Abstract:
A bidirectional power transistor formed horizontally in a semiconductor layer disposed on a heavily-doped semiconductor wafer with an interposed insulating layer, the wafer being capable of being biased to a reference voltage, the product of the average dopant concentration and of the thickness of the semiconductor layer ranging between 5·10 11  cm −2  and 5·10 12  cm −2 .

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application No. 09/58310, filed on Nov. 24, 2009, entitled “BIDIRECTIONAL POWER SWITCH CONTROLLABLE TO BE TURNED ON AND OFF,” which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a monolithic bidirectional power switch controllable on and off. 
     2. Discussion of the Related Art 
     The term “power switch” is here used to designate a switch capable of switching at voltages greater than 100 volts and/or at currents greater than 1 ampere. A power switch is for example intended to switch the power supply of a load connected to the mains. A power switch is generally formed vertically in a semiconductor substrate, that is, the current between the main electrodes of the switch flows between the upper and lower surfaces of the semiconductor substrate. This differentiates power switches from the components of an integrated circuit, which are generally horizontal, that is, the main electrodes of these components are located on a same surface of the semiconductor substrate and the current flows horizontally between these main electrodes. 
     An on and off controllable device, such as a vertical MOS transistor, may be used as a power switch. However, a vertical MOS transistor is unidirectional and it is necessary to associate it with a diode bridge if an A.C. current is desired to be conducted. This association has the disadvantage of having a non-negligible voltage drop in the on state. This is also true for a bipolar transistor. 
     A bidirectional component such as a triac, which has the advantage of having a very small voltage drop in the on state, may also be used. However, this component has the disadvantage of not being controllable to be turned off, unless a complex control circuit is associated thereto. 
       FIG. 1  shows a vertical bipolar NPN-type power transistor. At the surface of a lightly-doped N-type (N − ) semiconductor substrate  1  is formed a heavily-doped N-type (N + ) region  3  in a P-type well  5 . Substrate  1  and region  3  respectively form the collector and the emitter of the transistor. Well  5  forms the transistor base. On the lower surface of substrate  1  is formed a heavily-doped collector contact N-type layer  7 . Respective emitter, base, and collector metallizations E, B, and C are formed on regions  3 ,  5 , and  7 . 
     Substrate  1  is relatively thick, which provides a significant off-state breakdown voltage. Indeed, intrinsic silicon can withstand approximately 10 V per micrometer of silicon. Thus, a vertical structure comprising a substrate  1  having a thickness on the order of 60 μm may withstand, in the off state, a voltage on the order of 600 V. 
     If a power transistor with a bidirectional operation is desired to be obtained, it can be envisaged to make the structure of  FIG. 1  symmetrical, for example, as illustrated in  FIG. 2 . 
     The structure of  FIG. 2  comprises a stack of a first thick lightly-doped N-type semiconductor layer  11 , of a thin lightly-doped P-type layer  13 , and of a second thick lightly-doped N-type layer  15 . Layer  13  forms the base of the symmetrical transistor and layers  11  and  15  form its emitter/collector regions. Heavily-doped N-type emitter/collector contact regions  17  and  19  are formed, respectively, on the upper surface of layer  15  and on the lower surface of layer  11 . A heavily-doped P-type (P + ) well  21  crosses semiconductor layers  15  and/or  11  and contacts base region  13 . Respective emitter/collector and base metallizations A 1 , A 2 , and B are respectively formed on regions  17 ,  19 , and  21 . 
     The symmetrical structure of  FIG. 2 , however, cannot operate properly. Indeed, emitter/collector regions  15  and  11  are lightly N-type doped, which makes them efficient as collector regions (high breakdown voltage) but inappropriate as emitter regions, since they do not enable a proper injection of carriers in the on state. Thus, in both operating directions, the structure of  FIG. 2  has a low gain. To improve the on-state carrier injection, the doping of regions  15  and  11  should be increased, but this would decrease the breakdown voltage. Further, it is difficult to form a thin deeply buried base region  13  with current techniques, and the forming of a well  21  poses problems and may have side effects. 
     A bidirectional switch, controllable to be turned on and off, is thus needed. 
     SUMMARY OF THE INVENTION 
     An object of an embodiment of the present invention is to overcome at least some of the disadvantages of existing bidirectional power switches, controllable to be turned on and off. 
     Another object of an embodiment of the present invention is to provide a bidirectional power transistor having, in the on state, a high gain. 
     Another object of an embodiment of the present invention is to provide such a transistor having, when off, a high breakdown voltage. 
     Thus, an embodiment of the present invention provides a symmetrical power transistor formed horizontally in a semiconductor layer laid on a heavily-doped semiconductor wafer with an interposed insulating layer, the wafer being capable of being biased to a reference voltage, the product of the average dopant concentration and of the thickness of the semiconductor layer ranging between 5·10 11  cm −2  and 5·10 12  cm −2 . 
     According to an embodiment of the present invention, the product of the average dopant concentration and of the thickness of the semiconductor layer ranges between 9·10 11  and 2·10 12  cm −2 . 
     According to an embodiment of the present invention, the transistor comprises, in the semiconductor layer: 
     a lightly-doped semiconductor strip of a first conductivity type, at a doping level smaller than 5·10 15  at./cm 3 ; and 
     two doped regions of a second conductivity type at a doping level ranging between 10 15  and 8·10 17  at./cm 3 , located symmetrically with respect to the semiconductor strip. 
     According to an embodiment of the present invention, the doping level of each of the doped regions of the second conductivity type increases from the semiconductor strip. 
     According to an embodiment of the present invention, the doping of the doped regions of the second conductivity type gradually varies within a range having its lower limit between 10 15  and 5·10 15  at./cm 3  and having its upper limit between 5·10 16  and 8·10 17  at./cm 3 . 
     According to an embodiment of the present invention, the transistor further comprises heavily-doped fingers of the first conductivity type crossing the semiconductor strip and extending on either side of the strip in the doped regions of the second conductivity type. 
     According to an embodiment of the present invention, the semiconductor strip has a width smaller than 2 μm. 
     According to an embodiment of the present invention, the transistor further comprises a second insulating layer formed on the semiconductor layer and topped with a second conductive layer connected to the semiconductor strip. 
     According to an embodiment of the present invention, the transistor is of interdigited type. 
     According to an embodiment of the present invention, the transistor is of bipolar type, the lightly-doped strip of the first conductivity type forming a base and the doped regions of the second conductivity type forming emitter/collector regions. 
     According to an embodiment of the present invention, the transistor is of MOS type, the doped regions of the second conductivity type forming drain/source regions, the transistor further comprising an insulated gate formed on the lightly-doped semiconductor strip of the first conductivity type. 
     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, illustrates a known vertical power transistor; 
         FIG. 2 , previously described, illustrates a symmetrical vertical power transistor; 
         FIG. 3  is a perspective view of a bidirectional power transistor according to an embodiment of the present invention; 
         FIG. 4  is a top view of a variation of a transistor according to an embodiment of the present invention; 
         FIG. 5  is a cross-section view illustrating an effect advantageously obtained with a transistor according to an embodiment of the present invention; 
         FIG. 6  is a cross-section view illustrating a variation of a transistor according to an embodiment of the present invention; 
         FIGS. 7A and 7B  illustrate an advantage of a variation of a transistor according to an embodiment of the present invention; and 
         FIG. 8  illustrates an example of a transistor structure according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
       FIG. 3  is a perspective view of a bidirectional transistor according to an embodiment of the present invention. Unlike known power transistors, which are generally vertical, the transistor of  FIG. 3  is horizontal. This enables to do away with the issues of the forming of deep thin bases of the vertical symmetrical transistors and of the problems caused by the presence of wells of access to these bases. 
     The transistor is formed in a semiconductor layer  31  which extends on an insulating layer  33 , insulating layer  33  extending on a heavily-doped semiconductor wafer  35  biased to a reference voltage. In the shown example, the transistor is an NPN transistor. It should be noted that a PNP transistor can be manufactured in the same way, by inverting all conductivity types. As an example, insulating layer  33  may be made of silicon oxide. It should also be noted that insulating walls (not shown), crossing silicon layer  31  and reaching insulating layer  33 , may be formed all around the transistor of  FIG. 3 . 
     Semiconductor layer  31  is N-type doped. At the center of semiconductor layer  31  is formed a lightly-doped P-type strip  37  (P − ) which crosses layer  31  and separates it in two portions  38  and  39 . Portion  38  forms the emitter, respectively the collector, of the transistor, and portion  39  forms its collector, respectively its emitter. Strip  37  forms the transistor base and a metallization B is formed at the surface of strip  37 . On either side of strip  37 , at the opposite ends of portions  38  and  39 , heavily-doped N-type strips (N + ) forming emitter/collector contact region  40  and collector/emitter contact region  41  on which are formed metallizations A 1  and A 2  are formed symmetrically. 
     The fact for the transistor to be of horizontal type provides a direct access to base  37 . Further, as thin a baseband  37  as desired may be formed. As an example of numerical values, strip  37  may have a width smaller than 2 μm. 
     The transistor of  FIG. 3  is bidirectional and can thus be used as a controllable switch intended to conduct an A.C. current, for example, intended to be connected to the mains. Indeed, it will be shown that, for a selected doping of semiconductor layer  31 , provided to optimize the thickness of this layer, when the transistor is off, the transistor portion corresponding to its collector region is depleted. 
     More specifically, a doping level (n) for semiconductor layer  31 , which enables a satisfactory injection for the portion of this layer operating as an emitter is selected, and the thickness (t SOI ) of this layer is determined so that:
 
 n·t   SOI #∈ s   ·E   cr   /q# 10 12  cm −2  
 
     where ∈ s  is the dielectric permittivity of semiconductor layer  31 , E cr  is the critical field of semiconductor layer  31 , and q is the charge of the electron. 
     In other words, the thickness (t SOI ) of semiconductor layer  31  is selected according to the doping level (n) of this layer as follows:
 
 t   SOI #10 12   /n  cm,
 
     preferably
 
5·10 11   /n&lt;t   SOI &lt;5·10 12   /n  cm.
 
     Under such doping and thickness conditions, when heavily-doped N-type or P-type semiconductor wafer  35  is biased to a reference voltage and when the transistor formed in semiconductor layer  31  is in the off state, a so-called resurf effect (for REduced SURface Field) occurs, which results in that the semiconductor layer portion biased as an open collector fully depletes and thus acquires an apparent doping level much lower than its real doping level. 
       FIG. 4  is a top view of a variation of a transistor according to an embodiment of the present invention. It shows lightly-doped N-type emitter/collector regions  38 ,  39  located symmetrically with respect to strip-shaped base region  37 . Heavily-doped N-type emitter/collector contact regions  40 ,  41  are arranged on either side of emitter/collector regions  38 ,  39 . A wall of insulating material  51  crossing semiconductor layer  31  surrounds the transistor. In this variation, heavily-doped P-type fingers  53  cross strip  37  and extend, perpendicularly to strip  37 , on either side thereof. Fingers  53  extend in emitter/collector regions  38 ,  39  along a length which will be selected appropriately according to the distance between fingers  53 . There are many fingers  53 , which enable to protect base  37 , which is relatively thin, in the off state. Indeed, in this state, the presence of heavily-doped fingers  53  enables forming two opposite electric space charge areas between each finger  53  in the vicinity of base  37 . Such electric fields compensate for each other, which enables to generate, around base  37 , an area with no electric field. A breakdown in the vicinity of the base is thus avoided. 
       FIG. 5  is a cross-section view of one half of the transistor of  FIG. 4  along cross-section line A-A of this drawing. More specifically, the transistor half on the collector side when it is biased in the open state has been shown. The above-mentioned resurf effect results in that, in addition to depleted region  61  close to the collector-base junction, a depleted region designated by arrows  63  forms, which extends all the way to the surface of collector region  39  if its thickness complies with the above-mentioned conditions. Thus, when the transistor is off, it can withstand a significant voltage between its base and its collector. The depletion of collector region  39  in the open state allows the doping of layer  31  to increase. This enables a significant carrier injection between the emitter and the base when the transistor is desired to be turned on. As an example, the doping of semiconductor layer  31  may range between 10 15  and 8·10 17  at./cm 3 , and be preferably greater than 5·10 15  at./cm 3 . 
       FIG. 6  illustrates a variation of the structure of  FIG. 5 . In addition to the elements of  FIG. 5 , an insulating layer  71  is formed above semiconductor layer  31  and is covered with a conductive layer  73 , for example made of doped to semiconductor or metal, which contacts base region  37 . Thus, when the transistor is turned off, that is, when the base is at the lowest voltage and the collector is at the highest voltage, a resurf effect also emanates from the upper surface of layer  31 , due to the addition of insulating layer  71  and conductive layer  73 , as indicated by arrows  75 . Thus, a greater thickness of semiconductor layer  31  can be depleted, and thus a thicker layer  31  can be provided to enable the flowing of a greater current in the transistor, in the on state. Rather than increasing the thickness of layer  31 , the doping of layer  31  may also be further increased to enable, in the on state, the flowing of a greater current. 
     The present inventors have examined the distribution of the equipotential lines in a device such as that in  FIG. 4 , in the off state, and they have found that these lines were concentrated around the collector region. 
       FIG. 7A  is a cross-section view along line A-A, which in this case illustrates an example of distribution of equipotential lines. It is here considered that the collector contact is connected to a 600-V voltage and that the base is connected to a 0-V voltage. The equipotential lines (500 V, 400 V . . . ) concentrate close to the collector contact. Thus, the first micrometers of collector region  39  located close to collector contact  41  needs to withstand almost all of the 600 V. This may cause a breakdown. 
     To solve this problem, the present inventors provide forming emitter/collector regions  38  and  39  having a gradual doping between the base region and the respective emitter and collector, contact regions. 
       FIG. 7B  illustrates the distribution of the equipotential lines in such a structure. In this drawing, the 600-V voltage between the base and collector contact  41  substantially distributes along the entire length of collector region  39 . Thus, the risks of breakdown of this layer are limited. As an example of numerical values, the lowest doping (N 1 ) of collector region  39 , on the base side, may range between 10 15  and 5·10 15  at./cm 3 , and for example be on the order of 5·10 15  at./cm 3 , and the highest doping (N 2 ), on the side of collector contact  41 , may range between 5·10 16  and 8·10 17  at./cm 3 , and for example be on the order of 6·10 17  at./cm 3  for a length between the base and the collector contact on the order of 35 μm and a semiconductor layer  31  having a thickness of approximately 1.5 μm. 
     Thus, a bidirectional transistor having, in the on state, a high gain and a low voltage drop due to the high progressive doping of respective emitter and collector regions  38  and  39  (up to 8·10 17  at./cm 3 ), is obtained. In the off state, the transistor withstands high voltages since semiconductor layer  31  is fully depleted and that the equipotential lines between the base and collector contact regions are distributed along the entire length of the collector region. 
     As an example of numerical values, with a device having the following characteristics:
         thickness of insulating layer  33 : 3 μm;   thickness of insulating layer  71 : 3 μm;   doping of baseband layer  37 : 2.25·10 15  at./cm 3      doping of fingers  53 : 3·10 19  at./cm 3      minimum and maximum dopings of semiconductor layer  31 : N 1 =5·10 15  at./cm 3 , N 2 =6·10 16  at./cm 3      thickness of semiconductor layer  31 : 1.5 μm,       

     a transistor having a gain on the order of 44, losses on the order of 2.58 W for a 5-A current, and requiring a substrate surface area of no more than approximately 1.5 cm 2  is obtained. 
     Further, the transistor described herein can be obtained by a simple conventional method. 
       FIG. 8  is a top view of an embodiment of a power transistor having interdigited emitter, collector, and base regions. 
     An N-type doped semiconductor layer  81  extends on an insulating layer, not shown. An insulating wall  83  surrounds the transistor and crosses semiconductor layer  81 . Respective heavily-doped N-type emitter/collector and collector/emitter contact regions  85  and  87  are formed in semiconductor layer  81 . Regions  85  and  87  are formed of a main strip from which several thinner strips extend. The thinner strips of regions  85  and  87  are interdigited in layer  81 . A lightly-doped P-type base region  89  (P − ) is also formed in semiconductor layer  81 . Region  89  is also formed of a main strip from which thinner strips extend. Each thinner strip of base  89  is located, in semiconductor layer  81 , between the thin strips of emitter and collector contact regions  85  and  87 . Heavily-doped P-type fingers  91 , similar to fingers  53  of  FIG. 4 , cross the thinner strips of base region  89  and extend on either side of these strips. 
     Semiconductor layer  81  may have a progressive doping between each of thin base strips  89  and each of thin emitter/collector strips  85  and  87 , to promote the distribution of the field lines in semiconductor layer  81  when the transistor is off. 
     The interdigited structure of  FIG. 8  enables to form, on a small substrate surface area, a transistor made of several transistors in parallel which have a high on-state gain. The breakdown voltage across the main terminals of the transistor in the off state may also be high. 
     Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, it should be noted that all the conductivity types given herein may be inverted to form, instead of an NPN power transistor, a PNP power transistor. 
     Further, although the structure of a bipolar transistor has been described herein, it should be noted that, by forming an insulated gate at the surface of strip  37 , a MOS controlled bidirectional transistor may also be formed. 
     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.