Patent Publication Number: US-2002000565-A1

Title: Surface breakdown bidirectional breakover protection component

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to breakover protection components, currently called Shockley diodes or gateless thyristors, of bidirectional type.  
       [0003] 2. Discussion of the Related Art  
       [0004] The diagram of a bidirectional Shockley diode appears in FIG. 1. A first Shockley diode S 1  and a second Shockley diode S 2  are connected in antiparallel between terminals A 1  and A 2 .  
       [0005]FIG. 2 is a simplified cross-sectional view of a semiconductor component conventionally constituting a bidirectional Shockley diode.  
       [0006] The component is constructed on a very lightly-doped N-type substrate  1  and includes on its upper and lower surface sides P-type regions  3  and  4  occupying the major part of the component surface. Each of regions  3  and  4  contains, in a substantially complementary way, an N-type region, respectively  6  and  7 . Substantially facing each of regions  6  and  7 , at the interface between the respective P region and the substrate, are disposed lightly-doped N-type regions  8  and  9 . The upper and lower circumferences of the substrate are surrounded with a highly-doped N-type ring, respectively  11  and  12 . The circumferences of the upper and lower surfaces are coated with insulating layers, currently in silicon oxide, respectively  14  and  15 . An upper metallization M 1  corresponding to electrode A 1  covers regions  3  and  6  and a lower metallization M 2  covers regions  4  and  7 . These metallizations stop on the peripheral oxide layer, respectively  14 ,  15 .  
       [0007] A first vertical Shockley diode, the anode of which corresponds to the upper surface, includes regions  3 - 1 - 9 - 4 - 7 . A second Shockley diode, the cathode of which corresponds to the upper surface, includes regions  6 - 3 - 8 - 1 - 4 . For each of these diodes, the interface ( 3 - 8 ;  4 - 9 ) between the internal P-type region and the corresponding lightly-doped N-type region determines the breakdown voltage.  
       [0008] Generally, the lower surface of the component is intended to be mounted by brazing on a base plate forming a thermal distributor. In the case of the component shown in FIG. 2, this base plate  20  must exhibit a surface smaller than the surface of the component and of lower surface metallization M 2 . Indeed, when the component is mounted on this plate by a brazing layer  21 , the brazing tends to overflow laterally to form pads  22 . Should metallization M 2  extend on the entire lower surface of the component and plate  20  be wider than the component, the brazing pad would overflow on the lateral surface of the component and short-circuit regions  4  and  7  with substrate  1 . The function of the component would no longer be ensured, or would be very degraded.  
       [0009] Thus, a tendency of actual technology, for simplifying the assembly, is to implement so-called well components, that is, components having on their entire circumference a region of the same type of doping as one of the lower surface layers. Thus, a short circuit between the lower surface and the lateral surface no longer hinders the operation of the component.  
       [0010] The transformation of the component of FIG. 2 into a well component is illustrated in FIG. 3. Well  31  is formed by P-type drive-ins extending from lower and upper surfaces and joining lower surface P-type region  4 . It should be noted that, in this case, the transition from the simple structure of FIG. 2 to the well structure of FIG. 3 is relatively direct. In particular, the disposition of regions  8  and  9  determining the breakdown voltages of the component is not altered.  
       [0011] As shown in FIG. 3, the component can be mounted on a thermal distributor plate  33  which is wider than the component and a possible brazing pad  35  will not affect the operation of the component since it joins a region of the same type of doping as the useful regions which are desired to be connected by lower surface metallization M 2 .  
       [0012]FIG. 4 shows a simplified cross-sectional view of another embodiment according to the prior art of a double Shockley diode. The component is again formed based on a substrate  1 , including upper and lower P-type regions  3  and  4  wherein are formed upper and lower N-type regions  6  and  7 . Lightly-doped N-type regions  8  and  9  at the interface between the P-type regions and the substrate are replaced with lightly-doped N-type lateral regions  41  and  42 . This disposition enables reaching of a breakdown voltage lower than that of FIG. 2. Indeed, the breakdown voltage depends on the doping gradient at the junction likely to break down. In the case of FIG. 2, the bottom of a P diffusion, that is, a relatively lightly-doped region is located at this junction. Conversely, in the case of FIG. 4, the upper part of the P diffusion ( 3  or  4 ) is in contact with N-region  41  or  42 . As a result, lower breakdown voltages can be obtained, which is desired for some applications.  
       [0013] However, the component of FIG. 4 has the same assembling disadvantages as that shown in FIG. 2.  
       SUMMARY OF THE INVENTION  
       [0014] An object of the present invention is to provide a protection component of double Shockley diode type having a surface breakdown characteristic like the component described in relation with FIG. 4 and which can have a well structure to simplify its assembly.  
       [0015] While the transformation of the component of FIG. 2 into that of FIG. 3 seemed relatively simple, there exists a priori no simple way of transforming the component of FIG. 4 into a well structure. Thus, the present invention provides for significantly modifying the structure of this component.  
       [0016] More specifically, the present invention provides a bidirectional breakover component including a lightly-doped N-type substrate, an upper P-type region extending over practically the entire upper surface of the component except its circumference, a lower P-type uniform layer on the lower surface side of the component, substantially complementary N-type regions respectively formed in the upper region and in the lower layer, a peripheral P-type well; an overdoped P-type region at the upper surface of the well, and lightly-doped N-type regions between the circumference of the upper region and the well.  
       [0017] According to an embodiment of the present invention, the overdoped P-type region has the same level of doping as the upper region.  
       [0018] According to an embodiment of the present invention, the lightly-doped N-type regions have the same level of doping as the substantially complementary N-type regions.  
       [0019] These objects, characteristics and advantages as well as others, of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments of the present invention, in relation with the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0020]FIG. 1 shows the diagram of a double Shockley diode;  
     [0021] FIGS.  2  to  4  show cross-sectional views of several structures of double Shockley diodes according to prior art; and  
     [0022]FIG. 5 shows a cross-sectional view of an example of embodiment of a double Shockley diode according to the present invention.  
    
    
     DETAILED DESCRIPTION  
     [0023] As shown in FIG. 5, the component according to the present invention is formed from a very lightly-doped N-type substrate  1 . On the upper surface side and on most of its surface is formed a P-type region  3 . On the lower surface side and on its entire surface is formed a P-type region  4 . Substantially complementary N-type regions  6  and  7  are formed in respective regions  3  and  4 . The component circumference is surrounded with a P-type well  31  formed by drive-in from the upper and lower surfaces.  
     [0024] Between substantially opposite peripheral areas of region  3  and peripheral well  31  are formed lightly-doped N-type regions  51  and  52 . A continuous ring can also be formed instead of two distinct regions. It is however preferred to form distinct regions to improve the characteristics of the components and reduce their capacitance.  
     [0025] Besides, the upper surface of the well, especially at locations in contact with regions  51  and  52 , is coated with a P-type ring  55  of the same type as region  3  and, preferably, of the same level of doping.  
     [0026] Metallizations M 1  and M 2  are arranged like in the case of FIG. 3 and enable the same type of assembly on a large dimension base  33 .  
     [0027] The operating mode of the device of FIG. 5 is not exactly the same as that of FIG. 4, at least during its initial operating phase.  
     [0028] In case of a positive overvoltage on the upper metallization, the current flows from region  3  into region  52  through forward junction  3 - 52  and then, by avalanche, through the junction between regions  52  and  55 . Then, the current circuit flows through well  31  and lower surface  4  towards metallization M 2 . After which, forward junction PN between regions  4  and  7  becomes conductive and thyristor  3 - 1 - 4 - 7  triggers.  
     [0029] Conversely, when a positive overvoltage appears on the lower metallization, it passes through well  1  to region  55 , through the forward junction between regions  55  and  51 , and then through the avalanche junction between regions  51  and  3  towards metallization M 1 , after which the forward junction between regions  3  and  6  becomes conductive and thyristor  4 - 1 - 3 - 6  enters conduction.  
     [0030] Thus, in the case of a positive overvoltage on metallization M 1 , junction  52 - 55  determines the avalanche threshold whereas, in the case of a positive overvoltage on metallization M 2 , junction  51 - 3  determines the avalanche threshold. This is why a peripheral region  55  of same level of doping as region  3  has been provided to ensure the symmetry of the device. It should be noted that this constitutes a simple way of implementing a double Shockley diode with a distinct breakdown threshold in the two directions by choosing for region  55  a level of doping different from that of region  3 .  
     [0031] The structure of FIG. 5 achieves at least the objects aimed at, that is, making a surface breakdown double Shockley diode with a well. This structure is particularly simple. Especially, with respect to the various prior art structures, it includes lightly-doped N-type regions ( 51 ,  52 ) on one surface of the component only, which simplifies the fabrication.  
     [0032] In an embodiment of the present invention, the component of FIG. 5 has been made by using the following fabrication sequence:  
     [0033] diffusion of well  31 ,  
     [0034] simultaneous diffusion of P regions  3 ,  4 , and  55 ,  
     [0035] diffusion of N regions  51  and  52  (this step could be performed before the previous step),  
     [0036] diffusion of N regions  6  and  7 ,  
     [0037] opening of the oxide and metallization layers.  
     [0038] It should be noted that, in a certain range of breakdown voltages, regions  51  and  52  could be formed at the same time as regions  6  and  7 , which further simplifies the fabrication process.  
     [0039] Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.