Abstract:
A solid-state imaging device is provided which has preferable linearity of signal outputs according to light intensities and does not cause dark defects even at a low light intensity. The solid-state imaging device comprises: a ring gate having a non-uniform width; a source region formed inside the ring gate; a drain region formed surrounding a circumference of the ring gate; and a carrier pocket formed under the ring gate, wherein a region where (X divided by Y) is the smallest substantially coincides with a region where Z is the shortest; X is a pocket-to-source distance; Y is a pocket-to-drain distance; and Z is a source-to-drain distance.

Description:
RELATED APPLICATIONS 
   This application claims priority to Japanese Patent Application No. 2003-395615 filed Nov. 26, 2003 which is hereby expressly incorporated by reference herein in its entirety. 
   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to a solid-state imaging device providing high image quality and low power consumption and, especially, a solid-state imaging device, having a ring-shaped gate, which can obtain signal outputs by modulating threshold voltages (Vth) in accordance with subject light. 
   2. Related Art 
   As solid-state imaging devices to be built in cellular phones, etc., there are two types of image sensors: charge-coupled device (CCD) image sensors; and CMOS image sensors. CCD image sensors are superior in image quality, while CMOS image sensors consume lower power and process cost. In recent years, proposals have been made for MOS imaging devices employing threshold voltage modulation which provides both high image quality and low power consumption. A type of MOS imaging device providing threshold voltage modulation is disclosed in, for example, Japanese Patent Serial No. 2513981. 
   The image sensor disclosed in Japanese Patent Serial No. 2513981 obtains image outputs by arranging sensor cells, which correspond to unit pixels, in a matrix and repeating a three-state cycle of initialization, storage and read-out. Each unit pixel of the image sensor has a photodiode for storage, a modulation transistor for read-out, and an overflow drain gate for initialization. The gate of the modulation transistor is ring-shaped. 
   A light-generated charge generated by incident light entering the photodiode is transferred to a P-well region provided under the ring gate and stored in a carrier pocket formed in the region. The light-generated charge stored in the carrier pocket changes the threshold voltage (Vth) of the modulation transistor, which makes it possible to obtain a pixel signal corresponding to the incident light from a terminal (source contact) coupled to the source region of the modulation transistor. 
     FIG. 8  is a schematic top-view drawing of a modulation transistor unit of an image sensor. 
   The image sensor shown in  FIG. 8  has a photodiode  110  and a modulation transistor  101  adjacent to each other for each unit pixel on a board. A gate  102  of the modulation transistor  101  is formed in a ring. In an opening at the center of the ring gate  102 , a source region  104  is formed. A gate contact is formed, though not illustrated, on part of the surface of the ring gate  102 . Provided around the ring gate  102  is a drain region  106 . A light-generated charge (carrier) generated by incident light entering the photodiode  110  is stored in a carrier pocket  108 , which is a ring-shaped narrow band formed in a P-well region provided under the ring gate  102 . When the stored charge changes the threshold voltage (Vth) of the modulation transistor  101 , a pixel signal corresponding to the incident light can be output from a source contact  105  coupled to the source region  104  of the modulation transistor  101 . By applying bias voltages of, for example, 5V and 0V to a drain contact  107  and the source contact  105 , respectively, electric current flows between the drain and the source in accordance with the intensity of incident light entering the photodiode  110 , and a signal output is output from the source contact  105 . 
   If a locally low-potential region  109  is formed at part of the carrier pocket  108  under the ring gate  102  due to, for example, an uneven impurity concentration within the well, carriers (holes) are not stored uniformly in the carrier pocket  108  which is ring-shaped, but stored first in the locally low-potential region  109  when light enters the photodiode  110 . If there is no locally low-potential region  109 , an electric current-flow path (hereinafter referred to as “current path”) from all directions of the region (drain region  106 ) outside the circumference of the ring gate  102  toward the source region  104  is formed, and electric current flows uniformly on this path. In the latter case, a signal starts being output linearly from the time the amount of stored electric charges is still small (see line g in  FIG. 9 ). Whereas, if there is the locally low-potential region  109 , electric current does not flow from all directions of the region (drain region  106 ) outside the circumference of the ring gate  102  toward the source region  104 , but starts flowing into the locally low-potential region  109 . Therefore, low light intensity results in the slow rising of signal outputs, worsening linearity (see line h in  FIG. 9 ), and thereby causing dark defects. 
   Similar problems are disclosed in, for example,  FIG. 15  and  FIG. 17  of Japanese Unexamined Patent Publication No. 10-65138. However, in Japanese Unexamined Patent Publication No. 10-65138, a countermeasure is taken by forming a current path in a specific region in accordance with impurity distribution instead of a configuration forming a gate in a ring shape. Also, examples of forming a ring-shaped gate are shown in Japanese Unexamined Patent Publication No. 2002-164527, as well as Japanese Patent Serial No. 2513981 (discussed above). 
   As described above, local non-uniformity of potential in the carrier pocket on the ring gate has been causing a problem of dark defects when the amount of stored electric charges is small, that is, when the light intensity is low. 
   Accordingly, the present invention has been developed in view of the above problem and aims to provide a solid-state imaging device which has preferable linearity of signal outputs according to light intensities and does not cause dark defects even at a low light intensity. 
   SUMMARY 
   The above and other objects are provided by a solid-state imaging device comprising: a ring gate having a non-uniform width; a source region formed inside the ring gate; a drain region formed surrounding a circumference of the ring gate; and a carrier pocket formed under the ring gate, wherein a region where (X divided by Y) is the smallest substantially coincides with a region where Z is the shortest; X is a pocket-to-source distance; Y is a pocket-to-drain distance; and Z is a source-to-drain distance. 
   According to the present invention, a region where X/Y is the smallest is a region where a voltage potential is the lowest in the intermediate points of the pocket width. Therefore, the region where X/Y is the smallest is a region where carriers (holes) are stored most easily. 
   Further, a region where Z is the shortest, that is, the gate length L of the ring gate is the shortest, is a region where transistor capacity is the highest, that is, an electric current flows most easily. Therefore, coincidence between a region where carriers (holes) are stored most easily and a region having the highest transistor capacity is found. In such a region, it is considered that carriers effectively contribute to the modulation of a drain-to-source current because an electric current flows most easily (performance is the highest) and the largest amount of carriers is stored. In other words, the present invention, which provides a configuration having an electric current-flowing path (current path) basically in all directions on the entire circumference of the ring gate, achieves a setting which makes the amount of electric currents flowing partially larger by changing the shape of the ring gate to form a region where potential is lowered. 
   In the present invention, it is preferable that the ring gate has an oval contour and that the source region is formed in a round shape at the center of the ring gate. 
   With this configuration, supposing, for example, that the carrier pocket is provided in a ring shape with a roughly constant width so that the carrier pocket lies almost along the circumference of the ring gate by forming the ring gate in an oval shape and the source region in a round shape at the center of the ring gate, it becomes possible to achieve coincidence, on the ring gate, between a region where potential is the lowest, that is, carriers (holes) are stored most easily, and a region where an electric current flows most easily (performance is the highest). 
   Further, in the present invention, it is preferable that the ring gate has a round contour and that the source region is formed in an oval shape at the center of the ring gate. 
   With this configuration, supposing, for example, that the carrier pocket is provided in a ring shape with a roughly constant width under the ring gate, and near the circumference in the major-axis direction of the oval source region and, further, near the circumference of the ring gate in the minor-axis direction of the oval source region by forming the ring gate in a round shape and the source region in an oval shape at the center of the ring gate, it becomes possible to achieve coincidence, on the ring gate, between a region where potential is the lowest, that is, carriers (holes) are stored most easily, and a region where an electric current flows most easily (performance is the highest). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic top-view drawing of a modulation transistor of a censor cell in a solid-state imaging device of a first embodiment of the present invention. 
       FIG. 2  is a schematic view of a conventional modulation transistor. 
       FIG. 3  is a graph for comparing surface potential between a modulation transistor according to the present invention and a conventional modulation transistor. 
       FIG. 4  is a schematic top-view drawing of a modulation transistor of a censor cell in a solid-state imaging device of a second embodiment of the present invention. 
       FIG. 5  is a view showing the layout of sensor cells in a solid-state imaging device. 
       FIG. 6  is a vertical sectional view showing a modulation transistor unit in each sensor cell in  FIG. 5 . 
       FIG. 7A  is a graph showing potential among the drain, carrier pocket and source ( FIG. 7B ) of the modulation transistor in  FIG. 5  and  FIG. 6 . 
       FIG. 8  is a schematic top-view drawing of a modulation transistor unit of an image sensor. 
       FIG. 9  is a graph showing the relation between light intensities and signal outputs in a sensor cell. 
   

   DETAILED DESCRIPTION 
   The present invention will now be described referring to the accompanying drawings. 
   As embodiments of the present invention, a MOS solid-state imaging device employing threshold voltage modulation is explained. As shown in  FIG. 5 , the solid-state imaging device has a configuration having sensor cells, which make pixels on a board, arranged in a matrix of rows (horizontal direction) and columns (vertical direction).  FIG. 5  shows an example of the ring gate  102  having an octagonal contour (perimeter shape). For each unit pixel on the board, a solid-state imaging device has a photodiode  110  generating light-generated charges (carriers) by using incident light and the modulation transistor  101  outputting pixel signals based on the light-generated charges (carriers) corresponding to the incident light. By the modulation transistor  101 , which is provided adjacent to the photodiode  110 , light-generated charges generated in the photodiode  110  are transferred to a P-well region  113  (see  FIG. 6 ) provided under the ring gate  102  and stored in the carrier pocket  108 , which is a ring-shaped narrow band formed in the region. Further, the light-generated charges stored in the carrier pocket change the threshold voltage Vth of the transistor, and then a pixel signal corresponding to the incident light is output from the source contact  105 , which is coupled to the source region  104 , via a source line  111 . The carrier pocket  108  is provided in a narrow band under the ring gate  102  (the area enclosed by the two dotted lines). A gate line, that is, a selection line  112  is provided for selecting a line (a horizontal row) where pixel signals are to be read from the source region  104  by supplying a bias voltage to the gate  102  via a gate contact  103 . In addition, the drain region  106  forms the drain of the modulation transistor  101 , and also has a function of isolating sensor cells. 
     FIG. 6  is a vertical cross-sectional drawing showing the modulation transistor  101  in each sensor cell in  FIG. 5 . Numbers assigned to each part correspond to those in  FIG. 5 . The carrier pocket  108  is provided with a narrow width in the P-well region  113  under the gate  102 , and the drain region  106  is formed surrounding a channel under the gate  102 , and the P-well region  113 . 
     FIG. 7A  is a graph showing potential among the drain, the carrier pocket and the source of the modulation transistor in  FIG. 5  and  FIG. 6 .  FIG. 7B  is a schematic cross-sectional drawing of the modulation transistor corresponding to the horizontal axis in  FIG. 7A . Potential becomes the lowest at the center of the carrier pocket  108 . 
   First Embodiment 
     FIG. 1  is a schematic top-view drawing of a modulation transistor in a solid-state imaging device of a first embodiment of the present invention.  FIG. 2  is a schematic view of a conventional modulation transistor (along line A–A′ shown in  FIG. 6 , the photodiode is not illustrated).  FIG. 3  is a graph for comparing surface potential between the modulation transistor according to the present invention and the conventional modulation transistor. For parts that are the same as those shown in  FIG. 5 , the same numbers are assigned for explanation. In  FIG. 1 , a modulation transistor  101 A according to the first embodiment of the present invention configures a sensor cell together with a photodiode (shown by a two-dot chain line  110 ) which receives incident light. A ring gate  102 A has an oval contour (perimeter shape). The source region  104  is formed in a round shape at the center of the ring gate  102 A. A carrier pocket  108 A is formed in a ring-shaped narrow band with a roughly constant width under the ring gate  102 A and almost along the circumference of the ring gate  102 A. The source contact  105  is located at the center of the round source region  104 . The drain region  106  is provided outside the circumference of the ring gate  102 A. In part of the drain region  106 , the drain contact  107  is located. In addition, the source region  104 , which is formed in a round shape at the center of the ring gate  102 A, is described assuming that the part corresponding to the entire area inside the round shape is the source region. 
   In this configuration, when the photodiode  110  starts receiving incident light, the light-generated charges generated in the photodiode  110  are stored in the carrier pocket  108 A. The stored charges (holes) are stored first in regions whose potential is the lowest. 
   In the carrier pocket  108 A provided in a ring-shaped narrow band, intermediate points c and c′ of the carrier pocket width in the minor-axis direction of the oval shape are located in the approximate middle of the gate length (channel length) L 1  in the minor-axis direction of the ring gate  102 A. Potential at these intermediate points is approximately half the drain-to-source voltage. Further, in the carrier pocket  108 A, intermediate points d and d′ of the carrier pocket width in the major-axis direction of the oval shape are located at approximately three quarters of the gate length (channel length) L 2  (with reference to the source circumference) in the major-axis direction of the ring gate  102 A. Potential at these intermediate points is approximately three quarters of the drain-to-source voltage. Potential is the lowest at c and c′, and the highest at d and d′. 
   Assuming that drain voltage Vd=3.3 V, source voltage Vs=1.5 V, pocket width=constant (Const), and L 1 /L 2 =½, the potential at point c (and point c′) is given by: (Vd+Vs)/2=2.4 V, and the potential at point d (and point d′) is given by: 3 (Vd+Vs)/4=2.85 V, as shown by the dotted line in  FIG. 3 . 
   Therefore, the configuration having the oval ring gate  102 A, with the round source region  104  in its center, and a carrier pocket formed as illustrated makes a potential difference of 0.45 V between point c and point d in the carrier pocket  108 A as shown in  FIG. 3 . At the same time, the variation of surface potential on the carrier pocket sandwiched between point c and point d makes a curve which continuously shifts from 2.4 V at point c to 2.85 V at point d, as shown by the dotted line in  FIG. 3 . 
   During modulation, potential of the carrier pocket in the major-axis direction (near points d and d′) becomes higher due to the splitting ratio of the drain-to-source voltage, which lets the light-generated charges (carriers) stored in the carrier pocket  108 A flow into and be stored in the carrier pocket in the minor-axis direction (near points c and c′) as shown by the solid-line arrow. Further, the region having the largest capacity for making an electric current flow from the drain region  106  into a source region  104 A is the channel in the minor-axis direction (near points c and c′), as described above. Therefore, the first embodiment of  FIG. 1  has a layout achieving coincidence between a region where carriers are stored and a region where electric current flows most easily. This means that carriers effectively contribute to modulation even in a dark environment. As a result, the linearity of light intensities and signal outputs can be improved. 
   On the other hand, in the conventional modulation transistor  101  shown in  FIG. 2 , the ring gate  102  has a round contour (perimeter shape) and the source region  104  is also formed in a round shape at the center of the ring gate  102 . Further, the carrier pocket  108  is formed under the ring gate  102  in a ring-shaped narrow band with a roughly constant width substantially along the circumference of the ring gate  102 . The source contact  105  is located at the center of the round source region  104 . The drain region  106  is provided outside the circumference of the ring gate  102 . The drain contact  107  is located in part of the drain region  106 . 
   In the carrier pocket  108  provided in a ring-shaped narrow band, potential at two intermediate points a and b of the carrier pocket width located on the circumference at a 90-degree angle relative to each other, and the potential on the part of the carrier pocket between the two points a and b is all the same, which is given by (Vd+Vs)/2=2.4 V, as shown by the solid line in  FIG. 3 . This means that there is no low-potential region. Therefore the flow of electric current is not so smooth especially in a dark environment and, as explained in  FIG. 9 , the linearity of light intensities and signal outputs becomes poor (see line h in  FIG. 9 ). 
   As described above, in the conventional configuration, an electric current flows uniformly from all directions as shown in  FIG. 2 . Whereas in the present embodiment of the present invention having non-uniform and partially wider ring gate width, the amount of electric currents becomes larger where the current path is shorter (near points c and c′), and smaller where the current path is longer (near points d and d′). Therefore, signal outputs occur mainly in a region where a large amount of electric currents flows. Thus, even if threshold voltage Vth varies because of a potential dip (a decrease in potential), etc., in an area where the current path is short, its effect is small enough to control the variation among the sensor cells in a probabilistic manner. 
   In addition, in the modulation transistor  101  in  FIG. 2 , the carrier pocket  108  is provided under the ring gate  102  in a ring-shaped narrow band with a roughly constant width substantially along the circumference of the ring gate  102 . As another example of conventional configurations (not illustrated), by forming the carrier pocket  108  under the entire area of the ring gate  102 , that is, with the same width (or the same gate length) as that of the ring gate  102 , when the intermediate point is defined as the distance reference in measuring the pocket-to-source distance or the pocket-to-drain distance (the pocket here means the carrier pocket), the {pocket-to-source distance} to {pocket-to-drain distance} ratio is set to 1:1 uniformly on the entire circumference. Therefore, it can be said that no area with the lowest potential is formed. 
   Second Embodiment 
     FIG. 4  is a schematic top-view drawing of a modulation transistor in a solid-state imaging device of a second embodiment of the present invention. In the actual configuration of the sensor cells, there is a photodiode adjacent to the modulation transistor, which is omitted here. 
   In  FIG. 4 , a modulation transistor  101 B according to the second embodiment of the present invention configures a sensor cell together with a photodiode (not illustrated) which receives incident light. A ring gate  102 B has a round contour (perimeter shape). A source region  104 B is formed in an oval shape at the center of the ring gate  102 B. A carrier pocket  108 B is provided in an approximate oval shape with a roughly constant width under the ring gate  102 B and near the circumference in the major-axis direction of the oval source region  104 B and, further, near the circumference of the ring gate  102 B in the minor-axis direction of the oval source region  104 B. The source contact  105  is located at the center of the oval source region  104 B. The drain region  106  is provided outside the circumference of the ring gate  102 B. The drain contact  107  is located in part of the drain region  106 . In addition, the source region  104 B, which is formed in an oval shape at the center of the ring gate  102 B, is described assuming that the part corresponding to the entire area inside the oval shape is the source region. 
   During modulation with this configuration, potential of the carrier pocket in the minor-axis direction of the oval source region  104 B (near points f and f′) becomes higher due to the splitting ratio of the drain-to-source voltage, which lets the light-generated charges (carriers) stored in the carrier pocket  108 B flow into and be stored in the carrier pocket in the major-axis direction of the oval source region  104 B (near points e and e′). Further, a region having the largest capacity for making electric current flow from the drain region  106  into the source region  104 B is the channel in the major-axis direction of the oval source region  104 B (near points e and e′), where the drain-to-source distance is the shortest. Therefore, the second embodiment of  FIG. 4  has a layout achieving coincidence between a region where carriers are stored and a region where electric current flows most easily. This means that carriers effectively contribute to modulation even in a dark environment. As a result, the linearity of light intensities and signal outputs can be improved. 
   Moreover, although the above-described embodiments describe the shape of the ring gates and the oval or round source regions in the ring gates, the present invention allows any ring gate shape or source region shape if it satisfies the above conditions. Therefore, the ring gate or the source region can be formed in any polygonal shape approximating an oval or round shape such as an octagonal shape. 
   Field of Industrial Application 
   The present invention, which provides a configuration having an electric current-flowing path (current path) basically in all directions on the entire circumference of the ring gate, achieves a setting which makes the amount of electric currents flowing partially larger by changing the shape of the ring gate to form a region where the potential is lowered. Also, the present invention is especially effective when applied to a MOS solid-state imaging device employing voltage modulation.