Abstract:
A phototransistor used for an image sensor is provided. The phototransistor can reduce a dark current that occurs in the phototransistor and improve sensitivity at low luminance without crosstalk with a neighboring pixel or an image lag by including a buried collector. In the phototransistor including the buried collector, since the collector is not directly connected to outside, the phototransistor has a low dark current and a high photosensitive characteristic at low luminance. Since each image sensor is isolated, crosstalk between pixels or an image lag does not occur.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a phototransistor used for an image sensor, and more particularly, to a phototransistor capable of reducing a dark current that occurs in the phototransistor and improving sensitivity at low luminance without crosstalk with a neighboring pixel or an image lag by including a buried collector. 
         [0003]    2. Description of the Related Art 
         [0004]    An image sensor is used to measure strength of light. An image sensor with high sensitivity is constructed with a plurality of phototransistors. In general, the image sensor with high sensitivity uses phototransistors having a laminated or integrated structure. 
         [0005]    A phototransistor generates an output current proportional to strength of incident light. Photons which are absorbed in a junction region between a collector and a base or a junction region between an emitter and a base of the phototransistor generate electron-hole pairs collected by a PN junction. Minority carriers collected by the junction operate as a base current. The base current is multiplexed based on a transistor gain, thereby generating a collector current. In addition, an emitter current is a sum of the base current and the collector current and generally used as an output current. 
         [0006]      FIG. 1  illustrates a phototransistor having a laminated structure according to a conventional technique. 
         [0007]    As shown in  FIG. 1 , a conventional phototransistor having a laminated structure is constructed with an n+ emitter  115 , a p type base  113 , and an n type collector  112 . In addition, the conventional phototransistor may further include an intrinsic absorption layer  114  between the emitter  115  and the base  113 . This structure may be constructed with a p+ emitter, an n type base, and a p type collector. 
         [0008]    In the aforementioned structure, when a voltage is applied to the emitter  115  and the collector  112 , a weak current flows between the base  113  and the collector  112 . However, a barrier is formed between the emitter  115  and the base  113 . The current is limited due to the barrier. At this time, when photons are incident onto the base  113 , electron-hole pairs (EHPs) are generated due to the photons. Electrons move to the collector  112 . Holes fill the barrier formed in the base  113 . 
         [0009]    Accordingly, as the number of holes increases, the potential of the barrier is lowered due to the holes collected in the barrier, thereby generating a current flow from the emitter to the collector in proportion to the decrease in potential. Thus, a current flows. The current introduced into the collector is greater than an optical current due to the electron-hole pairs generated by the incident light, at all times. The aforementioned procedure may be understood by using  FIG. 2 . 
         [0010]      FIG. 2  illustrates operation mechanism of a general phototransistor. 
         [0011]    As shown in  FIG. 2 , when a voltage is applied between an emitter and a collector, a barrier is formed in a region between the emitter and a base (a of  FIG. 2 ). At this time, when light is incident onto the base, electrons move to the collector, and holes are collected in the barrier. 
         [0012]    The potential of the barrier is lowered due to the holes collected in the barrier (b of  FIG. 2 ). Accordingly, electrons of the emitter can easily move to the collector. That is, although the number of the collected holes is small, a large current flows. 
         [0013]    Since the conventional phototransistor having the laminated structure is manufactured by using a deposition method or a crystal growth method, it is very limited to apply a procedure of manufacturing the conventional phototransistor to a general procedure of manufacturing a complementary metal oxide semiconductor (CMOS). 
         [0014]    In addition, since it is difficult to deal with a dark current generated at a surface, a change between pixels is large. Accordingly, it is difficult to apply the conventional phototransistor to an element that requires a uniform pixel characteristic such as an image sensor. 
         [0015]      FIG. 3  illustrates a phototransistor having an integrated structure according to a conventional technique. 
         [0016]    Referring to  FIG. 3 , the conventional phototransistor having the integrated structure is constructed with two electrodes poly-1 and poly-2, an n+ emitter, a p type base, and an n type well or collector. In addition, a field oxide layer (FOX) for separating pixels from one another is formed. 
         [0017]    At this time, in a state where a negative voltage is applied to the emitter and a negative voltage is applied to an electrode poly-2, when light is incident onto the p type base, electron-hole pairs are generated. Electrons among the generated electron-hole pairs move to the n type well (collector), and holes remain in the base, thereby increasing potential of the base. 
         [0018]    When a negative voltage is applied to the electrode poly-2, holes remaining in the base are dragged into the electrode poly-2. Accordingly, holes proportional to an amount of light are collected in the electrode poly-2. In this state, since only a current caused by movement of electrons exists, a weak current flows. This state is referred to as a hole accumulation state. 
         [0019]    A procedure of reading an image is achieved by applying a positive or zero voltage to the electrode poly-2. That is, when the positive voltage is applied to the electrode poly-2, a junction between the base and the emitter is forwardly biased. Accordingly, holes collected in the hole accumulation state are pushed out due to repulsive force. At this time, most holes exist between the emitter and the collector. Accordingly, since the barrier between the base and the emitter is lowered due to the holes, a current related to the amount of holes collected in the base flows between the emitter and the collector. 
         [0020]    In a case where strong light is incident onto the base of the phototransistor having the aforementioned structure, the base potential is rapidly increased due to the holes generated by the light until the junction between the base and the emitter is weakly biased in the forward direction. In addition, holes that are generated in the base fill a region under the electrode poly-2 and flow to the emitter. Accordingly, a large current flows in an electrode poly-1 in the hole accumulation state. This large current is referred to as an overflow. This large current operates as noise of a neighboring pixel. In addition, since it is difficult to remove holes of a pixel exposed to strong light, an image lag in which bright light is dragged occurs, thereby deteriorating image quality. 
         [0021]    On the other hand, the emitter is connected to the base through the surface, and the base is connected to a ground through the surface. Accordingly, a dark current may occur in low luminance. 
       SUMMARY OF THE INVENTION 
       [0022]    The present invention provides a phototransistor used for an image sensor, which includes a buried collector, the phototransistor with high sensitivity in a dark state due to a small dark current without noise caused by a neighboring pixel and an image lag in a bright state. 
         [0023]    According to an aspect of the present invention, there is provided a phototransistor including a buried collector, the phototransistor comprising: a first conductive emitter; a second conductive base; and a first conductive collector buried in the base. 
         [0024]    In the above aspect of the present invention, the phototransistor may be separated from a neighboring phototransistor through the emitter. 
         [0025]    In addition, the collector may be electrically connected through a metal oxide semiconductor (MOS). 
         [0026]    In addition, the emitter may be doped with a high concentration. 
         [0027]    In addition, the base may be doped with a low concentration. 
         [0028]    In addition, the collector may be doped with a low concentration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  illustrates a phototransistor having a laminated structure according to a conventional technique. 
           [0030]      FIG. 2  illustrates operation mechanism of a general phototransistor. 
           [0031]      FIG. 3  illustrates a phototransistor having an integrated structure according to a conventional technique. 
           [0032]      FIG. 4  illustrates a structure of a phototransistor including a buried collector according to an embodiment of the present invention. 
           [0033]      FIG. 5  illustrates electronic potential of the phototransistor including the buried collector according to the embodiment of the present invention. 
           [0034]      FIG. 6  is a top plan view illustrating an array of phototransistors including a buried collector according to an embodiment of the present invention. 
           [0035]      FIG. 7  is a cross sectional view illustrating the array of the phototransistors including the buried collector according to the embodiment of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    Hereinafter, the present invention will be described in detail by explaining exemplary embodiments of the invention with reference to the attached drawings. 
         [0037]      FIG. 4  illustrates a structure of a phototransistor including a buried collector according to an embodiment of the present invention.  FIG. 5  illustrates electronic potential of the phototransistor including the buried collector according to the embodiment of the present invention. 
         [0038]    Referring to  FIG. 4 , the phototransistor including the buried collector according to the embodiment includes an N+ type emitter  430 , a P type base  440 , and an N type collector  450  which are sequentially formed on a P+ type substrate  410 . 
         [0039]    The N+ type emitter  430  encloses the base  440  and the collector  450 . The N+ type emitter  430  is separated from a neighboring phototransistor. In addition, the N type collector  450  is completely buried into the p type base  440 . 
         [0040]    The collector  450  is electrically connected through a transfer transistor gate  460  of transfer transistor. A floating diffusion region  470  is connected to a constant voltage Vdd through a gate  480  of a reset transistor. The emitter  430  is connected to an emitter voltage Vem. 
         [0041]    An example of doping concentration on a line that connects a point a of the collector  450  shown in  FIG. 4  to a point b of the P+ substrate  410  is shown in (1) of 
         [0042]      FIG. 5 . That is, the collector  450  is doped with N type dopants in low concentration on the substrate  450  that is doped with P+ type dopants in high concentration. The base is doped with P type dopants in low concentration. The emitter  430  is doped with N+ type dopants in high concentration. 
         [0043]    In the aforementioned doped state, in a case where an external voltage is not applied, a depletion region or a space charge region is formed in a PN junction region. Accordingly, distribution of the space charge is shown in (2) of  FIG. 5 . 
         [0044]    Here, in a case where a voltage Vtx of the gate  460  of the transfer transistor, a voltage Vrx of the gate  480  of the reset transistor, and the constant voltage Vdd are positive, and a zero voltage is applied to the voltage Vem of the emitter, the depletion region is extended. The distribution of the space charge is shown in (3) of  FIG. 5 . 
         [0045]    An electric field due to the distribution of the space charge is shown in (4) of  FIG. 5 . A conduction band and a valence band due to the distribution of the space charge are shown in (5) of  FIG. 5 . 
         [0046]    In the aforementioned energy band, when the voltage Vtx of the gate  460  of the transfer transistor is fixed to 0, the collector  450  is isolated. No current flows due to the barrier between the base  440  and the emitter  430 . 
         [0047]    In this state, in a case where light is incident onto regions of the emitter  430  and the collector  450 , electron-hole pairs are generated. Like a general phototransistor, electrons move to the collector  450 , and holes fill the barrier formed in the base  440 . The holes collected in the base  440  lowers potential of the barrier between the base  440  and the emitter  430 . Accordingly, in a case where a large amount of holes are collected, a large amount of electrons are introduced from the emitter  430  to the collector  450 . Accordingly, since electrons (photoelectrons) generated due to photons and electrons that move from the emitter  430  to the collector  450  are collected in the collector  450 , a larger amount of electrons than that of electrons generated due to incident light exist in the collector  450 . 
         [0048]    When the transfer transistor is turned on, the collected electrons move to the floating diffusion region  470 . The electrons are converted into a voltage through the same procedure as the photodiode. 
         [0049]    Since the phototransistor having the aforementioned structure is completely buried into the base, noise occurring at a surface is not introduced into the collector, thereby reducing noise. In addition, since the phototransistor is separated from a neighboring pixel by the emitter, it is possible to reduce crosstalk occurring between neighboring pixels. In addition, in a case where the phototransistor is exposed to strong light, the voltage of the isolated collector rapidly becomes 0. Accordingly, since no more current flow, an overflow does not occur. 
         [0050]      FIG. 6  is a top plan view illustrating an array of phototransistors including a buried collector according to an embodiment of the present invention.  FIG. 7  is a cross sectional view illustrating the array of the phototransistors including the buried collector according to the embodiment of  FIG. 6 . 
         [0051]    As shown in  FIG. 6 , emitters  631   a  and  631   b  of photodiodes located in the same row are electrically connected to each other and emitters  631   a  and  632   a  of photodiodes located in different row are separated from each other through a row separation region  620 . Referring to  FIG. 7 , in the same layout as that of 
         [0052]      FIG. 6 , cross sectional structures taken along the lines A-A′, B-B′, and C-C′ are known. Here, a shallow trench isolation (STI) region may be used as a row separation region. 
         [0053]    An emitter completely includes a base and a collector of a unit pixel. The emitter is separated from a neighboring pixel. Accordingly, no crosstalk occurs due to movement of electrons between neighboring pixels. 
         [0054]    Up to now, a phototransistor including an N+ type emitter, a P-type base, an N type buried collector, which are formed on a P type substrate, has been described. However, a phototransistor including a P+ type emitter, an N type base, and a P type buried collector on an N type substrate may be also available. 
         [0055]    In the phototransistor including the buried collector according to an embodiment of the present invention, since the buried collector is electrically connected to outside through a metal oxide semiconductor (MOS), less noise occur. In addition, since a pixel is separated from a neighboring pixel through an emitter region, crosstalk caused by movement of electrons from the neighboring pixel, which may generally occur in a general image sensor, does not occur. 
         [0056]    In addition, in a case where the phototransistor is exposed to strong light, when holes are introduced into the emitter, since the collector is isolated, the voltage of the collector is substantially zero. No more current flows. Thus, noise does not occur in the phototransistor. Since holes accumulated due to the strong light is removed by making the voltage of the emitter negative, an image lag in which an image of the strong light is dragged does not occur. 
         [0057]    While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.