Solid-state imaging device

Abstract: A solid-state imaging device of the invention is provided with a CCD-structured branching unit which selects one signal charge sensor having characteristics suitable for the conditions of use from among a plurality of signal charge sensors each having different characteristics and forms a signal charge transmission path leading from the horizontal CCD to the selected signal charge sensor. As a result, there is no need to switch over the external circuit of the solid-state imaging device according to the conditions of use, which makes it possible to hold down the cost and reduce the size of the external size.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a solid-state imaging device that utilizes charge-coupled devices (CCDs) that is used as an area sensor, a line sensor, or the like. 2. Description of the Prior Art: Signal charge sensing methods employed for a signal charge sensor that forms the output part of a solid-state imaging device constructed from CCDs include, for example, a current output method, a floating-diffusion amplifier method, and a floating-gate amplifier method. The characteristics such as the sensitivity and dynamic range differ among these signal charge sensing methods, each method having its own advantages and disadvantages. Even in the case of signal charge sensors employing the same sensing method, the characteristics such as the sensitivity and dynamic range vary from one type of sensor to another depending on how the sensors are configured. Figure 3 is a diagram showing the structure of a signal charge sensor, with its adjacent circuitry of a solid-state imaging device in which a floating-diffusion amplifier is used for signal charge sensing. A p-type substrate 31 and an n + layer 32 form a floating-diffusion diode FD , while an n + layer 33 forms a reset drain RD . A line RS indicates a reset gate. The floating-diffusion diode FD is connected to the gate of an amplifier transistor TR31 by a connecting line 34 . The floating-diffusion diode FD , the connecting line 34 , the transistor TR31 , the reset drain RD , and the reset gate RS constitute the signal charge sensor. The floating-diffusion diode FD senses the signal charge reaching the final stage (end portion) of a horizontal CCD formed below an output electrode OG and produces an output voltage V out to deliver to the connecting line 34 . The floating-diffusion diode FD has a floating capacitance C1 between the n + layer 32 and the output electrode OG , a floating capacitance C2 between the n + layer 32 and the reset gate RS , and a capacitance Cd between the n + layer 32 and the p-type substrate 31 . The line 34 has a floating capacitance Cg . Therefore, when the values of the floating capacitances C1 , C2 , and Cg are denoted as c1, c2, and cg, respectively, and the value of the capacitance Cd as cd, the capacitance cfd of a capacitor CFD formed in the signal charge sensor having the floating-diffusion diode FD and the connecting line 34 can be given by cfd = c1 + c2 + cg + cd On the other hand, the output voltage V out of the signal charge sensor can be given by V out = Q sig /cfd where Q sig is the amount of signal charge sensed by the signal charge sensor. Therefore, when the capacitance cfd of the capacitor CFD in the signal charge sensor is made variable, the output voltage V out of the signal charge sensor can be made variable. For example, when a variable capacitance diode is connected in parallel to the signal charge sensor, the signal charge sensor can be made to provide a variable output voltage. In this case, however, the capacitance of the signal charge sensor increases by the variable capacitance diode connected in parallel to it, and hence, the output voltage of the signal charge sensor decreases. Generally, solid-state imaging devices utilizing CCDs are provided with only one signal charge sensor that forms the output part thereof. Therefore, when it is desired to use the aforementioned solid-state imaging device in a darker place than usual, the external circuit provided external to the solid-state imaging device for amplifying the output signal of the signal charge sensor has to be switched over to another external circuit having a higher gain. Thus, the solid-state imaging device having only one signal charge sensor has the disadvantage in that the cost and size of the external circuit increase. A solid-state imaging device is shown in Figure 2 . The solid-state imaging device shown comprises vertical CCDs 21 arranged in rows and a horizontal CCD 22 connected to the vertical CCDs 21 . To the final stage of the horizontal CCD 22 are connected three signal charge sensors 23 , 25 , and 26 , arranged parallel to each other and having different sensitivities, dynamic ranges, etc. With this arrangement, one of the three signal charge sensors 23 , 25 , and 26 is selected that can provide the most suitable characteristics according to the conditions of use of the solid-state imaging device. As a result, there is no need to switch over the external circuit according to the conditions of use of the solid-state imaging device, which makes it possible to hold down the cost and reduce the size of the external circuit. However, in the above-mentioned solid-state imaging device, since the final stage of the horizontal CCD 22 is connected to the three signal charge sensors 23 , 25 , and 26 , arranged parallel to each other, the result is that the combined capacitances of the signal charge sensors 23 , 25 , and 26 are connected to the final stage of the horizontal CCD 22 . This means that a greater capacitance is connected to the final stage of the horizontal CCD 22 than when only one signal charge sensor is connected. The resulting problem is that the output voltage that each of the signal charge sensors 23 , 25 , and 26 produces in response to the signal charge from the horizontal CCD 22 drops because of the large capacitance, which can render the solid-state imaging device unserviceable. SUMMARY OF THE INVENTION This invention provides the solid-state imaging device defined by claim 1. The sub-claims 2 and 3 are directed to preferred embodiments of the invention. Using the invention as claimed in claim 1, it is possible to select one signal charge sensor having characteristics suitable for the conditions of use from among the plurality of signal charge sensors each having different characteristics and to form a signal charge transmission path leading from the horizontal CCD to the selected signal charge sensor. As a result, there is no need to switch over the external circuit of the solid-state imaging device according to the conditions of use, which makes it possible to hold down the cost and reduce the size of the external circuit. Furthermore, since the formation of the transmission path serves to connect only the selected signal charge sensor to the final stage of the horizontal CCD, the result is that only the capacitance of the selected signal charge sensor is connected to the final stage of the horizontal CCD. Accordingly, unlike the example in which a plurality of signal charge sensors, arranged in parallel to each other, are connected to the final stage of the horizontal CCD, the capacitance connected to the final stage of the horizontal CCD can be prevented from increasing when compared with the configuration having only one signal charge sensor. Therefore, the provision of the plurality of signal charge sensors does not contribute to decreasing the output voltage of each of the signal charge sensors. Thus, the invention makes possible the objective of providing a solid-state imaging device that is provided with a plurality of signal charge sensors having different characteristics and that can select one signal charge sensor having characteristics suitable for the conditions of use, while preventing the output voltage of the selected signal charge sensor from dropping because of the provision of the plurality of signal charge sensors. A branching unit for transmitting signal charges to only one out of 3 identical charge sensors is known from EP-A-0 282 150. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram showing a CCD-structured branching unit in a solid-state imaging device according to the present invention; Figure 2 is a schematic diagram showing a solid-state imaging device which comprises three signal charge sensors arranged parallel to each other and having different sensitivities, dynamic ranges, etc; Figure 3 is a diagram showing the structure of a signal charge sensor, with its adjacent circuitry, of a solid-state imaging device in which a floating-diffusion amplifier is used for signal charge sensing; and Figure 4 shows another CCD-structured branching unit in a solid-state imaging device according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The solid-state imaging device of the present invention will now be described in detail by way of example with reference to the accompanying drawings. Figure 1 is a block diagram showing signal charge sensors, with their adjacent circuitry, of the solid-state imaging device according to the present invention. As shown in Figure 1 , the solid-state imaging device of this embodiment has a CCD-structured branching unit 2 adjacent to the final stage (end portion) of a horizontal CCD 1 . The horizontal CCD 1 receives signal charges generated by photodiodes which are arranged in an array of rows and columns through vertical CCDs (not shown in Figure 1 ). The horizontal CCD 1 transfers the signal charges to the end portion of the horizontal CCD 1 according to potential levels of pulse signals 1 and  2 which are applied on gate electrodes of the horizontal CCD 1 . The CCD-structured branching unit 2 is configured so as to form a signal charge transmission path leading from the final stage (end portion) of the horizontal CCD 1 to one signal charge sensor selected from among the three signal charge sensors 4 , 5 , and 6 by applying pulse signals a , b ,  c , 1 , and 2 to transfer electrodes 3 , 3 , ··· of the branching unit 2 . The signal charge transmission path is formed as a CCD channel under the transfer electrodes 3 . The signal charges are transferred through the signal charge transmission path (CCD channel) according to the surface potential variation of a semiconductor chip. This surface potential variation is caused by applying the pulse signals a , b ,  c , 1 , and  2 to the transfer electrodes 3 . The three signal charge sensors 4 , 5 , and 6 each have the function of sensing the signal charge reaching the final stage of the horizontal CCD 1 . Also, the three signal charge sensors 4 , 5 , and 6 have respectively different sensitivities and dynamic ranges. The three signal charge sensors 4 , 5 , and 6 are connected to an external circuit via transistors TR14, TR15 , and TR16, respectively. The transistors TR14, TR15, and TR16 are made to conduct when respective gate voltages 14 , 15 , and 16 are applied. The solid-state imaging device is configured so that, when the pulse signals a , b , c , 1 , and 2 are applied to the transfer electrodes 3, 3 , ··· of the CCD-structured branching unit 2 , one signal charge sensor that suits the conditions of use is selected from among the three signal charge sensors 4, 5 , and 6 each having different characteristics, and a signal charge transmission path is formed leading from the final stage of the horizontal CCD 1 to the signal charge sensor selected as suitable for the conditions of use. Therefore, there is no need to switch over the external circuit according to the conditions of use, which makes it possible to hold down the cost and reduce the size of the external circuit. Furthermore, since the formation of the transmission path serves to connect only one signal charge sensor, selected as suitable for the conditions of use, to the final stage of the horizontal CCD 1 , the result is that only the capacitance of the signal charge sensor selected as suitable for the conditions of use is connected to the final stage of the horizontal CCD 1 . Thus, according to this embodiment, unlike the prior art example in which three signal charge sensors, arranged in parallel to each other, are connected to the final stage of the horizontal CCD, the capacitance connected to the final stage of the horizontal CCD 1 can be prevented from increasing when compared with the configuration having only one signal charge sensor. Therefore, the output voltages of the signal charge sensors 4, 5, and 6 can be prevented from dropping because of the provision of the three signal charge sensors 4, 5, and 6 . Figure 4 shows another CCD-structured branching unit in a solid-state imaging device of the invention. The CCD-structured branching unit is formed by two-branches-sections B1, B2 and B3, as shown in Figure 4. At the two-branches-section B1 , any one of two branches of the two-branches-section B1 is selected based on the potential of the pulse signal a . At the two-branches-sections B2 and B3 , any one of two branches of each of the two-branches-sections B2 and B3 is selected based on the potential of the pulse signal b . The CCD-structured branching unit is so configured that, when the pulse signals a , b , 1 , and 2 are applied to the transfer electrodes (not shown in Figure 4 ) of the CCD-structured branching unit, one signal charge sensor that suits the conditions of use is selected from among four signal charge sensors 21 , 22, 23 and 24 each having different characteristics, and a signal charge transmission path is formed leading from the final stage of the horizontal CCD to the signal charge sensor selected as suitable for the conditions of use. In the CCD-structured branching unit, when the pulse signals a and b are High, the signal charge sensor 21 is selected. When the pulse signals a and b are Low, the signal charge sensor 24 is selected. The relationship between the potential levels (High or Low) of the pulse signals a and b and the selected one of the signal charge sensors 21 , 22 , 23 and 24 is shown in Table 1. As seen from the above, according to the CCD-structured branching unit as shown in Figure 4 , the application of the pulse signal c to the transfer electrodes is not needed in order to select one among the four signal charge sensors 21 , 22, 23 and 24 . In the embodiment of the invention, the pulse signal a is made by adding a gate voltage 21 to a gate voltage 22, and the pulse signal b is made by adding a gate voltage 23 to a gate voltage 24 . As is apparent from the above description, since the solid-state imaging device of the invention is provided with a CCD-structured branching unit, it is possible to select one signal charge sensor having characteristics suitable for the conditions of use from among a plurality of signal charge sensors having different characteristics and to form a signal charge transmission path leading from the horizontal CCD to the selected signal charge sensor. As a result, there is no need to switch over the external circuit of the solid-state imaging device according to the conditions of use, which makes it possible to hold down the cost and reduce the size of the external size. Furthermore, since the formation of the transmission path serves to connect only the selected signal charge sensor to the final stage of the horizontal CCD, the result is that only the capacitance of the selected signal charge sensor is connected to the final stage of the horizontal CCD. Thus, according to the present invention, unlike the prior art example in which a plurality of signal charge sensors, arranged in parallel to each other, are connected to the final stage of the horizontal CCD, the capacitance connected to the final stage of the horizontal CCD can be prevented from increasing when compared with the configuration having only one signal charge sensor. Therefore, according to the present invention, the output voltage of each of the signal charge sensors can be prevented from dropping because of the provision of the plurality of signal charge sensors. The invention may be embodied in other specific forms without departure from its scope as defined by the claims.

Claims: A solid-state imaging device comprising: photoelectric converting means for receiving light and converting said light to signal charges; charge transfer means (1) for receiving said signal charges and transferring said signal charges to an end portion of said charge transfer means; a plurality of signal charge sensors (4, 5, 6; 21, 22, 23, 24) each having different characteristics, said signal charge sensors being for sensing said signal charges which are transferred to said end portion of said charge transfer means; and a CCD-structured branching unit (2, 3) comprising a plurality of branches, each branch extending between a common transfer electrode at the end portion of the charge transfer means (1) and a respective charge sensor (4, 5, 6), each branch comprising a first transfer electrode (3), arranged after the common transfer electrode, and a second transfer electrode (3), arranged between the first transfer electrode (3) and the respective signal charge sensor (4, 5, 6), and means for applying pulse signals to said transfer electrodes to provide a signal charge transmission path leading from said end portion of said charge transfer means to only one of said signal charge sensors (4, 5, 6), according to the conditions of use wherein a respective signal ( a ,  b ,  c ) is applied to each first transfer electrode to select the respective branch, and a common pulse signal ( 2 ) is applied to the second transfer electrodes of each branch. A solid-state imaging device according to claim 1, wherein the CCD-structured branching unit has a first stage containing one two-branch section ( B1 ) for providing two alternative paths, and a second stage containing two two-branch sections ( B2 , B3 ) each providing two alternative paths such that each path from the first stage has two alternative paths from the second stage, the device further comprising means for applying pulse signals ( a ,  b ,  1 ,  2 ) to each of the two-branch sections to select a signal charge transmission path leading from the end portion of the charge transfer means to a selected one of the signal charge sensors ( 21 , 22 , 23 , 24 ). A solid-state imaging device according to claim 1 or claim 2, wherein said charge transfer means comprises vertical charge-coupled devices arranged in rows and a horizontal charge-coupled device ( 1 ) connected to said vertical charge-coupled devices, and said end portion of said charge transfer means is an end portion of said horizontal charge-coupled device ( 1 ).

Inventors: NAKA SHUNICHI, WATANABE TAKASHI, NAKA, SHUNICHI, WATANABE, TAKASHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L27
IPC codes: H01L27, H04N5