Source follower circuit for image sensor

Abstract: The present invention is directed to a solid state image sensing device which can reduce a consumed amount of an electric power. During a horizontal blanking period, a vertical scanning pulse of 5V is supplied to a vertical selection line (2A) from a vertical scanning circuit (3). In synchronism therewith, a voltage (I) is changed from 5V to 0V and a voltage (VGG) is changed from 0V to 5V. Thus, a source switch transistor (6) is turned off and a load transistor (9) is turned on, whereby a source follower circuit formed by a pixel transistor (12) and the load transistor (9) of a unit pixel (1) is set in the operable state to read an electrical signal accumulated in the unit pixel (1). According to the present invention, the load transistor (9) is turned on by the application of the voltage (VGG) only during the electrical signal read-out period and the dark signal read-out period of the unit pixel within the horizontal blanking period. Since a current is flowed to the pixel transistor (12) by a power supply (7) only during this period, a consumed electric power can be reduced considerably as compared with the prior art.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a solid state image sensing device capable of reducing a power dissipation. Description of the Prior Art: In a floating well amplifier (FWA) type solid state image sensing device which per se is well-known (see JP-A-3 278 574), when holes (positive holes) obtained by photo-electric conversion of a photosensitive element, e.g., photo-diode are accumulated in a P-type well of an N-channel MOS (metal oxide semiconductor) transistor (pixel transistor) and this pixel transistor is turned on, a surface potential of the P-type well is fluctuated by the accumulated holes and a surface channel current is changed. The changed amount of the channel current is read as an output signal of each of the pixels (see FIG. 5). In this solid state image sensing device, a drain electrode of a load transistor such as an N-channel MOS transistor or the like is connected to the source electrode of the pixel transistor to form a source follower circuit. Thus, when the load transistor is turned on, a channel current flows to the P-type well of the pixel transistor in accordance with the source follower circuit. In the above FWA solid state image sensing device, in order to prevent a blooming from taking place when signal charges accumulated in the P-type well overflow into adjacent pixels, a gate voltage of the pixel transistor is constantly set to about 5V and the pixel transistor is set in its on state. Since the above load transistor is constantly set in its on state, a current flows constantly to the source follower circuit formed by the pixel transistor and the load transistor. Since a current constantly flows to the source follower circuit as described above, the above FWA solid state image sensing device consumes a large amount of an electric power. Document JP-A-3 162 183 describes a photoelectric converting apparatus in which output amplifiers are made inoperative, thereby reducing electric power consumption in the case where a sensor portion is not driven. Two MOS transistors connected to emitters of Darlington type bipolar transistors of the output amplifiers are provided to control the operations of these amplifiers. When a signal is being output, the two MOS transistors are made conductive by a pulse. When the photoelectric converting apparatus is in the operation standby mode, the MOS transistors are made non conductive. Therefore, electric power consumption only during the operation standby mode is suppressed and low electric power consumption can be realized only during standby mode. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved solid state image sensing device in which the aforesaid shortcomings and disadvantages of the prior art can be eliminated. More specifically, it is an object of the present invention to provide a sold state image sensing device of the FWA type which can reduce a consumed electric power. To solve this object the present invention provides a solid state image sensing device as specified in claim 1. Preferred embodiments of the invention are described in the subclaims. A solid state image sensing device comprises a plurality of pixels, each being formed of a photosensitive element and a pixel transistor, being arrayed in horizontal and vertical directions in a two-dimensional fashion, a gate electrode and a source electrode of the each pixel transistor being connected to a vertical selection line and a vertical signal output line, respectively, and a load transistor, a drain electrode of the load transistor being connected to the source electrode of the pixel transistor to thereby form a source follower circuit, wherein the load transistor is turned on only during a horizontal blanking period. In accordance with an embodiment of the present invention, the load transistor is turned on only during the pixel signal read-out periods within the horizontal blanking period. The above and other objects, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof to be read in conjunction with the accompanying drawings, in which like reference numerals are used to identify the same or similar parts in the several views. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a structure of a solid state image sensing device according to an embodiment of the present invention; FIG. 2 is a circuit diagram showing an equivalent circuit of the solid state image sensing device according to the present invention; FIG. 3 is a cross-sectional diagram showing a structure of a unit pixel used in the present invention; FIGS. 4A and 4B are respectively diagrams used to explain a potential distribution of a gate section in the unit pixel; and FIGS. 5A through 5H are diagrams used to explain timings of control signals and output signals, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will hereinafter be described with reference to the drawings. FIG. 1 of the accompanying drawings shows a structure of a solid state image sensing device according to an embodiment of the present invention. The solid state image sensing device according to the present invention will hereinafter be described also with reference to FIG. 2 of the accompanying drawings which shows an equivalent circuit of the solid state image sensing device shown in FIG. 1. As shown in FIG. 1, a plurality of unit pixels 1 are arrayed in the horizontal and vertical directions in a two-dimensional fashion. - Each of the unit pixels 1 comprises a photosensitive element 11 such as a photo-diode or the like and a pixel transistor 12 such as an MOS transistor or the like. The photosensitive element 11 is connected to a channel region of the pixel transistor 12 as will be described later on. An electrical signal generated when the photosensitive element 11 is energized is accumulated in a P-type well 33 (see FIG. 3) of the pixel transistor 12. A gate electrode 21 of the pixel transistor 12 is connected to vertical selection lines 2A, 2B, ... (only two selection lines 2A, 2B are shown in FIG. 1). The vertical selection lines 2A, 2B, ... are connected to a vertical scanning circuit 3. The vertical selection lines 2A, 2B, ... are sequentially scanned by vertical scanning pulses supplied thereto from the vertical scanning circuit 3. A source electrode 22 of the pixel transistor 12 is connected to a vertical signal output line 4. The other side (upper end side in FIG. 1) of the vertical signal output line 4 is connected through a source switch transistor 6 to a power supply 7. A voltage I is applied to a gate electrode 61 of the source switch transistor 6 at a predetermined timing, thereby the source switch transistor 6 being turned on. A drain electrode 23 of the pixel transistor 12 is also connected to the power supply 7 by way of a power supply line 8. To the source switch transistor 6 side on the vertical signal output line 4, there is connected a load transistor 9, i.e., a drain electrode 91 of an MOS transistor in this embodiment. As shown in FIG. 2, the load transistor 9 and the pixel transistor 12 constitute a source follower circuit. The other side (under side in FIG. 1) of the vertical signal output line 4 is connected to a sample and hold (S/H) circuit 10. Electrical signals of all unit pixels 1 connected to one vertical selection line 2A (2B, 2C, ...) are held in the sample and hold circuit 10. MOS switches 13 connected to the sample and hold circuits 10 are sequentially turned on by horizontal scanning pulses transmitted from a horizontal scanning circuit 14, whereby the electrical signals held in the sample and hold circuits 10 are sequentially output. When the above electrical signal output operation is sequentially effected on all the vertical selection lines 2A, 2B, ..., an image signal of this solid state image sensing device can be obtained. The electrical signal generated in the unit pixel 1 is read once per upper and lower two vertical selection lines 2A, 2B during the horizontal blanking period. These electrical signals are synthesized by the sample and hold circuit 10, thereby producing an electrical signal of one line. FIG. 3 of the accompanying drawings shows a cross section of the unit pixel 1. As shown in FIG. 3, a P-type silicon substrate 31 has formed on its surface an N-type well 32 on which the P-type well 33 is formed. On the surface of the P-type well 33, there is formed a gate electrode 21 through an insulating member (not shown). The gate electrode 21 is connected to the vertical selection lines 2 (FIG. 1) as described above. An N + source region and an N + drain region are formed as a proper spacing at both sides of the gate electrode 21 over the P-type well 33. A source electrode 22 and a drain electrode 23 are respectively connected to the N + source and N + drain regions. The source electrode 22 is connected to the vertical signal output line 4 and the drain electrode 23 is connected to the power supply line 8. Holes (positive holes) 40 generated by the photo-electric conversion are accumulated in the P-type well 33 and an accumulated state shown in FIG. 4A is presented. When the vertical scanning pulse is transmitted from the vertical scanning circuit 3 under this accumulated state, the pixel transistor 12 is turned on. At that time, as shown in FIG. 4B, a channel current 41 flowing into the pixel transistor 12 is fluctuated by the holes 40, thereby changing a source potential of the source follower circuit formed by the unit pixel 1 and the load transistor 9. A changed amount of the source potential at that time becomes an output signal of the unit pixel 1. FIGS. 5A through 5H are respectively diagrams showing operation timings of the above control signals and the output signal of the unit pixel 1. According to this embodiment, during a horizontal effective scanning period (horizontal scanning period - horizontal blanking period), the voltage I applied to the gate electrode 61 of the source switch 6 is set to 5V and a voltage VGG applied to the gate electrode 92 of the load transistor 9 is set to 0V. As will be described next, the voltage I and the voltage VGG are changed at a predetermined timing during the horizontal blanking period, whereby the operation state of the source switch 6 or the load transistor 9 is changed to read an electrical signal. During the horizontal blanking period, of the upper and lower two vertical selection lines, for example, 2A and 2B, a vertical selection pulse of 5V (Volts)is applied to the upper vertical selection line 2A from the vertical scanning circuit 3. In synchronism therewith, the voltage I which is applied to the gate electrode 61 of the source switch 6 (FIG. 2) is changed from 5V to 0V and the voltage VGG which is applied to the drain electrode 91 of the load transistor 9 is changed from 0V to 5V. Therefore, the source switch 6 is turned off and the load transistor 9 is turned on so that the electrical signal of the pixel unit 1 applied to the vertical selection line 2A is read out and stored in the sample and hold circuit 10. In synchronism with the application of 10V to the vertical selection line 2A, the voltage I is changed to 5V and the voltage VGG is changed to 0V. By this reset operation, the holes 40 accumulated in the P well 33 in the unit pixel 1 are discharged to the P-type silicon substrate 31 side. When a light whose intensity is higher than a certain value becomes incident, the holes 40 exceed the accumulation capacity of the P well 33 and then overflow through the N well 32 into the P-type silicon substrate 31, thereby restraining a blooming. Subsequently, the voltage 5V is applied to the vertical selection line 2A. In synchronism therewith the voltage I is changed to 0V and the voltage VGG is changed to 5V. Thus, a dark signal accumulated in the unit pixel 1 is read out. By subtracting this dark signal from the electrical signal read out before, it is possible to obtain an accurate signal component. After the read-out of the upper vertical selection line 2A is ended, the read-out of the lower vertical selection line 2B is effected and both electrical signals are synthesized as described above. While one vertical selection line 2A (2B) is operated, the voltage of 0V is applied to the other vertical selection line 2B (2A). Further, while either of the two vertical selection lines 2A, 2B is operated, the voltage of 0V is applied other vertical selection lines 2C, 2D, ... The above read-out operation is sequentially effected on all the vertical selection lines 2A, 2B, ..., thereby producing the image signal of all the pixels. In this solid state image sensing device, only when the electrical signal and the dark signal accumulated in the unit pixel 1 are read out, the voltage VGG applied to the load transistor 9 becomes 5V and is held at 0V in other cases. Since the read-out period of the electrical signal and the dark signal is extremely short, the channel current flows into the load transistor 9 in an extremely short period of time. Therefore, the consumed amount of the electric power can be considerably reduced as compared with the case that the load transistor 9 is turned on even during the horizontal effective scanning period (see FIG. 5H). While the load transistor 9 is turned on only during the read-out period according to the above embodiment of the present invention, in accordance with a second embodiment of the present invention, the voltage VGG is set to 5V over the whole horizontal blanking period. During other periods, the voltage VGG is set to 0V, thereby controlling the load transistor 9. According to the second embodiment of the present invention, even when the driving speed of the voltage VGG is slow, a ringing at a leading edge or trailing edge of the waveform of the voltage VGG is not produced during the read-out period of the electrical signal accumulated in the unit pixel 1. Therefore, a stable output can be obtained. In that case, even during the reset period, the load transistor 9 is turned on, consuming electric power uselessly. However, this reset period is very short period of time as compared with the horizontal scanning period so that the electric power consumed during this reset period is very small. Hence, there occurs no problem. As described above, according to the first embodiment of the present invention, the load transistor that forms the source follower circuit together with the pixel transistor in the unit pixel is turned on in synchronism with the read-out timing of the electrical signal in the unit pixel. Therefore, according to the first embodiment of the present invention, since the operation time of the load transistor is considerably short, the electric power amount generated by the channel current flowing to the load transistor can be reduced considerably. Further, according to the second embodiment of the present invention, the load transistor is turned on substantially over the entire period of the horizontal blanking period and is turned off during other periods. Therefore, according to the second embodiment of the present invention, even when the driving speed of the power supply that turns the load transistor on is low, the ringing at the leading edge or trailing edge of the voltage waveform can be prevented from being produced during the read-out period of the electrical signal accumulated in the unit pixel, thereby making it possible to obtain the stable output signal. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications could be effected therein by one skilled in the art.

Claims: A solid state image sensing device comprising: (a) a plurality of pixels, each being formed of a photosensitive element (11) and a pixel transistor (12), being provided in horizontal lines and vertical columns of an array in a two-dimensional fashion, a gate electrode and a source electrode of said each pixel transistor (12) being connected to a vertical selection line (2A, 2B, ...) and a vertical signal output line (4), respectively, and (b) a plurality of load transistors (9) equal in number to the number of columns of the array, wherein each load transistor (9) is connected to the source electrode of every pixel transistor (12) in the column direction to thereby form source follower circuits, wherein said load transistors (9) are turned on only during the horizontal blanking period, respectively. The solid state image sensing device according to claim 1, wherein said load transistor (9) is turned on only during the pixel signal read-out periods within the horizontal blanking period. The solid state image sensing device according to claim 1 or 2, wherein said load transistor (9) is turned on only during the pixel signal read-out periods and the dark signal read-out periods within said horizontal blanking period. The solid state image sensing device according to one of the claims 1 to 3, further comprising a power supply (7) and a source switch transistor (6) whose main current path is connected in series between said power supply (7) and said vertical signal output line (4), wherein said source switch transistor (6) is turned off when said load transistor (9) is turned on. The solid state image sensing device according to one of the claims 1 to 4, wherein said pixel transistor comprises a first well (32) of a second conductivity type (N-type) formed on a semiconductor substrate (31) of a first conductivity type (P-type), a second well (33) of the first conductivity type (P-type) formed on said first well (32), source and drain regions formed within said second well, a channel region formed between said source and drain regions, a source electrode (22) connected to said source region, a drain electrode (23) connected to said drain region, and a gate electrode (21) formed through an insulating layer on said channel region.

Inventors: KAZUYA YONEMOTO, KAZUYA, YONEMOTO, Kazuya, Yonemoto, c/o Sony Corporation
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: H04N3
IPC codes: H04N3, H04N5