Abstract:
A diode structure includes a rectangular first doping region, and a second doping region surrounds the first doping region wherein the first doping region and the second doping region are separated by a first isolation structure. A third doping region surrounds the second doping region wherein the second doping region and the third doping region are separated by a second isolation structure. The first isolation structure, the second doping region, the second isolation structure and the third doping region are arranged in a quadruple concentric rectangular ring surrounding the first doping region.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to a diode structure. More particularly, the present invention relates to a diode with multiple-concentric-rectangular-ring structure. 
         [0003]    2. Description of the Prior Art 
         [0004]    A charge pump is a kind of circuit design which is very common in a circuit system. The charge pump is able to generate an output voltage greater in magnitude than the input voltage. The charge pump is applied widely in different kind of chips. For example, a charge pump is usually integrated in an electrical erasable programmable read only memory (EEPROM), which usually needs a voltage for the programming and erasing operations higher than the voltage available from the peripheral circuit. The integrated charge pump may multiply the voltage from the circuit, to provide an appropriate voltage for the programming and erasing operations of the memory. 
         [0005]    With the integration of the semiconductor manufacturing technology and the micro-electromechanical systems (MEMS), a semiconductor chip (biochip) which is able to detect and monitor the biological signals is developed. Generally, the electrical signal generated by an organism is very small, such as only several millivolts (mV), or even only several microvolts (μV). To detect and analyze the biological signal, a charge pump is usually integrated in a biochip to multiply the voltage of the biological electrical signal to a magnitude within the operation voltage range of the semiconductor devices embedded in the chip. 
         [0006]    The typical charge pump circuit, for example, the Dickson Charge Pump, comprises serial connected clocked diode-capacitor voltage multipliers. By controlling the capacitors&#39; charging/discharging cycle and the diodes&#39; dis-conducting/conducting cycle, the charges are pumped and the total number of the charges is multiplied successively along the diode chain in the forward-biased direction, and therefore the current is multiplied. However, in real process of the charge pumping, a portion of the current would tend to flow to the substrate instead of the ideal path aforesaid when the diode is forward-biased and conducting. The current flowing to the substrate becomes the leakage current. This phenomenon would have negative influences on the efficiency of the charge pump. Therefore, there is still a need in the field to provide a diode with better performance, which has smaller leakage current and larger forward current. 
       SUMMARY OF THE INVENTION 
       [0007]    It is one objective of the invention to provide a diode structure, which has larger forward current and smaller leakage current and better performance. 
         [0008]    According to one aspect of the present invention, a diode structure is provided. The diode comprises a substrate. A first doping region is disposed in the substrate, wherein the first doping region has a first conductivity type and is a rectangle from the top view with an aspect ratio larger than 2. A second doping region surrounding the first doping region and having a second conductivity type. A first isolation structure is disposed between the first doping region and the second doping region. A third doping region surrounds the second doping region. A second isolation structure is disposed between the second doping region and the third doping region. The first isolation structure, the second doping region, the second isolation structure and the third doping region are arranged in a quadruple-concentric-rectangular-ring surrounding the first doping region. 
         [0009]    According to one embodiment of the invention, the diode structure further comprises a connecting structure, which connects the second doping region and the third doping region. 
         [0010]    According to one embodiment of the invention, the aspect ratio of the first doping region is between 2 and 10. 
         [0011]    According to one embodiment of the invention, the aspect ratio of the first doping region is between 10 and 20. 
         [0012]    According to one embodiment of the invention, the aspect ratio of the first isolation structure, the second doping region, the second isolation structure and the third doping region is substantially the same as that of the first doping region. 
         [0013]    According to one embodiment of the invention, the first doping region, the second doping region and the first isolation structure are disposed in a first well which has the second conductivity type. 
         [0014]    According to one embodiment of the invention, the first well, the third doping region and the second isolation structure are disposed in a deep well. 
         [0015]    According to one embodiment of the invention, the third doping region and the deep well have the first conductivity type. 
         [0016]    According to one embodiment of the invention, the third doping region and the deep well have the second conductivity type. 
         [0017]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The accompanying drawings are included to provide a further understanding of the embodiments, and are incorporated in and constitute apart of this specification. The drawings illustrate some of the embodiments and, together with the description, serve to explain their principles. In the drawings: 
           [0019]      FIG. 1  and  FIG. 2  are the schematic cross-sectional diagrams of the conventional diode of the prior art. 
           [0020]      FIG. 3  is a schematic top view of a diode according to one embodiment of the invention. 
           [0021]      FIG. 4  is the schematic cross-sectional diagram taken along the line A-A′ in  FIG. 3 . 
           [0022]      FIG. 5  is the schematic cross-sectional diagram of a diode according to one embodiment of the invention. 
           [0023]      FIG. 6  shows the experimental operation conditions of “stand-by” and “on” operations of the diode as shown in  FIG. 5 . 
           [0024]      FIG. 7  shows the variation of the cathode current I E  and the leakage current I sub  of the diode as shown in  FIG. 5  in response to the varying cathode voltage V E  applied to the cathode while the anode is coupled to a fixed anode voltage. 
           [0025]      FIG. 8  and  FIG. 9  illustrate the respective variation of the cathode current I E  and the leakage current I sub  in response to the varying cathode voltage V E  of three diodes according to the present invention, where the three diodes have different aspect ratios. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The present invention will now be described with reference to the attached drawings to provide a thorough understanding. Furthermore, some system configurations and process steps are not disclosed in detail, as these should be well-known to those skilled in the art. Other embodiments maybe utilized, and structural, logical, and electrical changes may be made without departing from the scope of the present invention. 
         [0027]      FIG. 1  and  FIG. 2  are the schematic cross-sectional diagrams of two types of conventional diodes of the prior art. 
         [0028]    Please refer to  FIG. 1 , which is the cross-sectional diagram of an N + /PW diode. The heavily doped N +  doping region  200  and the P +  doping region  220  are disposed in the P well  300 , and are separated by the isolation structure  500 . When the diode is forward-biased, that is, when the electrical potential of the P +  doping region  220  is relatively positive with respect to the electrical potential of the N +  doping region  200 , the potential barrier of the P/N junction between the N +  doping region  200  and the P well  300  is lowered, allowing a forward current I f  (not shown) to flow from the P +  doping region  220 , along the P well  300  under the isolation structure  500 , and across the P/N junction between the N +  doping region  200  and the P well  300 , and finally to the N +  doping region  200 . According to the flow direction as described above, the N +  doping region  200  is considered to be the cathode of the diode, and the P +  doping region  220  is the anode. This type of diode is usually used in an amplifier or a rectifier circuit. 
         [0029]    Please refer to  FIG. 2 , which is the cross-sectional diagram of a P + /NW diode. The heavily doped N +  doping region  200  and the P +  doping region  220  are disposed in the N well  400 , and are separated by the isolation structure  500 . When the diode is reverse-biased, that is, when the electrical potential of the N +  doping region  200  is relatively positive with respect to the electrical potential of the P +  doping region  220 , the potential barrier of the P/N junction between the P +  doping region  220  and the N well  400  is enlarged, blocking the current flow, and the diode is disconnected. When the reverse bias voltage is larger than the breakdown voltage of the P/N junction between the P +  doping region  220  and the N well  400 , the junction breaks down, allowing a reverse current I r  (not shown) to flow from the N +  doping region  200  to the P +  doping region  220 . This type of diode is usually used as in a constant voltage device or an ESD (electrostatic discharge) device. 
         [0030]    However, the leakage and the insufficient efficiency problems still exist in the conventional diodes as shown previously. One objective of the present invention is to provide a diode with the multiple-concentric-rectangular-ring structure, which has better performance and lower leakage current. 
         [0031]    Please refer to  FIG. 3  and  FIG. 4 , which are the schematic top view and the corresponding cross-sectional diagram according to one embodiment of the present invention. 
         [0032]    As shown in  FIG. 3 , the diode comprises a substrate  10 . A first doping region  20  is disposed in the middle of the substrate  10 . The first doping region  20  is rectangular and has a specific aspect ratio. According to one embodiment of the present invention, for example, the aspect ratio of the first doping region  20  is 2. The first doping region  20  is surrounded by multiple rectangular ring regions, including, from the inside to outside, the first isolation structure  50 , the second doping region  22 , the second isolation structure  52 , the third doping region  24 , the third isolation structure  54  and the fourth doping region  26 . These regions are arranged to form a multiple-concentric-rectangular-ring structure around the first doping region  20 . Respectively, each of these regions may have the same or different aspect ratio as the first doping region  20 . According to one embodiment of the present invention, the first isolation structure  50 , the second doping region  22 , the second isolation structure  52 , the third doping region  24 , the third isolation structure  54  and the fourth doping region  26  may have substantially the same aspect ratio as the first doping region  20 . 
         [0033]    Please refer to  FIG. 4 , which is the schematic cross-sectional diagram taken along the line A-A′ in  FIG. 3 . The first doping region  20  and the second doping region  22  are separated by the first isolation structure  50 . The third doping region  24  and the second doping region  22  are separated by the second isolation structure  52 . The fourth doping region  26  and the third doping region  24  are separated by the third isolation structure  54 . The first doping region  20 , the first isolation structure  50  and the second doping region  22  are disposed in the first well  30 . Meanwhile, the first well  30 , the second isolation structure  52  and the third doping region  24  are disposed in a deep well  40 . Optionally, a second well  32  maybe disposed between the third doping region  24  and the deep well  40 , and a third well  34  may be disposed between the fourth doping region  26  and the substrate  10 . 
         [0034]    The first isolation structure  50 , the second isolation structure  52  and the third isolation structure  54  maybe, for instance, shallow trench isolation structures and the depths may be the same or different, to provide a better isolation effect, reducing the leakage current I sub  and the reverse current I r . According to one preferred embodiment, the depth of the first isolation structure  50  is deeper than the depths of the first doping region  20  and the second doping region  22 , but is shallower than the depth of the first well  30 . Meanwhile, the depth of the second isolation structure  52  is deeper than the depths of the second doping region  22  and the third doping region  24 , but is shallower than the depth of the deep well  40 . 
         [0035]    The substrate  10  may comprise a semiconductor substrate, such as, for example, a silicon substrate, a silicon contained substrate, a silicon-on-insulator (SOI) substrate or other suitable semiconductor materials. The first doping region  20  may be of a conductivity type, for instance, the N-type. The second doping region  22  and the first well  30  may be the conductive type which is opposite to the first doping region  20 , for instance, the P-type. The third doping region  24 , the second well  32  and the deep well  40  may all have N-type conductivity or all have P-type conductivity according to different embodiments. The fourth doping region  26 , the third well  34  and the substrate  10  may have different conductive type from the third doping region  24  and the deep well  40 . For example, when the third doping region  24 , the second well  32  and the deep well  40  have the first conductivity, the fourth doping region  26 , the third well  34  and the substrate  10  may have the second conductive type. When the third doping region  24 , the second well  32  and the deep well  40  have the second conductivity, the fourth doping region  26 , the third well  34  and the substrate  10  may have the first conductive type. 
         [0036]    Please refer to  FIG. 5 , which is the schematic cross-sectional diagram according to one embodiment of the invention. According to one embodiment of the embodiment, the first doping region  20  has the N type conductivity. The second doping region  22  and the first well  30  have the P type conductivity, and a P/N junction  60  is between the first doping region  20  and the first well  30 . The third doping region  24 , the second well  32  and the deep well  40  have the N type conductivity, and another P/N junction  62  is between the deep well  40  and the first well  30 . The fourth doping region  26 , the third well  34  and the substrate  10  have the P type conductivity, and another P/N junction  64  is between the deep well  40  and the substrate  10 . One feature of the present invention is that the second doping region  22  and the third doping region  24  are electrically connected by a connecting structure  70 , to ensure that they are in the same electrical potential. 
         [0037]    When there is no potential difference between the first doping region  20  and the second doping region  22 , there is no obvious current flow in the diode. When a forward bias which is greater than the potential barrier of the P/N junction  60 , for example, 0.7V for Si substrate, is applied to the diode, a forward current I f  (not shown) flows from the second doping region  22 , along the first well  30  under the first isolation  50  and across the P/N junction  60 , to the first doping region  20 . The first doping region  20  is regarded as the cathode  1  of the diode, and the second doping region  22  is regarded as the anode  2 . 
         [0038]    Please refer to  FIG. 6 , which illustrates the experimental operation conditions of the diode as shown in  FIG. 5 . It should be understood that these are the preferred exemplary conditions, and should not be limitations on operating the diode in practice. During the “stand-by” operation, the first doping region  20  (cathode), the second doping region  22  (anode) and the third doping region  24  (electrode A) are coupled to a voltage V DD . The fourth doping region  26  (electrode B) is coupled to a voltage V GND . During the “stand-by” operation, there is no potential energy applied to the P/N junction  60  and the P/N junction  62  respectively, and the P/N junction  64  is reverse-biased. There is sustainably no current flowing in the diode. 
         [0039]    During the “on” operation, the first doping region  20  (cathode) is coupled to a voltage V on  while the second doping region  22  (anode) is still coupled to a voltage V dd , and the fourth doping region  26  (electrode B) is coupled to a voltage V GNN . The P/N junction  60  is forward biased, and the resulting forward current I f  flows from the second doping region  22 , along the first well  30  under the first isolation  50  and across the P/N junction  60 , to the first doping region  20 . It should be noted that during the “on” operation, the P/N junction  62  is zero-biased and the P/N junction  64  is reverse-biased. The potential barriers of the P/N junction  62  and the P/N junction  64  decrease the opportunity for the forward current I f  to flow to the substrate  10 , to become the leakage current. The P/N junction  62  and the P/N junction  64  provide an enhanced isolation effect between the first well  30  and the substrate  10 . 
         [0040]    It should be noticed that, in another exemplary embodiment, the third doping region  24 , the second well  32  and the deep well  40  may have the same conductivity type as the second doping region  22  and the first well  30 , and may be electrically coupled to the second doping region  22  and the first well  30  by the connecting structure  70 . In this case, both the second doping region  22  and the third doping region  24  are regarded as the anode. 
         [0041]    Another objective of the present invention is to provide a preferred range of the aspect ratio, at which the diode with the multiple-concentric-rectangular-ring structure as shown previously may have better performance. 
         [0042]      FIG. 7  to  FIG. 9  illustrate the characteristics of an exemplary diode according to the present invention when it is forward-biased. It should be noticed that the operation conditions as shown in the diagrams are preferred exemplary experimental conditions, and should not be limitations on operating the diode in practice. 
         [0043]      FIG. 7  is the characteristic curve of the exemplary diode according to the present invention, showing the variation of the cathode current I E  and the leakage current I sub  in response to the varying cathode voltage V E  while the anode is coupled to a fixed anode voltage. In the following description, the current flow to the cathode is regarded as the cathode current I E , and the current flow to the substrate is regarded as the leakage current I sub . The voltage coupled to the cathode is regarded as the cathode voltage V E . During the “stand-by” operation, the cathode and the anode are both coupled to a voltage at, for example, 5V. The measured cathode current I E  and the leakage current I sub  are very small, for example, 1E-15 ampere (A). It is substantially considered to have no current. As the cathode voltage decreases gradually from 5V and the anode voltage is kept at 5V, the diode is increasingly forward-biased. The cathode current I E  increases gradually but the leakage current is approximately kept at the same level. When the anode voltage is 5V and the cathode voltage V E  is smaller than a certain value, for example, 4.3V, the leakage current increases abruptly. According to the characteristic curve shown in  FIG. 7 , it is preferred to set the “on” voltage, which is the voltage applied to the cathode of the exemplary diode according to the embodiment, to be between 4.3V and 4.4V in order to get the larger cathode current I E  and the smaller leakage current I sub . 
         [0044]      FIG. 8  and  FIG. 9  illustrate the respective variation of the cathode current I E  and the leakage current I sub  in response to the varying cathode voltage V E  of three exemplary diodes according to the present invention, where the three diodes have different aspect ratios. According to the present embodiment, all of the three diodes have the multiple-concentric-rectangular ring structure as shown in  FIG. 3 . The widths of the each diode&#39;s first doping region  20  are the same, but the lengths are different. According to the embodiment, the width is 0.45 μm, and the length is 1 μm, 5 μm, and 10 μm respectively. The calculated aspect ratio of each diode&#39;s first doping region  20  is 2, 10 and 20 respectively. The width and length values aforesaid are a preferred experimental embodiment, and should not be a limitation to the invention. The first isolation structure  50 , the second doping region  22 , the second isolation region  52 , the third doping region  24 , the third isolation region  54  and the fourth doping region  26  of each diode may have substantially the same aspect ratio as the first doping region  20  of itself. 
         [0045]    As shown in  FIG. 8 , when the cathode voltage is between 4.3V and 4.4V, the diode with the 10 μm first doping region  20  has the largest cathode current I E , followed by the diode with the 5 μm first doping region  20 , and the diode with the 1 μm first doping region  20  has the smallest cathode current I E . According to the result of the present embodiment, it is known that the cathode current I E  increases as the aspect ratio of the diode increases. 
         [0046]    As shown in  FIG. 9 , when the cathode voltage is between 4.3V and 4.4V, the diode with first doping region  20  having width of 5 μm or 10 μm has smaller leakage current I sub  than the diode which&#39;s first doping region  20  has width of 1 μm. 
         [0047]    According to the experimental result as described above, it may be concluded that the diode may have larger cathode current I E , smaller leakage current I sub  and better performance when the aspect ratio of the first doping region  20  is larger. The tendency aforesaid may still be seen when the current is normalized by the cathode area of the diode. According to the tendency observed from the experiment result of the exemplary diodes according to the present invention, it is preferred that the aspect ratio of the first doping region  20  is between 2 and 10. According to a best embodiment, the aspect ratio of the first doping region  20  is between 10 and 20. 
         [0048]    The diode with the multiple-concentric-rectangular-ring structure according to the present invention may provide larger forward current and smaller leakage current when it is forward biased. Furthermore, when the aspect ratio of the diode is larger, the performance is better. 
         [0049]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.