Abstract:
A novel structure of photo sensor is disclosed. The equivalent circuit of the invented photo sensor comprises a photo transistor integrated with a surface photo sensor. The structure of the surface photo sensor is substantially identical to the base-emitter junction of the photo transistor and may be prepared in the same process. The junction depletion region of the surface photo sensor locates adjacent to the light incident surface, whereby decay of incident light is minimal and more electron-hole pairs are generated. The present invention also discloses semiconductor material containing the invented photo sensor assembly of the invented photo sensor and method for preparation of the photo sensor, the semiconductor material and their assemblies.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a novel photo sensor and its preparation method, especially to a novel structure of photo sensor and its preparation method. The present invention discloses semiconductor material for the invented photo sensor and its assembly, and assembly comprising the invented photo sensor, and method for preparation thereof. The present invention discloses a mechanism for photo sensor that provides additional photo sensing region adjacent to surface of photo transistor in order to enhance the sensitivity of photo sensor, as well as process for preparation of the mechanism.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the conventional photo transistor, taking the NPN type photo transistor as an example, the major photo sensing region locates at the base-collector junction. When the depletion region at the base-collector junction receives incident light, electron-hole pairs are generated. Holes with positive charge are swapped from the depletion region by the built-in electrical field and enter the base region, forming photo current. Such photo current excites greater emitter current and forward biases the base-emitter junction, thus generates a collector terminal current that has a gain.  
         [0003]      FIG. 1  depicts the structure of a conventional photo transistor. As shown in this figure, a conventional photo transistor has a substrate  10  and an N-P-N layer structure formed on the substrate, wherein each layer has different kind and concentration of dopants. The N-P-N layer structure includes an N layer  11  at bottom, a P layer  12  in middle and an N layer  13  at top. At the border of the P layer  12  and the N layers  11 ,  13  are junctions. Electrodes are prepared at the N layer  11 , the P layer  12  and the N layer  13 . A photo transistor is thus prepared. The final structure of such a conventional photo transistor is shown in  FIG. 2 .  
         [0004]     In the photo transistor as shown in  FIG. 2 , the upper N layer  11  functions as light receiving surface. The electrode  21  connected to this N layer is the emitter of the photo transistor. Electrode  23  of the N layer  13  at the opposite side functions as collector. Electrode  22  of the P layer  12  is its base. Junction  12   a  between the base layer  12  and the collector layer  13  functions as photo detecting junction. The operation of the photo transistor was as described above. If the photo transistor is a P-N-P type, it would have similar structure and operation as the N-P-N type.  
         [0005]     When light beams project to a semiconductor layer, power of the light would exponentially decay along its penetration depth. As a result, light with the strongest power may be detected at the surface region of the semiconductor layer. If the photo detection region  12   a  of the photo transistor is positioned at this surface region, better detection efficiency may be obtained. However, the photo detection region of the conventional photo transistor is the depletion region of its base-collector junction, which is not positioned at or close to the surface of the semiconductor layer. Improvements in the photo detection efficiency are needed.  
         [0006]     To estimate range of wavelength of detectable light of a semiconductor material, the following formula may be used:  
         λ   =     1.24     E   ⁢           ⁢   g         ,       
 
 wherein λ is wavelength of detectable light (μm); Eg represents energy band-gap of the semiconductor material (eV). 
 
         [0007]     From the above equation it may be known that, since the energy band-gap of single-crystalline silicon is about 1.12 eV, wavelength of light detectable by conventional pure silicon-based photo transistor is approximately smaller than 1,100 nm. In order to detect grater wavelengths such as 1,310 nm or 1,550 nm, as used in the optical fiber communication system, III-V semiconductor components are used. However, the III-V materials are expensive and their preparation process is not compatible with the most popular silicon base CMOS processes. It is thus necessary to provide a novel photo sensor that is able to detect wavelengths in a range covering what are used in the optical fiber communication systems. It is also necessary to provide a novel photo sensor whose preparation process may be compatible with the most popular silicon base CMOS processes.  
       OBJECTIVES OF THE INVENTION  
       [0008]     The objective of this invention is to provide a novel photo sensor, wherein photo detective region is provided at adjacent to its surface, so to improve the detection efficiency of photo sensors.  
         [0009]     Another objective of this invention is to provide a photo sensor with effectively extended range of detectable wavelengths.  
         [0010]     Another objective of this invention is to provide a photo sensor whose preparation process may be compatible with popular silicon-based semiconductor processes.  
         [0011]     Another objective of this invention is to provide a photo sensor with an enlarged base-emitter junction whereby its light detective region is enlarged.  
         [0012]     Another objective of this invention is to provide a photo sensor with modified base-emitter junction, whereby range of detectible wavelength is extended.  
         [0013]     Another objective of this invention is to provide a novel process for preparation of photo sensor that is compatible with popular silicon-based semiconductor processes.  
         [0014]     Another objective of this invention is to provide a method for preparation of photo sensor that is compatible with the standard Silicon-Germanium BiCMOS process.  
       SUMMARY OF THE INVENTION  
       [0015]     According to this invention, a novel photo sensor is disclosed. The invented photo sensor comprises: a first polar semiconductor layer; a second polar semiconductor layer exhibiting a polarity opposite to that of said first polar semiconductor layer, surrounded by said first polar semiconductor layer and having a junction with said first polar semiconductor layer and a region exposed to incident light; a third polar semiconductor layer exhibiting a polarity opposite to that of said second polar semiconductor layer, surrounded by said second polar semiconductor layer and having a junction with said second polar semiconductor layer and a region exposed to said incident light; a fourth polar semiconductor layer exhibiting a polarity opposite to that of said second polar semiconductor layer, surrounded by said second polar semiconductor layer and having a junction with said second polar semiconductor layer and a region exposed to said incident light; and necessary electrodes to pick up photo detection signals from said photo sensor; wherein said third polar semiconductor layer and said fourth polar semiconductor layer are isolated.  
         [0016]     The equivalent circuit of the invented photo sensor may be understood as a conventional photo transistor integrated with a surface photo sensor. The structure of the surface photo sensor is in substance identical with the emitter-base structure of the photo transistor and may be prepared in the preparation of the photo transistor. The depletion region at junction of the surface photo sensor is positioned at adjacent to the light incident surface of the element, so to detect incident light at its surface regions and to generate electron-hole pairs in a larger quantity. When an N-P-N type photo transistor is included in the photo sensor of this invention, holes generated by incident light may enter the base of the photo transistor directly. As a result, greater output current may be obtained at the collector of the photo transistor. This invention also discloses semiconductor material comprising the invented photo sensor, assembly comprising the invented photo sensor and methods for preparation of said photo sensor, said semiconductor material and said photo sensor assembly.  
         [0017]     These and other objectives and advantages of this invention may be clearly understood from the detailed description by referring to the following drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  depicts the structure of a conventional photo transistor.  
         [0019]      FIG. 2  shows the structure of a conventional photo transistor with electrodes.  
         [0020]      FIG. 3  depicts the structure of semiconductor material in the preparation of the photo sensor of this invention.  
         [0021]      FIG. 4  is the structure of the photo sensor of this invention with electrodes.  
         [0022]      FIG. 5  shows the flowchart of method for preparation of the photo sensor semiconductor material of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     The followings are detailed description of the photo sensor of the present invention, when implemented under the basic structure of a P-N-P type photo transistor.  FIG. 3  depicts the structure of semiconductor material in the preparation of the photo sensor of this invention. As shown in this figure, the semiconductor material comprises a substrate  30 ; a first P layer  33  prepared in or on top of the substrate  30 ; an N layer prepared above said first P layer  33  and within the area defined by said first P layer  33 ; a second P layer  31  prepared above said N layer  32  and within the area defined by said N layer  32 ; and a third P layer  34  prepared above said N layer  32  and within the area defined by said N layer  32  but isolated with said second P layer  31 . In addition, the semiconductor material further comprises collector electrode  43  connected to said first P layer  33 , base electrode  42  connected to said N layer  32 , emitter electrode  41  connected to said second P layer  31  and surface photo sensor electrode  44  connected to said third P layer  34 .  FIG. 4  is the final structure of the photo sensor of this invention. In this figure,  41  represents emitter of the invented photo sensor,  42  represents its base,  43  represents its collector and  44  represents the surface photo sensor electrode.  
         [0024]     As known by those skilled in the art, in the semiconductor material as shown in  FIG. 3 , if the P layers are replaced by N layers and the N layer is replace by a P layer, a circuit similar to that of  FIG. 4  may be formed. In other words, in the following detailed description, the structure as shown in  FIG. 3  will be used as an example of this invention. The present, however, shall not be limited to the structure of  FIG. 3 . For example, when the polarities shown in  FIG. 3  are reversed, a photo sensor exhibiting similar functions may also be prepared.  
         [0025]     Now refer to  FIG. 5 .  FIG. 5  shows the flowchart of method for preparation of the photo sensor semiconductor material of  FIG. 3 . Method for preparation of the semiconductor material of  FIG. 3  will be described below.  
         [0026]     In the preparation of the semiconductor material of  FIG. 3 , at first at  51  a semiconductor substrate  30  is obtained. Material for the substrate  30  may be silicon-based materials, such as materials containing silicon or its compositions, including SiGe, SiC etc., or other compound semiconductors including semiconductor materials prepared from III-V elements or II-VI elements. Then at  52  impurities are doped into selected areas of the substrate  30 , to form first P layer  33  in said areas. Any doping technology may be applied in this step. Applicable doping technologies include thermal diffusion and ion implantation. It is also possible to prepare a P type electrode layer as the substrate by doping the substrate when it is prepared. Dopants that may be added include Group III elements and other suited materials. As described above, in this embodiment a P-N-P type photo transistor will be prepared. If an N-P-N type photo transistor will be prepared, in this step the substrate shall be doped to form an N layer. As a result, dopants may be Group V elements and other materials suited in forming an N layer. As to concentration of dopants, reaction temperature, pressure and time, they may be determined according to practical needs. No particular requirements or limitations in these conditions, as long as the P layer  33  so prepared may exhibit standard features of the positive polarity. The P layer  33  so prepared will function as collector  43  of the photo sensor.  
         [0027]     Thereafter, at  53  an N layer  32  is formed in the first P layer  33  at selected areas. In this embodiment, N layer  32  is surrounded by the first P layer  33 . The N layer  32  may be formed by doping impurities using any available method, including thermal diffusion and ion implantation. It is also possible to form the N layer  32  above selected areas within the area defined by the first P layer  33 . When forming the N layer  32 , any applicable method may be used. For example, it is possible to deposit a material layer on the first P layer  33  and then dope in the added material layer to perform the negative polarity. It is also possible to dope the added material layer during its preparation, so to form the N layer  32  directly. Here, any available method in forming and doping the material layer may be applied. The process in this step is similar to that of step  52 . Detailed description is thus omitted. The N layer  32  so obtained will function as base  42  of the photo sensor.  
         [0028]     Further, at  54  second P layer  31  and third P layer  34  are formed in selected areas within the area defined by the N layer  32 . Method to form the second and third P layers  31 ,  34  may be similar with that of the previous step, provided that dopants used in this step are different from that of Step  53 . In the present invention, second and third P layers  31  and  34  are isolated without contacts between them.  
         [0029]     In the above-described process, all reaction conditions may be determined according actual needs. Materials of the substrates of the first P layer, the N layer, and second and the third P layers may be identical or different. Dopants added to first, second and third P layers may be identical or different. However, if substrate materials and dopants for second and third P layers are identical, number of steps in the process and preparation costs may be reduced. This, of course, is not any requirement or limitation. In some preferred embodiments, material of first P layer  33  may be crystalline silicon. Material for N layer  32  may be SiGe. Material for second and third P layers  31 ,  34  may be poly silicon. The second and third P layers  31 ,  34  may function as emitter layer and surface photo sensor electrode of the invented photo sensor, respectively, depending on electrodes connected thereto and concentrations of dopants.  
         [0030]     At  55 , electrodes  43 ,  42 ,  41  and  44  are connected to the first P layer  33 , the N layer  32  and the second and third P layers  31  and  34  of the photo sensing semiconductor material so prepared. The photo sensor is thus prepared. The electrodes may be connected to the related layers using any applicable method, including screen printing, deposition, spitting, vapor deposition, plating etc. The photo sensor so prepared may contain a plurality of photo sensor units prepared in wafer. Therefore, at  56  the wafer is cut to obtain units of photo sensor and the units are packaged at  57 .  
         [0031]     In some embodiments of this invention, electrodes are connected to the photo sensing semiconductor material after cutting. In addition, it is possible to form particular wires to connect a plurality of photo sensors before they are cut. Furthermore, in some other embodiments, the substrates are prepared from transparent materials. In some further embodiments a reflection layer (not shown) is provided at the lower surface (non-incident side) of the substrate  30  to further enhance its photo sensing effects. It is also preferable to prepare the electrodes using transparent materials such as ITO and TO.  
         [0032]     The photo sensor so prepared has two depletion regions to detect incident lights and to convert such lights into current outputs. If compared with the conventional photo transistor, the invented photo sensor provides an additional junction  32   b  between its N layer  32  and third P layer  34 , in addition to the junction  32   a  between its base and collector. As a result, no matter the incident light is visible light with short wavelengths (such as 400-700 nm) or long wavelength light (such as light waves with the wavelength of 1,310 nm as used in the optical fiber communication system), they may effectively detected by the invented photo sensor.  
         [0033]     Nevertheless, the equivalent circuit of the invented photo sensor includes a photo transistor (including first P layer, N layer and second P layer in the above example) and a surface photo sensor (including third P layer and N layer). In them, one terminal of the surface photo sensor happens to be base of the photo transistor. When the incident light reaches the surface photo sensor, carriers (electrons) so generated will enter the base directly, so that amplified currents are output from collector of the photo transistor. The photo reaction efficiency of this invention is thus far higher than that of the conventional photo transistors.  
         [0034]     The photo sensor of this invention may be prepared by using the standard SiGe BiCMOS process. No special process modification is needed. In the process, the structure of the surface photo sensor and the emitter-base structure of the photo transistor are identical and may be prepared simultaneously. The process is thus made simplified. With the invented structure, the junction depletion region of the SiGe surface photo sensor locates at the SiGe region. Since the energy band-gap of the SiGe material is smaller than that of pure silicon, the SiGe surface photo sensor of this invention may be used to detect lights with longer wavelengths. As a result, the detectable range may be extended to include infrared wavelengths, whereby the invented photo sensor may be used in the optical fiber communication system.  
         [0035]     In addition, in the circuit of  FIG. 4 , if base  42  and surface photo sensor electrode  44  are floating, the photo sensor may operate in the photo-voltage mode. On the other hand, if base  42  is floating and surface photo sensor electrode  44  is biased, the photo sensor may operate in the photo-current mode. More applications are thus provided.  
         [0036]     In the photo sensor of this invention, the emitter of the photo transistor does not provide any photo detection function. Therefore, it is preferable to reduce the area of the emitter in the light incident surface. On the other hand, the region of the surface photo sensor is preferably expanded to as much as possible in order to further enhance the photo detective effects. In some embodiments of the present invention, the region of the surface photo sensor electrode has a ring shape and surrounds the emitter region. Such design may further increase the photo detective effects of this invention.  
         [0037]     As the present invention has been shown and described with reference to preferred embodiments thereof, those skilled in the art will recognize that the above and other changes may be made therein without departing from the spirit and scope of the invention.