Patent Publication Number: US-6342920-B2

Title: Photoelectric conversion device

Description:
This application is a division of application Ser. No. 08/805,968, filed Feb. 25, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photoelectric conversion device, for example, to one-dimensional or two-dimensional photoelectric conversion devices for reading images in a video camera, X-ray imaging apparatus, infrared ray imaging apparatus, and the like and, more particularly, to a photoelectric conversion device using a skimming charge transfer scheme. 
     2. Related Background Art 
     Conventionally, CCDs are popularly used as imaging devices of video cameras, digital cameras, and the like, and such image reading device uses a “charge skimming transfer scheme” disclosed in, e.g., IEEE trans. Electron. vol. ED-29, p. 3, 1982 and Japanese Patent Publication No. 7-48826. 
     FIG. 13 shows an infrared ray imaging apparatus using the conventional charge skimming transfer scheme described in Japanese Patent Publication No. 7-48826. FIGS. 12A and 12B show changes in charge amount before and after charge skimming transfer, and FIGS. 11A to  11 C show the input circuit of a charge skimming transfer type infrared ray imaging element. 
     FIGS. 11A to  11 C and FIG. 13 illustrate a photodiode  101 , a silicon CCD  102 , infrared rays  104 , an output circuit  105 , an input gate electrode  110 , an accumulation electrode  111 , a skimming electrode  112 , a CCD electrode  113 , an overflow drain  114 , an overflow electrode  115 , and a skimming voltage input terminal  119 . 
     In FIGS. 12A and 12B, charges  116  are produced due to background radiation, charges  117  are ascribed to radiation from a signal source, and charges are skimmed at a skimming level  118 . 
     The operation of the charge skimming transfer type infrared ray imaging element with the above arrangement will be described below with reference to the drawings. 
     (1) As shown in FIG. 11A, infrared rays  104  are converted into photocurrents by the photodiode  101 , and the photocurrents are input to and accumulated in a portion beneath the accumulation electrode  111  via the input gate electrode  110 . 
     (2) Upon completion of accumulation, as shown in FIG. 11B, a pulse signal is applied to the skimming electrode  112  to change the height of the potential well to transfer some accumulated charges to a portion underneath the CCD electrode  113 . The amount of charges to be transferred is controlled by the pulse signal (to be referred to as a skimming voltage hereinafter) to be applied to the skimming electrode  112 . 
     (3) Thereafter, as shown in FIG. 11C, charges remaining in the portion beneath the accumulation electrode  111  are ejected to the overflow drain  114  via the overflow electrode  115 . 
     When charge skimming transfer is performed, as described above, since DC components  116  produced by the background radiation can be removed, as shown in FIGS. 12A and 12B, contrast can be emphasized, and the amount of charges to be transferred to the portion beneath the CCD electrode  113  can be reduced. 
     Charges transferred to the portion below the CCD electrode  113  are sequentially transferred by the CCD  102  in the same manner as in a case wherein the charge skimming transfer scheme is not adopted, and are output to an external circuit via the output circuit  105 . 
     As described above, in the conventional photoelectric conversion device, when the number of photons of background radiation is extremely larger than the number of photons radiated by the signal source, the obtained signal has low contrast, and such problem cannot be solved by merely extending the accumulation time of the silicon CCD. In the above-mentioned prior art, this problem is solved by performing skimming charge transfer. However, since the skimming voltage is calculated and generated by an external circuit on the basis of the signal voltages of the respective pixels, other problems such as a long processing time, high cost of the entire system, and the like are posed. 
     On the other hand, an X-Y address type photoelectric conversion device, which uses a CMOS sensor or the like in place of the CCD and comprises a charge skimming transfer means, has not reached practical application level yet. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an element comprising-a skimming charge transfer function in an X-Y address type photoelectric conversion device using, e.g., a CMOS sensor and having excellent characteristics. 
     It is an object of the present invention to provide a photoelectric conversion element and a control method therefor for transferring an optimum amount of photoelectric charge from a photoelectric conversion means and outputting a signal corresponding to the amount of photoelectric charge transferred. 
     In order to attain the above-described object,according to one aspect of the present invention, a photoelectric conversion device includes a plurality of pixels. Each pixel includes photoelectric conversion means for converting light into photoelectric charges, transfer means for transferring a portion of the photoelectric charges from the photoelectric conversion means, and reading means for non-destructively outputting a signal corresponding to the portion of the photoelectric charges transferred by the transfer means. The photoelectric conversion device further includes controlling means for controlling an amount of photoelectric charges transferred by the transfer means. 
     According to another aspect of the present invention, the controlling means includes pulse setting means for changing a pulse signal, and the controlling means controls the amount of photoelectric charges transferred in response to an output from the pulse setting means. 
     According to another aspect of the present invention, the controlling means controls an amount of photoelectric charge transferred based on the signal output by the reading means. 
     According to another aspect of the present invention, the controlling means controls the transfer means by a method that includes a first step of controlling the transfer means according to a first control signal and a second step of controlling the transfer means according to a second control signal that differs from the first control signal. 
     Other objects and features of the present invention will become apparent from the following description of the embodiments taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 C are circuit diagrams showing a circuit for one pixel and a skimming charge transfer scheme according to the first embodiment of the present invention; 
     FIG. 2 is a circuit diagram of the first embodiment of the present invention; 
     FIG. 3 is a timing chart of the first embodiment of the present invention; 
     FIG. 4 is a flow chart showing the operation according to the first embodiment of the present invention; 
     FIG. 5 is a circuit diagram of an automatic skimming voltage control circuit according to the first embodiment of the present invention; 
     FIGS. 6A and 6B are respectively a circuit diagram and a timing chart of a maximum value detection circuit for a pixel signal according to the first embodiment of the present invention; 
     FIG. 7 is a circuit diagram of the second embodiment of the present invention; 
     FIG. 8 is a circuit diagram of the third embodiment of the present invention; 
     FIG. 9 is a circuit diagram of a circuit for one pixel according to the fourth embodiment of the present invention; 
     FIG. 10 is a circuit diagram of a circuit for one pixel according to the fifth embodiment of the present invention; 
     FIGS. 11A to  11 C are circuit diagrams for explaining the conventional charge skimming scheme; 
     FIGS. 12A and 12B are graphs showing the relationship between the skimming operation and the charge amount; and 
     FIG. 13 is a circuit diagram for explaining the conventional charge skimming scheme. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described in more detail hereinafter with reference to the accompanying drawings. 
     FIGS. 1A to  1 C show the sectional structure and peripheral circuits of a photoelectric conversion element according to the first embodiment of the present invention, and also illustrate the charge skimming transfer processes. 
     Referring to FIGS. 1A to  1 C, a photoelectric conversion device comprises a photodiode  1 , which is illustrated as a circuit element on the circuit diagram for the sake of simplicity (however, an actual photodiode  1  is constituted by the p-n junction between a p-type semiconductor substrate and an n + -type diffusion layer  30 ; the same applies to the photodiode shown in the other figures). The device also comprises a driver MOSFET  2  for outputting a signal. The drain terminal of the MOSFET  2  is connected to an output line via a selection switch  13 , and a resistive load or constant current load (not shown) is inserted in the output line. The load element and the driver MOSFET  2  constitute an amplifier. The device further comprises an input gate electrode  10 , an accumulation electrode  11 , a skimming electrode  12 , and a MOS switch  16  for resetting the floating gate of the MOSFET  2 . 
     Such photoelectric conversion device accumulates photoelectric signal charges produced by a photoelectric conversion means  1  in the control electrode (gate) of a MOS transistor  2 , and is characterized by comprising: 
     a skimming electrode  12  serving as means for transferring skimming charges of those produced by the photoelectric conversion means  1 ; 
     an n + -type region  31  serving as means for accumulating the transferred skimming charges; 
     a MOS transistor  2  serving as means for reading out potential changes caused by the skimming charges; and 
     means (to be described later with reference to FIG. 2) for automatically controlling the amount of skimming charges to be transferred. 
     Also, a photoelectric conversion device is characterized by comprising: 
     photoelectric conversion means consisting of a first diffusion layer  30  which is formed on a semiconductor substrate of one conductivity type and is doped with an impurity of a conductivity type opposite to the one conductivity type; 
     a transfer electrode  10  for transferring photoelectric charges produced by the photoelectric conversion means; 
     an accumulation electrode  11  for accumulating the transferred photoelectric charges; 
     a skimming electrode  12  for transferring skimming charges of the accumulated photoelectric charges; 
     a second diffusion layer  31  of the opposite conductivity type for accumulating the skimming charges; 
     means  2 , connected to the second diffusion layer  31 , for outputting potential changes caused by the skimming charges onto an output line; and 
     means  14  and  15  for resetting charges which remain after the skimming charges are transferred. 
     The charge skimming transfer operation of this embodiment will be described below with reference to the drawings. 
     (1) As shown in FIG. 1A, light  4  is converted into photocurrents by the photodiode  1  as the photoelectric conversion means, and the photocurrents are input to and accumulated in a portion beneath the accumulation electrode  11  via the input gate electrode  10 . 
     (2) Upon completion of accumulation, as shown in FIG. 1B, a pulse signal is applied to the skimming electrode  12  to change the height of the potential well, and some accumulated charges (skimming charges) are transferred to the n + -type diffusion layer  31  connected to the floating gate of the MOS transistor  2 . The amount of charges to be transferred is controlled by the pulse signal (skimming voltage) to be applied to the skimming electrode  12 . 
     (3) Thereafter, as shown in FIG. 1C, residual charges underneath the accumulation electrode  11  are ejected to the overflow drain  14  via the overflow electrode  15 . 
     When charge skimming transfer is performed, as described above, since DC components  116  produced by the background radiation can be removed, as shown in FIGS. 12A and 12B, contrast can be emphasized, and a signal with a high S/N ratio can be obtained. 
     By appropriately repeating the operations (1) to (3), the background signal components can be removed, and a signal with a higher S/N ratio can be obtained. 
     In this embodiment, since signal charges can be non-destructively read out, the skimming amount can be automatically optimized by reading signals during the accumulation period, and setting the skimming voltage based on, e.g., a voltage corresponding to the maximum signal of skimming charges of all the pixels. 
     In pixels having the photoelectric conversion means, signals may be read out in units of pixel blocks, and the skimming amount can be controlled by a skimming voltage corresponding to the minimum voltage of the readout signals in units of pixel blocks. Such control in units of clocks can be realized by controlling scanning circuits for scanning the respective pixels to locally scan the pixels. 
     FIG. 2 is a schematic circuit diagram showing a MOS linear sensor as an example of the photoelectric conversion device of the first embodiment described above with reference to FIGS. 1A to  1 C. FIG. 2 shows circuits for four pixels, and a dotted line portion G 1  as a peripheral circuit for one pixel will be explained below. Note that the selection switch  13  in FIGS. 1A to  1 C is not shown in FIG.  2 . 
     In FIG. 2, the circuit comprises a photodiode  1  serving as a photoelectric conversion means, which is illustrated as a circuit element on the circuit diagram for the sake of simplicity (however, an actual photodiode  1  is constituted by the p-n junction between a p-type semiconductor substrate and an n + -type diffusion layer  30 ; the same applies to the photodiode shown in the other figures). The circuit also comprises a driver MOSFET  2  for outputting a signal. The drain terminal of the MOSFET  2  is connected to an output line, and a MOSFET  17  serving as a load resistor is inserted in the output line. The load element  17  and the driver MOSFET  2  constitute a source-follower amplifier for each pixel. The circuit further comprises an input gate electrode  10 , an accumulation electrode  11 , a skimming electrode  12 , and a MOS switch  16  for resetting the floating gate of the MOSFET  2 . 
     The circuit further comprises an overflow drain  14  and a MOSFET  15  serving as an overflow control switch. 
     Moreover, the sensor comprises an automatic skimming voltage control circuit  51 , and a circuit  52  for detecting the maximum value of skimming charges of each pixel. 
     The circuits  51  and  52  as means for automatically controlling the skimming charge amount detect the maximum signal of skimming charges, and compare it with a proper signal level. When the maximum signal of the skimming charges is smaller than the proper signal level, the means controls the skimming voltage to increase the amount of skimming charges; otherwise, the means controls the skimming voltage to decrease the amount of skimming charges. 
     In FIG. 2, the skimming electrode  12  serves as means for transferring skimming charges of those produced by the photoelectric conversion means, 
     the automatic skimming voltage control circuit  51  and the maximum value detection circuit  52  for detecting the maximum value of skimming charges of each pixel serve as means for controlling the amount of skimming charges to be transferred, 
     an n + -type region  31  serves as means for accumulating the transferred skimming charges, and 
     the MOS switch  15  serves as means for resetting charges which remain after the skimming charges of those produced by the photoelectric conversion means are transferred. 
     FIG. 3 is a timing chart of the circuit in FIG. 2, and FIG. 4 is a flow chart showing the control scheme and operation of the circuit in FIG.  2 . 
     The operation of this embodiment will be briefly described below with reference to FIGS. 3 and 4. 
     At time t 0 , pulses φ R , φ T , and φ SKM  are applied to reset the residual charges in the pixels. Upon completion of resetting (t 1 ), accumulation of photoelectric charges is started. 
     At time t 2 , when pulses φ T  and φ S  are applied, and a pulse φ SKM  of an initial voltage is applied to the skimming electrodes, skimming charges corresponding to the applied voltage are read out to the floating gates of the transistors  2  of the respective pixels. At this time, when pulses φ L  and φ TR  are also applied, the source-follower amplifiers of the respective pixels operate to read out pixel signals to capacitors C T1  to C T4 . 
     Thereafter, a scanning circuit  21  is operated from times t 3  to t 4  to serially read out the signals from the capacitors C T1  to C T4  to an output terminal  53 , and the circuit block  52  obtains, e.g., the maximum value of these signals. 
     The circuit block  51  generates an optimal skimming voltage φ SKM  in accordance with the obtained maximum signal amount, and applies it to the respective pixel at time t 5  to perform skimming transfer and signal read again. 
     By repeating the series of operations described above, an optimal skimming signal can be obtained. 
     In the above description, after skimming transfer, the pixels continue charge accumulation without resetting the accumulation electrodes. Alternatively, the accumulation electrode and pixel portions may be reset in each scan and thereafter, accumulation may be resumed. 
     FIG. 5 shows an example of an automatic setting circuit of a skimming voltage during the accumulation period, which is denoted by  51  in FIG.  2 . In this circuit, an input signal (the signal output from an output amplifier  54  in FIG. 2) is inverted and amplified by an inverting amplifier  61 , and its output voltage is reflected in the amplitude of the pulse φ SKM  More specifically, as the input signal is smaller, the amplitude of the pulse φ SKM  becomes larger to automatically increase the amount of skimming charges to be transferred. 
     FIGS. 6A and 6B show an example of a circuit for detecting the maximum value of skimming charge signals of the respective pixels, which circuit is denoted by  52  in FIG.  2 . FIG. 6A is a circuit diagram, and FIG. 6B is a timing chart thereof. In the circuit operation, a pulse φ INIT  changes to Hi-level to turn on a MOS switch  71 , thereby resetting a capacitor  72 . Thereafter, every time a signal is input, the input signal is compared with a voltage in the capacitor  72 . If the input signal is lower than the voltage in the capacitor  72 , a pulse OUT changes to V CC  level shown in FIG. 6B; otherwise, the pulse OUT changes to GND level. As a consequence, a PMOS  73  is turned on, and the input signal is written in the capacitor  72 . Upon completion of a series of operations, a pulse φ READ  changes to high-level to turn on a MOS switch  74  so as to form a feedback loop, thereby outputting the voltage in the capacitor  72 . 
     FIG. 7 is a circuit diagram showing the second embodiment, which is substantially the same as the first embodiment, except for means for detecting a maximum value of pixels. 
     In FIG. 7, an NPN transistor  23  is inserted in the output line of each pixel, and its emitter is commonly connected. With this arrangement, when signals are read out from the respective pixels to capacitors C 1  to C 4 , the maximum value of the signals can be immediately obtained. 
     In this case, a circuit  52 ′ may be a simple buffer amplifier. 
     FIG. 8 shows the third embodiment, which is substantially the same as the first embodiment, except that the means for detecting the maximum signal of pixels is replaced by means for detecting background light. 
     In FIG. 8, for example, a pixel G 4  enclosed in the dotted line serves as a dummy pixel, which is arranged at a position where no signal light is incident, and the voltage of background light is obtained based on the output from this pixel, thereby controlling the skimming voltage. 
     FIG. 9 is a circuit diagram of a photoelectric conversion element according to the fourth embodiment of the present invention. 
     This embodiment is substantially the same as the first embodiment, except that the output amplifier in the device described in the first embodiment is replaced by an NMOS inversion circuit. With this arrangement, the gain can be set to be 1 or more. Note that the same reference numerals in FIG. 9 denote the same parts as those in FIGS. 1A to  1 C, and a detailed description thereof will be omitted. 
     FIG. 10 is a circuit diagram of a photoelectric conversion element according to the fifth embodiment of the present invention. 
     In this embodiment, the skimming electrode  12  alone is arranged to also play roles of the input gate electrode and the accumulation electrode in the above-mentioned embodiment. 
     After skimming charges of those produced by a photodiode are transferred to an n + -type region  31 , the residual charges are ejected by a MOS switch  15  arranged in an n + -type region  30 . 
     The amount of skimming charges is controlled by the skimming electrode  12  in the same manner as in the above embodiment. 
     In the above embodiments, the photoelectric conversion device which reads out a photoelectric signal by transferring it to the gate electrode of the MOS transistor has been described. The present invention can also be applied to other photoelectric conversion devices such as a FGA (floating gate array device), an AMI (amplified MOS intelligent imager), a BASIS (base-stored image sensor), and the like. 
     In the above embodiments, a one-dimensional photoelectric conversion device has been exemplified. However, the same effect can be obtained even when the present invention is applied to a two-dimensional photoelectric conversion device. 
     In the above embodiments, n-type layers are used as the diffusion layers  30  and  31 . Alternatively, an n-type semiconductor substrate and p-type diffusion layers may be used. 
     As described above, according to the present invention, since, for example, the MOS transistor is used as the accumulation means of skimming charges in place of a CCD which is used in the conventional device, a charge skimming transfer type photoelectric conversion device which can remove background signal components as in the conventional device, and can yet provide a signal with a high S/N ratio and, hence, high contrast, can be realized. 
     Also, according to the present invention, since the skimming voltage can be automatically controlled in real time, an optimal skimming charge amount can be obtained. 
     Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.