Abstract:
The invention concerns a voltage-controlled triac-type component, formed in a N-type substrate ( 1 ) comprising first and second vertical thyristors (Th 1 , Th 2 ), a first electrode (A 2 ) of the first thyristor, on the front side of the component, corresponding to a first N-type region ( 6 ) formed in a first P-type box ( 5 ), the first box corresponding to a first electrode (A 2 ) of the second thyristor, the first box containing a second N-type region ( 8 ); and a pilot structure comprising, above an extension of a second electrode region ( 4 ) of the second thyristor, a second P-type box ( 11 ) containing third and fourth N-type regions, the third region ( 12 ) and a portion of the second box ( 11 ) being connected to a gate terminal (G), the fourth region ( 13 ) being connected to the second region ( 8 ).

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of medium power bidirectional switches, and for example of bidirectional switches connectable on the electric A.C. supply network, capable of withstanding voltages of several hundreds of volts. 
   2. Discussion of the Related Art 
   Among known medium-power bidirectional switches, triacs have the advantage of being able to withstand voltages of one polarity or the other, that is, they can be directly placed in a circuit supplied by an A.C. supply network, and further have the advantage of being able to be controlled by a positive or negative gate signal. 
   However, a disadvantage of triacs is that they are controlled by injection of a current. In many cases, it would be preferred for this control to be performed by a voltage, that is, for the triac to turn on when the voltage applied to its control terminal becomes greater, in absolute value, than a determined threshold. 
   A known solution to overcome this disadvantage is to arrange, in series with the gate terminal of a triac, a diac or bidirectional Schottky diode, which turns on when the voltage thereacross exceeds a determined threshold. However, despite the many attempts performed, the monolithic integration of a triac or of a diac has never been achieved in a commercially exploitable manner. 
   SUMMARY OF THE INVENTION 
   Thus, an object of the present invention is to provide a monolithic component of voltage-controlled triac type. 
   To achieve this and other objects, the present invention provides a monolithic component of voltage-controlled triac type, formed in a substrate of a first conductivity type, including first and second vertical thyristors, a first main electrode of the first thyristor, on the front surface side of the component, corresponding to a first region of the first conductivity type formed in a first well of the second conductivity type, said first well corresponding to a first main electrode of the second thyristor, the first well containing a second region of the first conductivity type; and a pilot structure including, on the front surface side, above an extension of a second main electrode region of the second thyristor, a second well of the second conductivity type containing third and fourth regions of the first conductivity type, the third region and a portion of the second well being connected to a gate terminal, the fourth region being connected to the second region. 
   According to an embodiment of the present invention, the component is surrounded at its periphery with a wall of the second conductivity type extending from one surface to the other of the component. 
   According to an embodiment of the present invention, on the front surface side, the first well includes an extension which surrounds the second well. 
   According to an embodiment of the present invention, the external periphery of the first well and of its extension is surrounded by a lightly-doped ring of the second conductivity type. 
   The foregoing object, 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, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an embodiment of a bidirectional switch according to the present invention; 
       FIG. 2  is intended to explain the triggering of a bidirectional switch according to the present invention in quadrants Q 1  and Q 4 ; 
       FIG. 3  is intended to explain the triggering of a bidirectional switch according to the present invention in quadrants Q 2  and Q 3 ; 
       FIG. 4  shows the assembly of a bidirectional switch according to the present invention in an application to a light dimmer; and 
       FIG. 5  is a simplified top view of an embodiment of a bidirectional switch according to the present invention. 
   

   DETAILED DESCRIPTION 
   As illustrated in  FIG. 1 , a monolithic voltage-controlled bidirectional switch according to the present invention is formed in a portion  1  of a lightly-doped N-type semiconductor substrate delimited by a heavily-doped P-type isolating wall  2 . The component includes a proper switch structure corresponding to thyristors Th 1  and Th 2 . This structure includes, on the lower surface side, a P-type layer  3  in a portion of which is formed an N + -type region  4  and, on the upper surface side, a P-type well  5  in which is formed an N + -type region  6 . Thus, thyristor Th 1  includes from its anode to its cathode portions of regions and layers  3 - 1 - 5 - 6 . Thyristor Th 2  includes from its anode to its cathode portions of regions and layers  5 - 1 - 3 - 4 . 
   The entire lower surface of the component is coated with a metallization M 1  connected to a first main terminal A 1  of the switch, terminal A 1  being intended to be connected to an A.C. power supply. The upper surfaces of well  5  and of N +  region  6  are covered with a metallization M 2  connected to a terminal A 2  forming the second main terminal of the component and normally grounded. 
   To achieve a voltage control of this switch, the present invention provides, on the one hand, an additional N + -type region  8  formed in P-type well  5  and coated with a metallization M 3 . It should be noted that metallization M 3  is only in contact with additional region  8  and not with well  5 . Further, the present invention provides a pilot structure including on the upper surface side a P-type well  11  in which are formed separate N + -type regions  12  and  13 . Region  13  is coated with a metallization M 4 . Region  12  and a portion of the upper surface of well  11  are covered with a metallization M 5  connected to a gate terminal G. 
   Further, the component includes various means intended to guarantee its breakdown voltage. A given lateral distance of the N well is left free between peripheral wall  2  and the lateral ends of the elements described hereabove of the switch and of the pilot structure. In this free area is preferably provided an N +  ring  15  having a channel stop function, this ring being possibly coated with a metallization not connected to an external terminal. Further, a P well  16  connected to ground like P well  5  is provided outside of well  11  of the pilot area. In top view, well  16  forms an extension of well  5  which surrounds well  11 . The external periphery of P wells  5  and  16  is bordered with a lightly-doped P-type well ring  17 . 
   The operation of the bidirectional voltage-controlled switch according to the present invention in the four possible starting quadrants will now be discussed in relation with  FIGS. 2 and 3 . 
     FIG. 2  illustrates the operation of a switch according to the present invention controlled in quadrant Q 1 , that is, when terminal A 1  is positive with respect to terminal A 2  and the voltage on the gate is positive. In this configuration, main thyristor Th 1  is likely to be turned on. 
   When the voltage on gate terminal G becomes greater than the sum of the forward voltage drop of the diode corresponding to the junction between P well  11  and N +  region  13  and of the avalanche voltage of the zener diode corresponding to the reverse voltage between N + -type region  8  and P well  5 , a current flows from metallization M 5  to metallization M 4 , from metallization M 3  to metallization M 4 , and from metallization M 3  to metallization M 2 . This current turns on a lateral pilot thyristor SCR 1  having its anode corresponding to well  11  and its cathode corresponding to region  6 , that is, which includes regions and layers  11 - 1 - 5 - 6 . The starting of pilot thyristor SCR 1  generates carriers at the level of the junction between substrate  1  and well  5 , and thus starts main thyristor Th 1  ( 3 - 1 - 5 - 6 ). 
   In the fourth quadrant, in which the voltage on the gate is positive and in which terminal A 1  of the triac is negative with respect to terminal A 2 , the operation is similar to that of the first quadrant as concerns the starting of pilot thyristor SCR 1 . However, given the biasing of the main electrodes, the turning-on of pilot thyristor SCR 1  turns on thyristor Th 2 . 
     FIG. 3  illustrates the operation of the device according to the present invention in the second quadrant, that is, when electrode A 1  is positive with respect to electrode A 2  and the gate electrode is negative with respect to electrode A 2 . Then, as soon as the negative voltage on terminal G exceeds a given threshold, a current flows from terminal A 2  to terminal G, through well  5 , the forward junction between this well and region  8 , the connection between metallization M 3  and metallization M 4 , the reverse junction between region  13  and well  11 , and flows into well  11  towards metallization M 5  under region  12 . Due to the resistance of the P well under region  12 , a voltage drop is created which, when it exceeds 0.6 volts, turns on PN +  junction  11 – 12 . This starts a lateral thyristor SCR 2  having its anode corresponding to P well  5  and its cathode corresponding to N + -type region  12 , and which includes regions and layers  5 - 1 - 11 - 12 . The turning-on of this pilot thyristor creates a carrier generation at the interface between well  5  and substrate  1  and turns on thyristor Th 1 . 
   In quadrant Q 3 , in which electrode A 1  is negative with respect to electrode A 2 , and in which a negative voltage is applied on the gate, the operation is similar as concerns the starting of lateral thyristor SCR 2 , but this time, thyristor Th 2  is turned on, the carrier generation in the substrate turning on the junction between substrate  1  and P-type layer  3 . 
     FIG. 4  shows an example of application of a component according to the present invention to the forming of a light dimmer. 
   An A.C. voltage is connected to terminal A 1  via a load L, for example, an electric light bulb having a power of some hundred watts, terminal A 2  being grounded and forming the second terminal of the supply voltage. The A.C. voltage is also applied to gate terminal G via an adjustable resistor R. The gate is also grounded via a capacitor C. Thus, at the beginning of a halfwave, capacitor C progressively charges with a time constant which depends on the setting of resistor R. When the voltage on capacitor C reaches the positive or negative threshold voltage corresponding to the voltage of a forward diode and of a reverse diode, one or the other of thyristors Th 1  or Th 2  turns on according to the biasing of the considered halfwave. A dimmer has thus very simply been made with a single semiconductor component. It should be noted that the starting occurs in quadrant Q 1  or in quadrant Q 3  according to whether the considered halfwave is positive or negative. Given that the dopings of P wells  5  and  11  and of N +  regions  8  and  13  are respectively identical, the turn-on threshold is substantially identical for negative halfwaves and for positive halfwaves. 
   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, as concerns the dimensions of the various elements, it should be clear that the drawings are very simplified and that most of the component surface area is taken up by the switch structure corresponding to thyristors Th 1  and Th 2  and that the surface area of the pilot thyristor is relatively small. Further, all conductivity types could be inverted, the polarities of the various voltages being modified accordingly. 
     FIG. 5  is a simplified top view of an embodiment of a bidirectional switch according to the present invention. In this drawing, the same elements as in  FIG. 1  will be designated with the same references. In the illustrated embodiment, well  16  forms an extension of well  5  which surrounds well  11 . 
   Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.