Abstract:
A low-capacitance bidirectional device of protection against overvoltages, intended to be used at high frequencies, including first and second discrete one-way Shockley diodes, the cathode and the anode of the first diode being respectively connected to the anode and to the cathode of the second diode, the break-over voltages of each diode ranging between 50 and 125 V.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a low-capacitance bidirectional device of protection against overvoltages. More specifically, the present invention relates to a bidirectional protection device intended for high-frequency applications. 
   2. Discussion of the Related Art 
     FIG. 1  shows a conventional structure of a bidirectional protection device  10 . As usual in the representation of integrated circuits, the different regions are not drawn to scale. 
   Device  10  is formed of a monolithic circuit in which are formed two vertical Shockley diodes in antiparallel. The first vertical Shockley diode  11  is shown to the left of  FIG. 1 , and the second vertical Shockley diode  12  is shown to the right of FIG.  1 . 
   The monolithic circuit includes a lightly-doped substrate  15  of conductivity type N. Substrate  15  includes a heavily-doped well  16  of conductivity type P on the side of its upper surface  17 , and a heavily-doped well  18  of conductivity type P on the side of its lower surface  19 . 
   First Shockley diode  11 , located to the left of  FIG. 1 , includes an intermediary buried region  20 , of conductivity type N, more heavily doped than substrate  15  but much more lightly doped than P-type well  16 , arranged at the interface between P-type well  16  and substrate  15 , and a heavily-doped N-type cathode region  21 , arranged on the side of upper surface  17  of substrate  15  in P-type well  16 . Cathode region  21  consists, for example, of spaced apart concentric rings, in parallel strips, or in islands separated according to a network. 
   Similarly, second Shockley diode  12 , located to the right of  FIG. 1 , includes a buried region  22  of conductivity type N, and an N-type heavily-doped cathode region  23 . 
   Upper and lower insulating regions  26  and  27  respectively cover the periphery of upper and lower surfaces  17  and  19  of substrate  15 . 
   Upper and lower metallization layers  30  and  31  respectively cover the upper and lower surfaces  17  and  19  of substrate  15 . Upper metallization layer  30  acts as the cathode electrode of first Shockley diode  11  by being connected to N-type cathode region  21 , and of the anode electrode of second Shockley diode  12  by being connected to P-type well  16 . Lower metallization layer  31  acts as the role of the anode electrode of first Shockley diode  11  by being connected to P-type well  18 , and of the cathode electrode of second Shockley diode  12  by being connected to N-type cathode region  23 . Upper and lower metallization layers  30  and  31 , respectively, are connected to terminals A and B of device  10 . 
   Conventionally, when the voltage applied across bidirectional protection device  10  is included between positive and negative break-over voltages, device  10  is non-conductive, and said to be in the off state. When the voltage is greater than the positive break-over voltage or smaller than the negative break-over voltage, the device is conductive. For the device to switch from the on state to the off state, the current flowing therethrough must fall below a threshold level. 
   For the two break-over voltages to have substantially the same absolute value, the dopant concentrations respectively of buried regions  20  and  22 , and of P-type wells  16  and  18 , must be identical. 
   In the off state, device  10  of  FIG. 1  exhibits a general capacitance that may be high, which is a disadvantage upon use of protection device  10  for high-frequency applications, for example, applications in telecommunications. 
   A possibility, to decrease the general capacitance of device  10  in the off state, is to replace a device  10  having the desired break-over voltage by two identical sub-devices  10  assembled in series, each having a break-over voltage equal to half the desired total break-over voltage. The capacitances of the elementary sub-devices being assembled in series, the total capacitance is equal to the capacitance of one sub-device divided by two. However, when the break-over voltage of the bidirectional device of  FIG. 1 , which can be obtained, for example, by increasing the dopant concentration of buried regions  20 ,  22  (the dopant concentrations of wells  16 ,  18  remaining constant) is divided by two, its capacitance appears to increase. Thus, the decrease in capacitance obtained by the series assembly is less than expected. 
   SUMMARY OF THE INVENTION 
   The present invention aims at obtaining a bidirectional protection device having a low capacitance. 
   To achieve this and other objects, the present invention provides a low-capacitance bidirectional device of protection against overvoltages, for use at high frequencies, including first and second discrete one-way Shockley diodes, the cathode and the anode of the first diode being respectively connected to the anode and to the cathode of the second diode, the break-over voltages of each diode ranging between 50 and 125 V. 
   According to an embodiment of the present invention, each monolithic circuit includes a lightly-doped substrate of a first conductivity type, a first heavily-doped well of a second conductivity type including a first heavily-doped region of the first conductivity type, a second intermediary region of the first conductivity type located between the substrate and the first well, more heavily doped than the substrate and less heavily doped than the first well, and a second heavily-doped well of the second conductivity type, the capacitance, when the diode is non conductive, associated with the junction between the first well and the second region, being greater than the capacitance associated with the junction between the substrate and the second well. 
   According to an embodiment of the present invention, the dopant concentration of the second P-type well is smaller than the dopant concentration of the first P-type well. 
   According to an embodiment of the present invention, the dopant concentration of the substrate is approximately 5*10 15  atoms/cm 3  and, in the vicinity of the junction between the first well and the second region, the dopant concentration of the second region is greater than 1*10 16  atoms/cm 3  and the dopant concentration of the first well is 4*10 18  atoms/cm 3 . 
   The present invention also provides a method for manufacturing a bidirectional device of protection against overvoltages, for high-frequency applications, including the steps of forming two Shockley diodes on two distinct monolithic circuits, each diode having a break-over voltage ranging between 50 and 125 V; and connecting the cathode and the anode of the first diode respectively to the anode and to the cathode of the second diode. 
   The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , previously described, schematically shows a monolithic circuit integrating a conventional bidirectional protection device; 
       FIG. 2  shows an equivalent capacitance circuit of the device of  FIG. 1 ; 
       FIG. 3  schematically shows the bidirectional protection device according to the present invention; 
       FIG. 4  shows an equivalent capacitance circuit of the device of  FIG. 3 ; 
       FIG. 5  illustrates a curve representative of the ratio of the total capacitances of the device according to the present invention and of the device of  FIG. 1  as a function of the junction capacitance ratio of the device according to the present invention; and 
       FIG. 6  illustrates a curve representative of the ratio of the total capacitances of the device according to the present invention and of the device of  FIG. 1  according to the break-over voltage of the device according to the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention is the result of a study on the distribution of the capacitances in the internal structure of device  10  of  FIG. 1 , which contribute to the total capacitance, C 1 , of device  10 . 
   In the off state, on the side of first Shockley diode  11 , the junction between P-type well  16  and buried N-type region  20  exhibits a first capacitance, C 1 , while the junction between substrate  15  and P-type well  18  exhibits a second capacitance, C 2 . Similarly, on the side of second Shockley diode  12 , the junction between P-type well  16  and substrate  15  exhibits capacitance C 2  while the junction between P-type well  18  and buried region  22  exhibits capacitance C 1 . For the two Shockley diodes, portions of P-type wells  16 ,  18  which contact metal layers  30 ,  31  at the level of associated N-type cathode regions  21 ,  23  conventionally act as emitter short-circuits so that no capacitance is associated with the junction between N-type cathode regions  21 ,  23  and P-type wells  16 ,  18 . 
     FIG. 2  shows an equivalent electric diagram of device  10  of  FIG. 1  only considering capacitances. The equivalent circuit is formed by the two capacitances in parallel C 1  and C 2 , corresponding to the junctions located on the side of upper surface  17 , assembled in series with the two capacitances in parallel C 2  and C 1 , corresponding to the junctions located on the side of lower surface  19 . The equivalent total capacitance C t  of device  10  is then provided by the following relation:
   C   t =( C   1   +C   2 )/2 
   Qualitatively, if capacitance C 1  is much greater than capacitance C 2 , C t  is close to C 1 /2. 
   As shown in  FIG. 3 , the present invention provides using as a protection device  40  an antiparallel assembly of first and second one-way vertical discrete Shockley diodes  41  and  42 . Diodes  41 ,  42  have an identical structure, second diode  42 , located to the right of  FIG. 3 , being shown upside down with respect to first diode  41 , located to the left of FIG.  3 . In the following description, only first diode  41  will be described in detail. 
   First diode  41  includes a lightly-doped N-type substrate  43 . Substrate  43  includes, on its upper surface side  44 , a heavily-doped P-type well  45 . A buried N-type region  46  is arranged at the interface between substrate  43  and P-type well  45 . Buried region  46  has a dopant concentration greater than the dopant concentration of substrate  43  and smaller than that of P-type well  45 . Substrate  43  includes, on its lower surface side  47 , a heavily-doped P-type anode well  48 . On the side of upper surface  44  is formed an N-type cathode region  49 , in P-type well  45 , for example, in the form of separate concentric rings, of spaced apart parallel strips, or of an island network. 
   Upper and lower insulating layers  52  and  53 , respectively, cover the peripheries of upper and lower surfaces  54  and  57 , respectively, of substrate  43 . 
   An upper cathode metal layer  56  contacts, one the side of upper surface  44 , P-type well  45  and N-type cathode region  49 . A lower anode metal layer  57  contacts, on the side of lower surface  47 , P-type anode well  48 . 
   As previously indicated, second diode  42  has a structure similar to that of first monolithic circuit  41 , but is shown upside down in FIG.  3 . 
   The anode of the first Shockley diode is connected by an electric connection  58  to the cathode of the second Shockley diode. The cathode of the first Shockley diode is connected by an electric connection  59  to the anode of the second Shockley diode. The terminals of device  40  are designated with references A and B. The Shockley diodes are thus interconnected in the same way as the device  10  of FIG.  1 . The general operation of device  40  according to the present invention is similar to that of device  10  of FIG.  1 . 
   However, as will be detailed hereafter, the total equivalent capacitance, C T , of device  40  according to the present invention, when the Shockley diodes are off, is different from that of device  10  of FIG.  1 . 
     FIG. 4  shows an equivalent electric diagram of device  40  according to the present invention only considering capacitances. Each monolithic circuit  41 ,  42  includes, in series, a first capacitance, corresponding to the capacitance of junction J 1  between N-type buried region  46  and P-type well  45 , and a second capacitance corresponding to the capacitance of junction J 2  between P-type well  48  and substrate  43 . Portions of P-type well  45  contacting the upper cathode metal layer  56 , at the level of cathode region  49 , act, in a known manner, as emitter short-circuits so that no capacitance is associated with the junction between cathode region  49  and P-type well  45  for the two diodes  41 ,  42 . When the dopant concentrations of the various regions of diodes  41 ,  42  are the same as those of device  10  of  FIG. 1 , the corresponding capacitances are substantially equal. To simplify the comparison, these capacitances are called C 1 , C 2  in  FIG. 4  as in FIG.  2 . 
   The total equivalent capacitance C T  of device  40  according to the present invention corresponds to the arranging in parallel of the two series capacitances C 1  and C 2  of monolithic circuit  41  with the two series capacitances C 1  and C 2  of monolithic circuit  42 . The total equivalent capacitance C T  is thus provided by the following formula:
 
 C   T =2*( C   1   *C   2 )/( C   1   +C   2 )
 
   Qualitatively, if capacitance C 1  has a much greater value than capacitance C 2 , C T  is close to 2*C 2  and is much smaller than above-mentioned capacitance C 1 , which was close to C 1 /2. 
     FIG. 5  illustrates a curve  60  representative of the ratio of total capacitances C t  and C T  according to the ratio of capacitances C 1  and C 2 . Capacitance C 1  being always greater than capacitance C 2 , curve  60  is drawn for values of C 1 /C 2  greater than 1. 
   The higher capacitance ratio C 1 /C 2 , the higher capacitance ratio C t /C T . This means that the smaller capacitance C 2  is with respect to capacitance C 1 , the smaller the total equivalent capacitance C T  of device  40  according to the present invention is with respect to total equivalent capacitance C t  of device  10  of FIG.  1 . 
   Device  40  according to the present invention exhibits, by the sole fact that the Shockley diodes are formed in a discrete manner, a total capacitance C T  smaller than that of device  10  of  FIG. 1  in which the two Shockley diodes are integrated on a single monolithic circuit. Thus, although the current tendency is to integrate several components in a same monolithic circuit as soon as this is technically possible, the present invention shows that, to solve the problem posed, it is particularly advantageous to have a structure where each component is formed discretely. 
     FIG. 6  illustrates a curve  61  representative of the ratio between total capacitances C t /C T  according to the break-over voltage of the Shockley diodes (assumed to be identical for the two diodes). 
   For each Shockley diode, the break-over voltage is essentially determined by the difference of dopant concentration on either side of junction J 1 . Curve  61  is drawn assuming that, for each Shockley diode of device  40 , the concentrations of P-type wells  45  and  48  are fixed and identical and that the concentration of N-type buried region  46  varies according to the desired break-over voltage. It is also assumed that, for device  10  of  FIG. 1 , the concentrations of P-type wells  16  and  18  are fixed and identical and that the concentrations of buried regions  20  and  22  are identical and selected according to the desired break-over voltage. Capacitances C 1  and C 2  are determined by the dopant concentrations of the regions forming the junctions with which they are associated. Thus, curve  61  is plotted for a constant capacitance C 2  and variable capacitance C 1 . 
   Curve  61  illustrates the fact that the ratio between total capacitances C t /C T  is at its highest for break-over voltages close to 50 V, and decreases as the break-over voltage increases (which corresponds to a decrease in capacitance C 1 ). Device  40  according to the present invention thus exhibits a total capacitance C T  which decreases as compared to total capacitance C 1  of device  10  of  FIG. 1  as the break-over voltage decreases. For a break-over voltage of 125 V, total capacitance C T  is already smaller by more than 20% than C t . For a break-over voltage of 58 V, total capacitance C T  is smaller by more than 40% than C 1 . The present invention thus finds a particularly advantageous application for break-over voltages ranging between 50 V and 125 V. 
   As an example, a capacitance ratio C t /C T  of approximately 1.7 is obtained for a break-over voltage of 58 V corresponding to a dopant concentration of substrate  15 ,  43  of 5*10 15  atoms/cm 3 , a dopant concentration of N-type buried layers  20 ,  22 ,  46  resulting from a surface concentration ranging between 2*10 17  and 6*10 17  atoms/cm 3 , and dopant concentrations of P-type well  16 ,  18 ,  45 ,  48  resulting from a surface concentration ranging between 2*10 18  and 8*10 18  atoms/cm 3 . 
   For a determined break-over voltage, it is possible to further decrease ratio C t /C T  by decreasing capacitance C 2  of the junction between substrate  43  and P-type well  48 . Indeed, given the expressions of total capacitances C T  and C t , a decrease in capacitance C 2  causes a significant decrease in total capacitance C T  while it has less effect upon total capacitance C t . 
   To decrease capacitance C 2 , it is desired to obtain an avalanche voltage of the junction between substrate  43  and P-type well  48 . For this purpose, it is possible to decrease the dopant concentration of P-type well  48  in the vicinity of junction J 2 . This may be obtained by forming a deeper P-type well  48 . As an example, for a dopant surface concentration of approximately 4*10 18  atoms/cm 3 , and a depth of well  48  of 20 μm, a ratio C t /C T  of 1.7 is obtained, for a dopant surface concentration of approximately 1*10 18  atoms/cm 3 , and a depth of well  48  of 110 μm, a ratio C t /C T  of 2.12 is obtained, and for a dopant surface concentration of approximately 3*10 16  atoms/cm 3 , and a depth of well  48  of 30 μm, a ratio C t /C T  of 1.95 is obtained. 
   The fact that Shockley diodes  41 ,  42  of device  40  according to the present invention are formed on distinct monolithic circuits  41 ,  42 , enables setting different dopant concentrations for P-type wells  45  and  48 , and thus decreasing C 2 . When the bidirectional device is formed on a single monolithic circuit, it is not possible in practice to locally modify the dopant concentrations of P-type wells  16  and  18  to reduce capacitance C 2 , which, further, as already mentioned, results in a small decrease only in total capacitance C t . 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the monolithic circuits may include guard rings on the upper and lower surface sides surrounding the P-type wells to avoid short-circuiting the substrate and the P-type wells upon deposition of the metal layers. Further, the device according to the present invention has been described with break-over voltages of the same absolute value. Clearly, by forming Shockley diodes  41 ,  42  in a discrete manner, it is possible to easily set different dopant concentrations for P-type wells  45 , and thus obtain a bidirectional device  40  having positive and negative break-over voltages of different values. 
   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 only as defined in the following claims and the equivalents is not intended to be limiting. The present invention is limited thereto.