Abstract:
A linear image sensor using phototransistors as light receiving elements has improved after-image characteristics and reduced production costs by providing a light-receiving MOS diode proximate each photosensor, placing the MOS diode in an inversion state during the accumulation of photo-charge so that the accumulated photo-charge is accumulated at a base region of the phototransistors, placing the MOS diode in an accumulating state during a reset operation to return the phototransistors to an initial state after a readout operation has been performed, so that residual charge without reading out is transferred to the base region of the phototransistors and an after-image caused by residual charge is reduced through an emitter of the phototransistors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an image sensor which reads picture-information and electrically transmits it, and which is suitable for a facsimile or image scanner apparatus. 
     In a prior art linear image sensor, the light-receiving element of the linear image sensor is typically as disclosed in Japanese Patent Disclosure S61-124171. The structure is shown in FIG.  5 . In FIG. 5, symbol  4  is a separated (or separation) layer, symbol  3  is an epitaxial layer and collector, symbol  6  is a base, and symbol  9  is an emitter. 
     However, since the junction capacitance is large and the base region is in a floating state because the junction portion between the base and collector is wide, there has been a problem of an after-image in which the last signal of a bright state remains in the readout signal even when the read out image changes from a bright state to a dark state so that a distinct picture is not obtained. 
     SUMMARY OF THE INVENTION 
     To solve the above problem, the sensor of the present invention comprises a light-receiving MOS diode in which a part of a light-receiving element is covered with a light-transmissive electrode for transmitting a part of light, and letting the light-receiving MOS diode operate in an inversion state while accumulating photo-charge, so that the generated photo-charge is accumulated in the inversion region thereof and base region of phototransistor. By placing the light-receiving MOS diode in an accumulating state during a reset time after readout to the outside and resetting through the emitter in a state in which photo-charge density of the base region is raised to move residual charge without performing readout to the base region, residual charge causing an after-image is reduced. By placing the light-receiving MOS diode in the accumulating state even during readout lets photo-charge accumulated under the light-receiving MOS diode move to the base region, and enlarging the voltage between the base and emitter makes bipolar transistor operation of the phototransistor easy. This characteristic is improved even when the read picture changes from a dark state to a bright state or, conversely from the above-mentioned operation, from a bright state to a dark state, in which rising of a readout signal is a little lower than rising of a signal of enough bright state. 
     By forming the inversion region of the light-receiving diode and the base of the phototransistor with the same conductivity type, elements constructing a unit light-receiving element are reduced. In order to transfer photo-charge accumulated at the inversion region under the light-receiving MOS diode to the base of the phototransistor uniformly and surely between a plurality of light-receiving elements, the gate electrode of the light-receiving MOS diode covers at least a part of the base of the phototransistor through isolating coating. By forming a MOS transistor for switching to read a signal out from the light-receiving elements outside on a substrate of conductive (or conductivity) type which is different from the conductive type of the substrate that the light-receiving MOS diode is formed with and by making the gate electrodes common, element area is reduced. 
     By forming the base of the phototransistor with a well of a C-MOS transistor of a peripheral circuit, a base process added to C-MOS manufacturing process is reduced so as to realize a low cost linear image sensor. 
     By not applying constantly electric field of more than 0.7 MV/cm to the gate oxide film of the light-receiving MOS diode, influence of surface recombination current having impact is stable. Further by reducing threshold voltage of the light-receiving MOS diode, the diode operates keeping inversion state even at readout, and surface recombination current of oxide film boundary which is active by depletion becomes small. 
    
    
     BRIEF DESCRIPTION THE DRAWINGS 
     FIG. 1A is a sectional structural view showing a light-receiving element of a linear image sensor of the present invention at photo-charge accumulation. 
     FIG. 1B is a sectional structural view showing a light-receiving element of a linear image sensor of the present invention at photo-charge accumulation. 
     FIG. 1C is a sectional structural view showing a light-receiving element of a linear image sensor of the present invention at readout and reset time. 
     FIG. 2A is a sectional structural view showing a light-receiving element and a switching element of a linear image sensor of the present invention. 
     FIG. 2B is a circuit diagram of a light-receiving element and a switching element in a linear image sensor of the present invention. 
     FIG. 2C is a timing chart of a light-receiving element and a switching element in a linear image sensor of the present invention. 
     FIG. 3 is a sectional structural view showing a light-receiving element and a switching element of an another embodiment in a linear image sensor of the present invention, which are formed with p well base. 
     FIG. 4 is a block diagram of a linear image sensor of the present invention. 
     FIG. 5 is a sectional structural view showing a light-receiving element of the prior linear image sensor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the attached figures, embodiments of the present invention will be described bellow. FIG. 1A is a sectional structural view showing an embodiment of a light-receiving element in a linear image sensor of the present invention. 
     Symbol  101  is an n-type silicon semiconductor substrate, symbol  102  is an n-type region formed to supply voltage to the semiconductor substrate and being high in impurity density, symbol  112  is a metal electrode thereof, symbol  103  is a p-type base region of a bipolar transistor, symbol  113  is a metal electrode thereof, symbol  104  is an n-type emitter region of the bipolar transistor, symbol  114  is a metal electrode thereof, and symbol  105  is a gate electrode of a light-receiving MOS diode made of polycrystalline silicon transmitting at least a part of light. Symbol  106  shows an inversion region when negative voltage is applied to the gate electrode  105  of the light-receiving MOS diode and the surface of the n-type silicon semiconductor substrate  101  turns in inversion state and the n-type silicon semiconductor substrate  101  forms a substrate region of the light-receiving MOS diode and a collector region of the phototransistor. Symbol  107  is a shielding layer, symbol  108  is a hyaline protection layer for transmitting light, and symbol  110  is a light-receiving element region formed with the phototransistor and the light-receiving MOS diode. Next, referring to FIG. 1B, a state in which light is applied will be described. When a light is applied to the light-receiving element region  110  during accumulation in which an original picture is being read out, positive holes of photo-charge generated in the n-type silicon semiconductor substrate  101  is accumulated in the base  103  or the inversion region  106 . Although a part of the positive holes of the generated photo-charge reaches the base  103  in the light-receiving element region  110  so as to accumulate at the base, the rest does not reach the base  103  and distinguishes, and diffuses or drifts to near other light-receiving element region or negative voltage region. As a result, photo-charge generated at the light-receiving element region is not accumulated and can not be read out to the outside. By forming the inversion region  106  under the light-receiving MOS diode and accumulating the charge, similar photoelectric transfer efficiency with a light-receiving element comprising a phototransistor having a wide base region is obtained. Next, in FIG. 1C, a readout operation of the picture signal and a reset operation for returning the light-receiving element to an initial condition are shown. After the accumulation operation, a readout switch is placed in an ON state and a picture signal is read out to the outside. At readout time, by applying positive voltage to the gate electrode  105 , placing the surface of the n-type silicon semiconductor substrate  101  under the light-receiving MOS diode in an accumulating state from an inversion state, driving the positive holes of the photo-charge accumulated by the previous state in the inversion region  106  to the base region  103 , and enlarging the voltage between the base and emitter, bipolar transistor operation of the phototransistor becomes easy. As a result, the characteristic is improved in which rising of a readout signal from a dark state to a bright state is a little smaller than rising of a signal of enough bright state when the read picture changes from dark state to bright state. After readout to the outside, at a reset time for returning the light-receiving element to the initial state, positive voltage is applied to the gate electrode  105  continuously, and the surface of the n-type semiconductor substrate under the light-receiving MOS diode is kept in the accumulating state. In that manner, residual charge without- readout to the outside is reset to a reset voltage through the emitter  104  while keeping a current moving in the base region  103  and increasing the photo-charge density of the base region, and residual charge causing after-image is reduced. 
     By forming the inversion region  106  of the light-receiving diode and base  103  of the phototransistor with the same conductive type, elements constructing a unit light-receiving element are reduced. In order to transfer photo-charge accumulated at the inversion region  106  under the light-receiving MOS diode to the base  103  of the phototransistor surely, the gate electrode  105  of the light-receiving MOS diode covers at least a part of the base of the phototransistor through isolation layer. 
     FIG. 2A is a sectional structural view of another embodiment of the present invention, which includes switching elements or reset elements comprising MOS transistors. Adding to the light-receiving element region  110  shown FIG. 1A, symbol  109  is a metal thin film for shielding, symbol  121  is a p-well conductive type of which is different from the n-type silicon semiconductor substrate  101 , symbols  123  and  124  are a drain region and source region of an n-channel MOS transistor for switching formed in the p-well  121 , symbol  128  is a gate electrode of the MOS transistor, and symbol  127  as a leading electrode from the drain region connected to the metal electrode of said emitter  104  so as to be same voltage. Symbols  125  and  126  are a drain region and a source region of the n-channel MOS transistor for reset formed in the p-well  121 , symbol  130  is a gate electrode of the MOS transistor, symbol  131  is a leading electrode from the source region  126  connected to a diffusion region  122  applying Gnd voltage being reset voltage. The conductive type of the diffusion region  123  is the same as the p-well  121 , and voltage is applied the p-well too. The source region  124  of said MOS transistor for switching and the drain region  125  of the MOS transistor for reset are connected with a metal wiring  129 . The metal wiring  129  is connected to a circuit for reading the signal obtained at the light-receiving element out too. Referring to a circuit diagram of FIG. 2B and a timing chart of FIG. 2C, operation of the construction well be described. Readout switching pulse φSWi for reading the signal generated at the light-receiving portion out to outside is applied to the gate electrode  105  of the light-receiving MOS diode  105  and the gate electrode  128  of n-channel MOS transistor for switching, and picture signal is read out to outside as the signal φSWi for signal readout period. After that, reset pulse φRST is applied to the gate  130  of the n-channel MOS transistor for reset to initialize the light-receiving-element for reset period of the later half in high state of switching pulse φSWi. The photo-charge accumulating period is a period that the switching pulse φSWi is applied again and output to signal φSIG as a picture signal. In this manner, the gate electrode  105  of the light-receiving MOS diode and the gate electrode  128  of the n-channel MOS transistor for switching are connected at readout and reset time. At photo-charge accumulating, negative voltage is applied to the gate electrode  105  of the light-receiving MOS diode and the gate electrode  128  of the n-channel MOS transistor for switching at the same time so as to form the inversion region  106 , the switching MOS transistor is turned in non-conductive state, and photo-charge can be accumulated at the base  103  and the inversion region  106 . At readout and reset time, positive voltage is applied to the gate electrode  105  of the light-receiving MOS diode and the gate electrode  128  of the n-channel MOS transistor, for switching at the same time, the n-channel MOS transistor for switching is turned in conductive state. By changing the inversion region  106  under the light-receiving MOS diode to accumulating region, moving the photo-charge accumulated in the inversion region  106 , and enlarging voltage between the base and emitter, readout and reset becomes easy. As the gate electrode  105  of the light-receiving MOS diode covers at least a part of the base region of the phototransistor through an isolation layer, photo-charge accumulated in the inversion region  106  can be moved surely to the base region  103  so that uniform photoelectric efficiency is obtained between a plural of light-receiving elements. Thus, the gate electrode  105  of the light-receiving MOS diode and the gate electrode  128  of the n-channel MOS transistor for switching are controlled at the same scanning circuit, and numbers of elements constructing the circuit of the linear image sensor can be reduced. In other words, by forming the MOS transistor for switching reading the signal from the light-receiving element out to outside on the substrate of conductive type which is different from conductive type of substrate in which the light-receiving MOS diode is formed, and by making the gate electrodes common, size of the circuit becomes small and uniform photoelectric efficiency is obtained between a plural of light-receiving elements. Reset operation is carried out by the following steps: to keep the MOS transistor for switching in conductive state after end of readout; to make the MOS transistor for reset conductive state from non-conductive state of readout time; and to apply reset voltage to the emitter  102  of the phototransistor through the MOS transistor for reset and MOS transistor for switching. Although the image sensor is described using the n-channel silicon semiconductor substrate  101  for convenience sake, a p-channel silicon semiconductor substrate too is useful. Although polycrystalline silicon is used for the gate electrode  105  of the light-receiving MOS diode, transparent conductive thin film as ITO or the like too is useful. 
     FIG. 3 is a sectional structural view of another embodiment of the present invention, which includes a switching element and a reset element comprising MOS transistor. a p-well base  121   b  formed, at the same time when the p-well  121  used for C-MOS transistor is formed replaces the base of FIG.  1 A and FIG. 2A, and a light-receiving element is used, in which a diffusion region  121   c  has the same conductive type as the well diffusion region on at least a part of surface of the p-well base and high impurity density. Thus, by forming the base of the phototransistor with a well of a C-MOS transistor of a peripheral circuit at the same time, a base process added to C-MOS manufacturing process is reduced so as to realize a low cost linear image sensor. Although a diffusion region  121   c  may be excluded by replacing the diffusion region  121   c  with the p-well base  121   b , the existence of the diffusion region  121   c  shown in FIG. 3 makes better performance in the temperature characteristic and the like. 
     The way of operation accords with FIG.  2 B and FIG.  2 C. 
     Although two states are described here, which generate by voltage of the gate electrode  105  of the light-receiving diode, and which are the inversion state and the accumulating sate of silicon surface under the gate electrode  105  of the light-receiving diode, more stable operation for operation voltage and operation temperature is realized by varying electric field intensity and threshold of the MOS diode forming the light-receiving diode. 
     Surface recombination generating at the inversion region of the light-receiving diode and surface of isolation formed thereon has an impact to accumulated photo-charge value and causes unstable photoelectric characteristic. Thickness of the isolation layer between the n-type silicon semiconductor substrate  101  and the gate electrode  105  is more than 800 angstrom, and voltage between the gate electrode  105  and the n-type silicon semiconductor substrate  101  is 5.5 V maximum. By that, the maximum electric field intensity is less than 0.7 MV/cm, and charge value flowing in the layer is held down so as to hold increase of surface recombination down. By introducing n-type impurity moderately to surface of the n-type silicon semiconductor substrate, the inversion region  106  is formed under the light-receiving MOS diode even at readout and reset time. the inversion region  106  always exists at surface of the n-type silicon semiconductor substrate so as to hold activation of surface recombination occurring in depletion. By holding the surface recombination down and making stable, a stable photoelectric characteristic is obtained as the result. 
     FIG. 4 is a block diagram showing an embodiment of the present invention. The circuit comprises a light-receiving element array having a plural of phototransistors, an analog switch array having a plural of switching elements, a shift resistor being a scanning circuit, a scanning input circuit driving the shift resistor, a clock buffer, and an output circuit outputting the output signal of the scanning circuit and a picture signal obtained at the light-receiving element. 
     As above-mentioned, by using the present invention, the following are realized: 
     charge accumulating at the base region of the phototransistor after readout is reduced so as to improve after-image characteristic; 
     after-image characteristic is improved and photoelectric efficiency is kept; 
     capacity between base and collector and readout of signal charge becomes easy so as to improve rising characteristic at rime turning to bright state from dark state; 
     uniformity of photoelectric efficiency between a plural of the light receiving elements is improved; 
     the number of the elements of the scanning circuit is reduced so as to realize a low cost linear image sensor; and 
     the base process formed adding to the C-MOS manufacturing process is cut so as to realize a low cost linear image sensor.