG.
13
. In the drawing, PD denotes a photodiode and Tr
1
denotes an n-channel MOS transistor having a drain connected to the cathode of the photodiode PD, a source connected to an output signal line Lsig, and a gate connected to a column select line Lvsel. When light is incident upon the photodiode PD, charges proportional to the quantity of incident light are generated and accumulated (integrated) in the pn junction capacitance of the photodiode PD. At the completion of the integration process, a high voltage is applied to the column select line Lvsel to bring the transistor Tr
1
into the ON state. Upon the turning ON of the transistor Tr
1
, the charges accumulated in the pn junction capacitance of the photodiode PD are outputted to the output signal line Lsig to be read, out.
However, the aforesaid CCD solid-state image pickup device has the drawback of a narrow dynamic range because only the potential of the charges generated in the photodiode is accumulated and transferred. On the other hand, the conventional MOS solid-state image pickup device has several drawbacks including: an insufficiently wide dynamic range—albeit wider than the dynamic range of the CCD solid-state image pickup device—because the charges generated in proportion to the quantity of incident light are accumulated in the pn junction capacitance of the photodiode; a degraded S/N ratio (signal-to-noise characteristic), due to switching noise which results from the reading operation performed by switching the MOS transistor; and, differences in the time of integration between pixels because the reading operation is performed by switching the MOS transistor for each of the pixels sequentially. Specifically, when the respective MOS transistors for two photodiodes are sequentially activated, as shown in
FIG. 12
, the turning ON of a second MOS transistor (e.g., Tr
5
), which corresponds to a column select line that is subsequent to the column select line Lvsel connected to the transistor Tr
1
, the turning on of transistor Tr
5
lags behind the turning ON of the transistor Tr
1
. Although integration in a photodiode connected to the transistor Tr
5
is initiated simultaneously with integration in the photodiode PD connected to the transistor Tr
1
, provided that light is incident concurrently, the reading operation from the transistor Tr
1
leads the reading operation from the transistor Tr
5
due to the switching time difference, so that an integration time in the transistor Tr
1
is different from an integration time in the transistor Tr
5
.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a two-dimensional solid-state image pickup device with a sufficiently wide dynamic range. Another object of the present invention is to provide a two-dimensional solid-state image pickup device wherein all pixels can be integrated with the same timing.
These and other objects of the present invention are achieved by a two-dimensional solid-state image pickup device comprising pixels arranged to form a matrix, each of the pixels including: a photodiode; a first MOS transistor having a first electrode and a gate electrode each connected to one electrode of the photodiode, the first MOS transistor operating in a subthreshold region; a second MOS transistor having a gate electrode connected to the gate electrode of the first MOS transistor, the second MOS transistor operating in the subthreshold region; a capacitor having one terminal connected to a second electrode of the second MOS transistor and the other terminal connected to a fixed voltage, the capacitor being reset via the second MOS transistor when a reset voltage is supplied to a first electrode of the second MOS transistor; and a third MOS transistor for read operation having a first electrode connected to the one terminal of the capacitor, a second electrode connected to an output signal line, and a gate electrode connected to a column select line.
In this arrangement, a photoelectric current generated in the photodiode is logarithmically converted in the first MOS transistor and the gate voltage thereof becomes proportional to the current through logarithmic conversion. The voltage is used to charge the capacitor through the second MOS transistor. At the completion of integration, the third MOS transistor is turned ON and a voltage on the capacitor is led out onto the output signal line. Afterward, when the reset voltage is applied to the first electrode of the second MOS transistor, the voltage on the capacitor is reset to a reset value and integration in the capacitor is initiated again.
The objects of the present invention may also be achieved by a two-dimensional solid-state image pickup device comprising pixels arranged to form a matrix, each of the pixels including: a photodiode; a first MOS transistor having a first electrode and a gate electrode each connected to one electrode of the photodiode, the first MOS transistor operating in a subthreshold region; a second MOS transistor having a gate electrode connected to the gate electrode of the first MOS transistor and a first electrode connected to a direct-current voltage, the second MOS transistor operating in the subthreshold region; a capacitor having one terminal connected to a second electrode of the second MOS transistor and the other terminal connected to a fixed voltage, the capacitor integrating a signal based on photoelectric charges generated in the photodiode; a third MOS transistor for read operation having a first electrode connected to the one terminal of the capacitor, a second electrode connected to an output signal line, and a gate electrode connected to a column select line; and a fourth MOS transistor having a first electrode connected to the one terminal of the capacitor and a second electrode connected to a direct-current voltage, the fourth MOS transistor being turned ON in response to a reset pulse applied to a gate electrode thereof to reset the capacitor to an initial state.
In this arrangement, the integration in the capacitor and the reading of a voltage from the capacitor are performed in the same manner as in the case of the first embodiment; however, the resetting of the capacitor is performed by turning ON the fourth MOS transistor that is connected in parallel with the capacitor. Accordingly, the resetting of the capacitor is performed more accurately at a higher speed.
The objects of the present invention may also be achieved by a two-dimensional solid-state image pickup device comprising pixels arranged to form a matrix, each of the pixels including: a photodiode; a first MOS transistor having a first electrode and a gate electrode each connected to one electrode of the photodiode, the first MOS transistor operating in a subthreshold region; a second MOS transistor having a gate electrode connected to the gate electrode of the first
Hagihara Yoshio
Miyatake Shigehiro
Nakamura Satoshi
Takada Kenji
Garber Wendy R.
Minolta Co. , Ltd.
Sidley Austin Brown & Wood LLP
Wilson Jacqueline
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