Abstract:
A vertical voltage-controlled bidirectional monolithic switch formed between the upper and lower surfaces of a semiconductor substrate surrounded with a peripheral wall, including: a first multiple-cell vertical IGBT transistor extending between a cathode formed on the upper surface side and an anode formed on the lower surface side; and a second multiple-cell vertical IGBT transistor extending between a cathode formed on the lower surface side and an anode formed on the upper surface side, in which the cells of each transistor are arranged so that portions of the cells of a transistor are active upon operation of the other transistor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to voltage-controlled bidirectional vertical components and more specifically to voltage-controlled bidirectional vertical switches for medium-power applications enabling, for example, switching of loads connected to the mains, such as electro-domestic appliances. 
     2. Discussion of the Related Art 
     FIG. 1 is a very simplified cross-section view of a voltage-controlled vertical bidirectional switch formed from two vertical transistors of IGBT type (isolated gate bipolar transistor). It should be clear, as usual in the representation of semiconductor components, that the drawing is not to scale. 
     Switch  10  includes two vertical IGBT transistors  11 A,  11 B. Each transistor is comprised of a lightly-doped N-type substrate  13 A,  13 B delimited by a P-type isolating wall  14 A,  14 B. The lower surface of substrate  13 A,  13 B is uniformly coated with a P-type layer  15 A,  15 B. 
     On the upper surface side of substrate  13 A,  13 B is arranged a cell network  16 A,  16 B. For clarity, only two cells  16 A,  16 B are shown for each transistor  11 A,  11 B. Each cell  16 A,  16 B is comprised of a well  17 A,  17 B including a heavily-doped P-type central region  18 A,  18 B and a more lightly doped P-type peripheral region. A heavily-doped N-type ring  19 A,  19 B is formed in well  17 A,  17 B. The portion of well  17 A,  17 B outside of ring  19 A,  19 B is covered with an isolated gate  20 A,  20 B. Each transistor  11 A,  11 B also includes a P-type ring  21 A,  21 B, which surrounds cell network  16 A,  16 B. A peripheral heavily-doped N-type channel stop ring  22 A,  22 B is located at the periphery of transistor  11 A,  11 B. 
     A cathode metallization M 1 A, M 1 B contacts central region  18 A,  18 B of well  17 A,  17 B and N-type ring  19 A,  19 B of each cell  16 A,  16 B, as well as P-type ring  21 A,  21 B. An anode metallization M 2 A, M 2 B covers P-type layer  15 A,  15 B. A metal ring M 3 A, M 3 B is connected to peripheral channel stop ring  22 A,  22 B to make it equipotential. Similarly, a metal ring M 4 A, M 4 B is connected to isolating wall  14 A,  14 B to make it equipotential. 
     The terminals associated with isolated gates  20 A,  20 B of each transistor  11 A,  11 B are respectively designated with references G 1 , G 2 . 
     The lower surface of transistor  11 A, located on the left-hand side of FIG. 1, directly rests on a radiator  23 . The lower surface of transistor  11 B, located to the right of FIG. 1, rests on radiator  23  with an interposed insulator  24 . 
     The vertical bidirectional switch is obtained by connecting the two vertical IGBT transistors  11 A,  11 B as follows. Metallization M 1 A of transistor  11 A, located to the left of FIG. 1, is connected by an electric connector  28  to the anode, formed by metallization M 2 B, of transistor  11 B located to the right of FIG.  1 . The cathode formed by metallization M 1 B of this latter transistor  11 B is connected by an electric connector  29  to radiator  23 . The two IGBT transistors  11 A,  11 B are thus connected in antiparallel. The main terminals of bidirectional switch  10  correspond to radiator  23  and to metallization M 1 A. According to the voltages on gates G 1 , G 2 , one or the other of the two transistors, or none of them, can be turned on. A voltage-controlled bidirectional switch is thus obtained. 
     The above voltage-controlled bidirectional switch has the disadvantage of not being monolithic. Conversely, it includes two transistors formed on separate chips. It thus has a relatively significant bulk and requires use of wirings to connect the two transistors. Further, one of the transistors is laid on the radiator via an insulator. It is often difficult to obtain an insulator both ensuring a good electric insulation between the transistor and the radiator and a satisfactory heat exchange between the two elements. 
     SUMMARY OF THE INVENTION 
     The present invention aims at monolithically forming a voltage-controlled bidirectional switch. 
     To achieve this and other objects, the present invention provides a vertical voltage-controlled bidirectional monolithic switch formed between the upper and lower surfaces of a semiconductor substrate surrounded with a peripheral wall, including a first multiple-cell vertical IGBT transistor extending between a cathode formed on the upper surface side and an anode formed on the lower surface side; and a second multiple-cell vertical IGBT transistor extending between a cathode formed on the lower surface side and an anode formed on the upper surface side, in which the cells of each transistor are arranged so that portions of the cells of a transistor are active upon operation of the other transistor. 
     The present invention also provides a vertical voltage-controlled bidirectional monolithic switch formed in a substrate of a first conductivity type surrounded with a peripheral wall of a second conductivity type, including a network of upper cells, formed on the upper surface side of the substrate, each upper cell being formed of a ring of the first conductivity type formed in a well of a second conductivity type, the well region outside of the ring forming a channel ring covered with an upper isolated gate; an upper metallization, forming a first main electrode, connected to the central region of the well and to the ring of each upper cell; a network of lower cells similar to the upper cells, formed on the lower surface side of the substrate, opposite to the network of upper cells; and a lower metallization forming a second main electrode, connected to the central region of the well and to the ring of each lower cell. 
     According to an embodiment of the present invention, the lower isolated gate is connected to a sink which crosses a region of the substrate from the lower surface to the upper surface, the sink being connected to a gate contact formed on the upper surface side. 
     According to an embodiment of the present invention, the lower isolated gate is connected to the sink by a metallization isolated from the lower metallization. 
     According to an embodiment of the present invention, the lower metallization covers the entire lower surface. 
     According to an embodiment of the present invention, the lower metallization is connected to the peripheral wall. 
     According to an embodiment of the present invention, the substrate region crossed by the sink is isolated from the substrate regions where the networks of upper and lower cells are formed by the peripheral wall which extends in an auxiliary wall of the second conductivity type. 
     According to an embodiment of the present invention, the peripheral wall extends on the lower surface side in a lower ring of the second conductivity type surrounding the network of lower cells. 
     According to an embodiment of the present invention, on the upper surface side, an upper ring of the second conductivity type surrounds the network of upper cells and is connected to the upper metallization. 
     According to an embodiment of the present invention, on the upper surface side, a heavily-doped channel stop ring of the first conductivity type surrounds the upper ring of the second 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, shows a simplified cross-section view of a bidirectional switch according to prior art; 
     FIG. 2 shows a simplified cross-section view of a bidirectional switch according to the present invention; and 
     FIG. 3 shows a simplified top view of an embodiment of the bidirectional switch of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     As illustrated in FIG. 2, switch  10  is formed in an N-type substrate  40  surrounded with a P-type isolating wall  41 . On the upper surface side of substrate  40  is formed a network of cells  42 , only three cells being shown in FIG.  2 . Each cell  42  includes a well  43  comprised of a heavily-doped P-type region  44  and of a more lightly doped P-type region. A heavily-doped N-type ring  45  is formed in well  43 . The portion of well  43  outside of ring  45  is covered with an isolated gate  46 . 
     On the lower surface side of substrate  40  is formed an similar arrangement of cells  48 . The structure of cells  48  is identical to that of cells  42 . On FIG. 2, only three lower cells  48  are shown. A lower isolated gate  49  covers the well portion outside of the ring of each lower cell  48 . 
     Wall  41  includes a ring-shaped extension  50  on the lower surface side of the substrate which surrounds the network of lower cells  48 . 
     As in prior art, on the upper surface side, a P-type upper ring  51  having a lightly-doped external periphery surrounds the network of upper cells  42 . A heavily-doped N-type channel stop ring  52  is arranged around upper P-type ring  51 . 
     A metallization M 1 ′ contacts central region  44  of well  43  and the N-type ring  45  of each upper cell  42 , as well as P-type ring  51 . A metallization M 2 ′ contacts on the lower surface side the central region of the P-type well and the N-type ring of each lower cell  48 . Metallization M 2 ′ covers the entire lower surface of substrate  40 . It is further connected to peripheral wall  41 . 
     A metal ring M 3 ′ in contact with channel stop ring  52  equalizes the voltage on channel stop ring  52 . Similarly, a metal ring M 4 ′ in contact with peripheral wall  41  equalizes the voltage thereon to the level of the upper surface. 
     Metallization M 2 ′ is assembled on a conductive support  54 , for example, a radiator, with which it is in electric contact. 
     Lower isolated gate  49  is connected by a metallization M 5 ′ to a conductive sink  55  which thoroughly crosses the monolithic circuit to reach the upper surface where a gate contact  56  is made. Sink  55  is formed in a portion  57  of the substrate which is isolated from the active switch portion by peripheral wall  41 , which extends in a complementary isolating wall  58 . Sink  55  may be formed by a standard through wall manufacturing method. Sink  55  may also correspond to a heavily-doped P-type area formed according to a so-called temperature gradient zone melting (TGZM) technique, to a wall formed from trenches or to a metal via. Gate terminals G 1 , G 2  are respectively connected to isolated gate  46  and to gate contact  56 . 
     FIG. 3 shows in a very simplified manner an example of distribution of the different elements located on the upper surface level of the monolithic circuit. Upper cells  42  are represented by simple squares. The limits of P-type ring  51  are shown with the lines bearing references  51 - 1  and  51 - 2 . The limit of the upper isolated gate  46  is shown by a dotted line  59 . The limit of metallization M 1 ′ is shown by line  60 . Contact pad  56  of lower isolated gate  49  is shown to the bottom left of FIG. 3. A contact pad  61  of upper isolated gate  46  is shown to the top center of FIG. 3 by a square surface. The contacts between metallization M 1 ′ and P-type ring  51  are symbolized by rectangles in dotted lines  62 . Contact pad  61  of upper gate  46  may include a ring portion, not shown, which surrounds upper cell network  42 . 
     The operation of the bidirectional switch according to the present invention is the following. 
     The switch behaves as two IGBT-type transistors arranged in antiparallel. 
     Main terminals A 1 , A 2  shown on FIG. 2 of the switch are taken on the one hand on metallization M 1 ′ and on conductive support  54 . The switch is controlled by contact terminals G 1 , G 2  of upper and lower gates  46  and  49 . 
     As an example, when terminal A 2  is positively biased with respect to terminal A 1 , when gate terminal G 1  is controlled and when gate terminal G 2  is not controlled, the current path from terminal A 2  to terminal A 1  schematically is the following. From metallization M 2 ′, the current crosses, for each lower cell  48 , the forward junction between the heavily-doped P-type region and N-type substrate  40 , then crosses, for each upper cell  42 , the channel formed in the region of well  43  outside of ring  45  to join metallization M 1 ′. The current can flow through upper cells  42 , due to the application on gate terminal G 1  of a control voltage enabling, in each upper cell  42 , formation of a channel in the region of well  43  external to ring  45 . 
     When terminal A 1  is positively biased with respect to terminal A 2 , when gate terminal G 2  is controlled and when gate terminal G 1  is not controlled, the current path from terminal A 1  to terminal A 2  schematically is the following. From metallization M 1 ′, the current crosses, for each upper cell  42 , the forward junction between the heavily-doped P-type region and N-type substrate  40 , then crosses, for each lower cell  48 , the channel formed in the region of well  43  external to ring  45 , to join metallization M 2 ′. The current can flow through lower cells  48 , due to the application on gate terminal G 2  of the control voltage which enables, in each lower cell  48 , formation of a channel in the well region external to the ring. 
     A component which behaves as a voltage-controlled vertical bidirectional switch is thus obtained. 
     The switch according to the present invention is particularly well adapted to medium-power applications, for example, to withstand voltages on the order of 600 V, and currents ranging between 1 and 50 A. As an example, for currents on the order of 8 A, the monolithic circuit may have general dimensions of 3.6 millimeters by 4.2 millimeters. Wall  41 ,  58  surrounding sink  55  delimits a surface of approximately 200 μm by 200 μm. Each cell may have dimensions of 25 μm by 25 μm and may be spaced apart from the next cells by 25 μm. 
     Those skilled in the art should note that the off-state breakdown voltage of the switch according to the present invention is essentially defined by the junctions between the substrate and the cell wells. The doping of these wells will be optimized to reach the selected breakdown voltage while enabling satisfactory control of the channel regions of the cells. 
     The present invention has many advantages. 
     The two IGBT transistors forming the switch are integrated in a single monolithic component directly assembled on a support on the side of a metal layer which completely covers the lower surface of the monolithic component. The switch assembly is thus simplified as compared to prior art since the steps of attaching one of the transistors on an insulating layer as well as of forming various internal connections of the switch by electric wirings have been eliminated. 
     Further, in the general case where the support is a radiator, a good thermal exchange is obtained between the monolithic circuit and the radiator due to the lower metallization covering the entire lower surface of the monolithic circuit. 
     Further, the forming of cell networks on both surfaces of the monolithic circuit enables obtaining a component having its surface area approximately divided by two with respect to that of a switch of prior art. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. Thus, the conductivity types of the different elements forming the switch may be inverted. Further, the upper and lower cells have been shown to be perfectly opposite to one another. They may be shifted to optimize the path followed by the current. 
     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.