Abstract:
A light receiving element is provided with a phototransistor and a light receiving MOS diode proximate thereto and having a gate electrode covering a portion of the base region of the phototransistor. The gate electrode permits transmission of a portion of received light. The light receiving MOS diode forms an inversion layer in a substrate adjacent the base of a phototransistor during the time photo charges are stored, and generated photo charges are stored in the inversion region and the base region of the phototransistor. During the storage state, the potential of the inversion region and the base region of the phototransistor is limited, so that the intensity of an electric field applied to an insulating film between the electrode and the semiconductor substrate is 0.7 MV/cm or less. Alternatively, the potential of the electrode in a waiting state is fixed or made floating, so that an electric field is not applied, and recombination at the surface of the semiconductor substrate is made stable.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a linear image sensor suitable for a facsimile or an image scanner which reads out image information and transmits it. 
     In a conventional linear image sensor, a light receiving element of a linear image sensor as disclosed in Japanese Patent Application Laid-open No. Sho 61-124171 is known. FIG. 8 shows its structure. In FIG. 8, reference numeral  4  denotes a separation layer,  3  denotes an epitaxial layer and is a collector,  6  denotes a base, and  9  denotes an emitter. 
     SUMMARY OF THE INVENTION 
     However, since a base/collector junction portion is wide, junction capacitance is also large, and further, since a base region is in a floating state, there is a problem of persistence of vision that even if read images are changed from a bright state to a dark state, a signal of the previous bright state remains in the read signal, and there has been a problem that a clear image can not be obtained. 
     In order to solve the foregoing problem, in the present invention, a part of a light receiving element is made a light receiving MOS diode covered with an electrode permitting transmission of part of light, and when photo charges are stored, the light receiving MOS diode is operated in an inversion state, and generated photo charges are stored in its inversion region and a base region of a phototransistor, and at the time of resetting after reading to the outside, the light receiving MOS diode is made a storage state so that residual charges which were not able to be read to the outside are transferred to the base region, and in the state where the photo charge density of the base region is raised, resetting is made through an emitter so that the residual charges which become persistence of vision are reduced. At the time of reading as well, the light receiving MOS diode is made the storage state, so that photo charges stored under the light receiving MOS diode are transferred to the base region and the voltage between the base and emitter is made large, as a result of which, the bipolar transistor operation of the phototransistor is made easier, and improvement has been made to the characteristics contrary to the foregoing, that is, the rising characteristics that even when read images are changed from a dark state to a bright state, a read signal is slightly lower than a signal in a sufficiently bright state. 
     Moreover, to prevent an electric field of 0.7 MV/cm or more from being applied to a gate oxide film of the light receiving MOS diode, a base potential is limited, so that the influence of surface recombination current applied by an interface of the gate oxide film is stabilized, and further, improvement has been made to the leak of photo charges, which were not able to be absorbed, into the vicinity of an adjacent pixel in the case where light with high illumination is incident on a pixel under sunlight or fluorescent light. Moreover, at a waiting state, the potential of a gate of the light receiving MOS diode is fixed to a high voltage or made floating, so that improvement has been made such that an electric field is prevented from being applied to the gate oxide film of the light receiving MOS diode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional structural view of a light receiving element of a linear image sensor of the present invention. 
     FIG. 2 is a sectional structural view of a light receiving element and a switching element of the linear image sensor of the present invention. 
     FIG. 3 is a block diagram of a linear image sensor of the present invention. 
     FIG. 4A is a first circuit diagram of a light receiving element of a linear image sensor of the present invention. 
     FIG. 4B is a second circuit diagram of a light receiving element of a linear image sensor of the present invention. 
     FIG. 5A is a first circuit diagram of a limiter circuit of a linear image sensor of the present invention. 
     FIG. 5B is a second circuit diagram of a limiter circuit of a linear image sensor of the present invention. 
     FIG. 5C is a third circuit diagram of a limiter circuit of a linear image sensor of the present invention. 
     FIG. 6 is a circuit diagram of a linear image sensor showing a second embodiment of the present invention. 
     FIG. 7A is a first time chart diagram of a unit light receiving element of a linear image sensor showing a second embodiment of the present invention. 
     FIG. 7B is a second time chart diagram of the unit light receiving element of the linear image sensor showing the second embodiment of the present invention. 
     FIG. 8 is a sectional structural view of a light receiving element of a conventional linear image sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional structural view of a light receiving element of a linear image sensor showing an embodiment of the present invention. Reference numeral  101  denotes an n-type silicon semiconductor substrate,  102  denotes an n-type region which has a high impurity concentration and is formed to apply a potential to the semiconductor substrate,  112  denotes its metal electrode,  103  denotes a p-type base region of a bipolar phototransistor,  113  denotes its metal electrode,  104  denotes an n-type emitter region of the bipolar phototransistor,  114  denotes its metal electrode, and  105  denotes a gate electrode of a light receiving MOS diode, which is made of polycrystal silicon or the like to permit transmission of at least a part of light. Reference numeral  106  denotes an inversion region at the time when a positive potential is applied to the gate electrode  105  of the light receiving MOS diode so that the surface of the n-type silicon semiconductor substrate  101  becomes an inversion state, and the n-type silicon semiconductor substrate  101  becomes a substrate region of the light receiving MOS diode and a collector region of the phototransistor. Reference numeral  107  denotes a light-shielding film,  108  denotes a transparent protective film to permit transmission of light, and  110  denotes a light receiving element region where the phototransistor and the light receiving MOS diode are formed. When the light receiving element region  110  is irradiated with light in a storage period when an original image is read, holes of photo charges generated in the n-type silicon semiconductor substrate  101  are stored in the base  103  and the inversion region  106 . Here, in the case where the inversion region  106  is not formed, although a part of the holes of the generated photo charges reaches the base  103  in the light receiving element region  110  and is stored, the remainder does not reach the base  103  and disappears, or is diffused or drifted to another light receiving element region or negative potential region in the vicinity, and as a result, the photo charges generated in the light receiving element region are not sufficiently stored in the light receiving element, and it becomes impossible to read them to the outside. When the inversion region  106  is formed under the light receiving MOS diode and charges are stored, it has become possible to obtain a photoelectric conversion efficiency comparable with a conventional light receiving element composed of a phototransistor having a wide base region. After the operation of storage, although an image signal is read out to the outside, at the time of resetting after reading to the outside, a positive potential is applied to the gate electrode  105 , so that the surface of the n-type silicon semiconductor substrate  101  under the light receiving MOS diode is made a storage state from the inversion state. By doing so, the residual charges which were not able to be read to the outside are transferred to the base region  103 , and in the state where the photo charge density of the base region is raised, resetting to a reset potential is made through the emitter  104 , so that the residual charges which become persistence of vision are decreased. At the time of reading as well, the surface of the n-type silicon semiconductor substrate  101  under the light receiving MOS diode is made the storage state so that holes of photo charges previously stored in the inversion region  106  are transferred to the base region  103 , and a voltage between the base and emitter is increased so that the bipolar transistor operation of the phototransistor is made easier, as a result of which, improvement has been made to the characteristics contrary to the foregoing problem of the persistence of vision, that is, the characteristics of rising from dark to bright that when read images are changed from the dark state to the bright state, the read signal is slightly smaller than the signal in the sufficiently bright state. 
     Besides, by making the conductivity of the inversion region  106  of the light receiving diode equal to the conductivity of the base  103  of the phototransistor, the number of elements constituting a unit light receiving element is made small. Besides, in order to certainly transfer the photo charges stored in the inversion region  106  under the light receiving MOS diode to the base of the phototransistor, the gate electrode  105  of the light receiving MOS diode is made to cover at least a part of the base of the phototransistor through an insulating film. 
     FIG. 2 is a sectional structural view showing another embodiment of the present invention, which includes a switching element and a reset element made of a MOS transistor. In addition to the light receiving element region  110  shown in FIG. 1, reference numeral  109  denotes a metal thin film for light-shielding,  121  denotes a p well with conductivity different from the n-type silicon semiconductor substrate  101 ,  123  and  124  denote a drain region and a source region of a switching n-channel MOS transistor formed in the p well  121 ,  128  denotes a gate electrode of the MOS transistor, and  127  denotes a lead electrode from the drain region  123 , which is connected to the metal electrode  114  of the emitter  104  and has the same potential. Reference numerals  125  and  126  denote a drain region and a source region of a reset n-channel MOS transistor formed in the p well  121 ,  130  denotes a gate electrode of the MOS transistor, and  131  denotes a lead electrode from the source region  126 , which is connected to a diffusion region  122  to give the Gnd potential as a reset potential. This diffusion region  123  has the same conductivity as the p well  121 , and gives a potential also to the p well. The source region  124  of the switching MOS transistor is connected to the drain region  125  of the reset MOS transistor through a metal wiring line  129 . The metal wiring line  129  is also connected to a circuit which reads a signal obtained by the light receiving element to the outside. By doing so, at the time of reading and resetting, the gate electrode  105  of the light receiving MOS diode can be connected to the gate electrode  128  of the switching n-channel MOS transistor, and at the time of storing photo charges, the gate electrode  105  of the light receiving MOS diode and the gate electrode  128  of the switching n-channel MOS transistor are made a negative potential at the same time to form the inversion region  106 , and the switching MOS transistor is made a non-conduction state, so that the photo charges can be stored in the base  103  and the inversion region  106 . At the time of reading and resetting, the gate electrode  105  of the light receiving MOS diode and the gate electrode  128  of the switching n-channel MOS transistor are made a positive potential at the same time, the switching n-channel MOS transistor is made a conduction state, the inversion region  106  under the light receiving MOS diode is changed to the storage region, the photo charges stored in the inversion region  106  are transferred to the base region  103 , and the voltage between the base and emitter is raised, so that reading and resetting are made easier. At this time, since such a structure is adopted that the gate electrode  105  of the light receiving MOS diode covers at least a part of the base region  103  of the phototransistor through an insulating film, the photo charges stored in the inversion region  106  can be certainly transferred to the base region  103 , so that uniform photoelectric conversion efficiency can be obtained among a plurality of light receiving elements. From the above, the gate electrode  105  of the light receiving MOS diode and the gate electrode  128  of the switching n-channel MOS transistor can be controlled by the same scanning circuit, so that the number of elements constituting the circuit of the linear image sensor can be reduced. That is, the switching MOS transistor for reading a signal from the light receiving element to the outside is formed on the substrate with conductivity different from conductivity of the substrate on which the light receiving MOS diode is formed, and the gate electrode is made common, so that the circuit scale can be made small, and uniform photoelectric conversion efficiency among a plurality of light receiving elements are obtained. Incidentally, the reset operation is made in such a manner that after reading is ended, while the switching MOS transistor is made to keep the conduction state, the reset MOS transistor is made the conduction state from the non-conduction state at the time of reading, and a reset potential is applied to the emitter  102  of the phototransistor through the reset MOS transistor and the switching MOS transistor. For convenience, although the description has been made while using the n-type silicon semiconductor substrate  101 , a p-type silicon semiconductor substrate may be used. Although polycrystal silicon is used for the gate electrode  105  of the light receiving MOS diode, a transparent conductive thin film of ITO or the like may be used. 
     FIG. 3 is a block diagram showing an embodiment of the present invention, which includes a light receiving element array  301  composed of a plurality of phototransistors, an analog switch array  302  composed of a plurality of switching elements, a shift register  303  as a scanning circuit, a scanning input circuit  304  and a clock buffer  305  for driving the shift register, and an output circuit  306  for outputting an output signal of the scanning circuit and an image signal obtained in the light receiving element. 
     FIG. 4A is a first circuit diagram of a light receiving element of a linear image sensor showing an embodiment of the present invention. A unit light receiving circuit is structured in the manner described below. A collector electrode of a phototransistor  201  as a light receiving element is connected to a power source voltage VDD, and a base region of the phototransistor  201  is connected to a first electrode of a p-channel MOS transistor  202  of a light receiving MOS diode through a common diffusion region. The first electrode of the MOS transistor  202  is floating. A first electrode of a switching n-channel MOS transistor  203  and a limiter circuit  205  are connected to an emitter region of the phototransistor  201 . A second electrode of the MOS transistor  203  is connected to a common signal line  204 . A third electrode gate of the MOS transistor  202  and a third electrode gate of the MOS transistor  203  are connected to ΦSch. When ΦSch becomes low, the MOS transistor  203  is turned off, and the MOS transistor  202  is turned on, so that a storage period starts. Here, when the phototransistor  201  is irradiated with light, like FIG. 1, holes of photo charges generated in the n-type silicon semiconductor substrate are stored in the base  103  and the inversion region  106 . However, in the case where irradiation of intense light is made, although a part of holes of the generated photo charges reach the base  103  in the light receiving element region  110  and are stored, the remainder is diffused or drifted to another light receiving element or negative potential region in the vicinity. Besides, the generated recombination center of the interface of the insulating film between the gate electrode  105  of the light receiving MOS diode and the inversion region  106  influences the amount of stored photo charge, and becomes an unstable factor for photoelectric characteristics. In order to prevent this, a potential Vbc of the base  103  and the inversion region  106  is made 2.5 V at the maximum. By this, the maximum electric field intensity in the insulating film becomes 0.7 MV/cm or less, and the amount of electric charge flowing in the film is suppressed. FIG. 4A shows a case where a voltage of the emitter region of the phototransistor  201  is limited, and when a voltage between the base and collector of the phototransistor  201  is Vbc, a capacitance between the base and collector is Cbc, a current amplification coefficient is hfe, and a capacitance value of wiring capacitance  206  is CL, a limit voltage Vlim of the limiter circuit  205  is set so as to be expressed by the following equation. 
     
       
           V lim= Vbc /(1 +CL/hfe·Cbc )=2.5/(1 +CL/hfe·Cbc ) 
       
     
     FIG. 4B is a second circuit diagram of a light receiving element of a linear image sensor showing an embodiment of the present invention. Like this, such a structure is adopted that the voltage of the base region of the phototransistor  201  is directly controlled. For convenience, although the explanation has been made on the case where the light receiving MOS diode has the electrode, it is also acceptable to make such a case that the light receiving MOS diode has no gate and the base region of the phototransistor  201  is floating, and it is possible to suppress diffusion or drift of photo charges to another light receiving element region or negative potential region in the vicinity. 
     FIG. 5A is a first circuit diagram of a limiter circuit of a linear image sensor showing an embodiment of the present invention. A source of an n-channel MOS transistor  207  and a substrate are connected to the GND, a gate electrode and a drain electrode are connected to a terminal  208 , and the voltage between the terminal  208  and the GND is limited. The added threshold voltage of this MOS transistor  207  becomes the limit voltage Vlim. In this case, since a unit light receiving circuit includes one MOS transistor, the area of a cell can be reduced. FIG. 5B is a second circuit diagram of a limiter circuit of a linear image sensor showing an embodiment of the present invention. A p-type region of a diode  209  formed of a pn junction is connected to the GND, an n-type region is connected to a terminal  210 , and the voltage between the terminal  209  and the GND is limited. 
     The breakdown voltage of this diode in a reverse direction becomes the limit voltage Vlim. FIG. 5C is a third circuit diagram of a limiter circuit of a linear image sensor showing an embodiment of the present invention. Plural stages of n-channel MOS transistors are connected in series to each other, and only in a MOS transistor  211 , a source and a substrate are connected to the GND, and a gate electrode is connected to a drain electrode. A source of some n-channel MOS transistor between the MOS transistor  211  and a terminal  213  is connected to a drain of a next n-channel MOS transistor. A gate electrode and a drain electrode of an n-channel MOS transistor  212  at the side nearest the terminal are connected to the terminal  213 , and the voltage between the terminal  213  and the GND is limited. The threshold voltage for the number of stages of the MOS transistors from the MOS transistor  211  becomes the limit voltage Vlim. Thus, the circuit of FIG. 5A can be constructed by the same diffusion as a C-MOS region like FIG. 2, FIG. 5B can be constructed by N-channel MOS transistors with the same threshold voltage, and formation can be made without change of the number of process steps. 
     FIG. 6 is a circuit diagram of a linear image sensor showing a second embodiment of the present invention. This is constituted by a phototransistor  201  and a p-channel MOS transistor  202  of a light receiving MOS diode for reading image information, a switching n-channel MOS transistor  203  for reading a signal obtained by the phototransistor  201  and the MOS transistor  202  to the outside, a scanning circuit  216  for driving a shift register by receiving a start pulse ΦSI inputted from a terminal  214  for scanning and by receiving a clock ΦCLK inputted from a terminal  215  for scanning, a waiting state detecting circuit  217  for detecting a waiting state by receiving the start pulse ΦSI inputted from the terminal  214  for detection of the waiting state and by receiving the clock ΦCLK inputted from the terminal  215  for detection of the waiting state, and a control circuit  218  for controlling the MOS transistor  202  by receiving a control pulse ΦCTRL outputted from the waiting state detecting circuit  217 . Here, there are included blocks each having the same structure as the block of a unit light receiving circuit constituted by the phototransistor  201 , the MOS transistor  202 , the MOS transistor  203 , and the control circuit  218 , the number of blocks being equal to at least that of output bits. 
     FIG. 7A is a first time chart diagram of a unit light receiving element of the linear image sensor showing the second embodiment of the present invention. When the start signal ΦSI is inputted, synchronously with the clock pulse ΦCLK, ΦSch- 1  to ΦSch-n are sequentially inputted to the gate of the MOS transistor  202 , and in the same manner, synchronously with the clock pulse ΦCLK, ΦQ 1  to ΦQn are sequentially inputted to the gate of the MOS transistor  203 , and the output of the phototransistor  201  of each bit is read to the common signal line  204 . When the output of all bits is ended, a waiting state starts from a period Ti, and the potentials of the clock pulse ΦCLK and the start signal ΦSI are fixed to VDD and GND, and when the control pulse ΦCTRL of the waiting state detecting circuit  217  becomes high, ΦSch- 1  to ΦSch-n become high, and VDD is given to the gate of the MOS transistor  202 . By this, like FIG. 1, an electric field comes not to be applied to the interface of the insulating film of the gate electrode  105  of the light receiving MOS diode. FIG. 7B is a second time chart view of a unit light receiving element of the linear image sensor showing the second embodiment of the present invention. A waiting state starts from a period T 1 , and when the potentials of the clock pulse ΦCLK and the start signal ΦSI are fixed to VDD and GND, and when the control pulse ΦCTRL of the waiting state detecting circuit  217  becomes high, ΦSch- 1  to ΦSch-n become floating, and the gate of the MOS transistor  202  becomes floating. By this, like FIG. 1, an electric field comes not to be applied to the interface of the insulating film of the gate electrode  105  of the light receiving MOS diode. For convenience, although the explanation has been made on the case where the control pulse ΦCTR-L is high in the waiting state, it is also possible to control ΦSch- 1  to ΦSch-n by making low in the waiting state and high in other states. 
     As described above, by using the present invention, charges remaining in the base region of the phototransistor after reading were decreased and characteristics of persistence of vision were improved. 
     Moreover, by using the present invention, the characteristics of persistence of vision were improved, and it was possible to keep photoelectric conversion efficiency. 
     Moreover, by using the present invention, it was possible to reduce the capacitance between base and collector, it became easy to read signal charges, and it was also able to improve the rising characteristics at the time when the dark state was changed to the bright state. 
     Moreover, by using the present invention, it was possible to improve the uniformity of photoelectric conversion efficiency among a plurality of light receiving elements. 
     Moreover, by using the present invention, it was possible to decrease the number of elements of the scanning circuit, and to realize the inexpensive linear image sensor. 
     Moreover, by using the present invention, it was possible to reduce crosstalk between light receiving elements due to intense external light, and it was possible to suppress the surface recombination of the interface of the insulating film between the gate electrode of the light receiving MOS diode and the inversion region.