Abstract:
The invention concerns a bidirectional electronic switch of the pulse-controlled bistable type comprising a monolithic semiconductor circuit including a vertical bidirectional switch structure (TR; ACS) provided with a gate terminal (G 1 ), first (Th 1 ) and second (Th 2 ) thyristor structures whereof the anodes are formed on the front face side, the first thyristor anode region containing a supplementary P-type region ( 6 ), and a metallization (A 1,  A 2 ) connected to the main surface of the front face of the vertical bidirectional component and to the second thyristor anode; a capacitor (C) connected to the first thyristor anode and to the second thyristor supplementary N-type region; and a switch (SW) for short-circuiting the capacitor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a bistable bidirectional switch, that is, a switch capable of being turned on for several halfwaves of the A.C. voltage applied thereto, after a single control pulse. This bistable switch can then be turned off by application of a new pulse and remain off until it receives a new turn-on pulse. 
   2. Discussion of the Related Art 
   A first category of bidirectional switches is formed of switches of triac-type or other bidirectional switches corresponding to thyristor associations. A common feature of these components is that they are turned on in a given halfwave by a pulse, and then automatically turn off when the current flowing therethrough falls under a determined threshold, generally called the hold current i H . Then, to turn such bidirectional switches back on, a pulse has to be applied again upon each halfwave during which the component is desired to be on. Thus, such switches are not bistable. 
   Another category of bidirectional switches is formed of components of MOS or bipolar transistor type, which turn on when a signal is applied to their control terminal, but for which this control signal must be continuously maintained for the component to remain on. Such components of transistor type are not controllable with pulses. 
   In prior art, it has been provided to associate semiconductor components and passive components in circuits enabling obtaining a pulse-controlled bistable bidirectional switch. However, such circuits are relatively complex and generally require association of several semiconductor components and of several passive components. 
   SUMMARY OF THE INVENTION 
   The present invention aims at the manufacturing in essentially monolithic form of such a pulse-controlled bistable bidirectional switch. 
   To achieve this and other objects, the present invention provides a bidirectional switch of pulse-controlled bistable type, including: 
   a monolithic semiconductor circuit formed from a lightly-doped N-type substrate having 
   a rear surface coated with a metallization, including: 
   a vertical bidirectional switch structure provided with a gate terminal; 
   first and second thyristor structures, having respective anodes formed on respective front surface sides, the anode region of the first thyristor containing an additional P-type region; 
   a metallization connected to a main front surface of the vertical bidirectional component and to the anode of the second thyristor; 
   a capacitor connected to the anode of the first thyristor and to an additional N-type region of the second thyristor; 
   a switch for short-circuiting the capacitor. 
   According to an embodiment of the present invention, the monolithic semiconductor circuit includes an N-type substrate; and 
   on a front surface side: 
   a first P-type region in which is formed a second N-type region corresponding to a first main electrode of the bidirectional switch, 
   a second P-type region corresponding to an anode of the first thyristor, 
   a third P-type region corresponding to an anode of the second thyristor and containing an additional N-type region; 
   on the rear surface side: 
   a P-type layer; 
   in this P-type layer, N-type regions, interrupted at locations where the bidirectional component includes an N-type region on its upper surface side. 
   According to an embodiment of the present invention, a gate contact is connected with the first P-type region and with an N-type region formed therein, the bidirectional switch structure being of triac type. 
   According to an embodiment of the present invention, the switch includes an isolating wall connecting the upper surface to the lower P-type surface and containing an N-type region on its upper surface side, with which a gate terminal is connected, the bidirectional switch structure being of ACS type. 
   According to an embodiment of the present invention, the rear surface electrode is connected to an A.C. voltage, the front surface electrode of the bidirectional switch structure being grounded. 
   According to an embodiment of the present invention, the front surface electrode is connected to an A.C. voltage, the front surface electrode of the bidirectional switch structure being grounded. 
   The foregoing and other 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, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified cross-section view of a first embodiment of a bistable bidirectional switch according to the present invention, 
       FIGS. 2  to  4  illustrate various operating phases of the bistable bidirectional switch of  FIG. 1 , 
       FIG. 5  is a simplified cross-section view of a second embodiment of a bistable bidirectional switch according to the present invention, and 
       FIGS. 6 and 7  are simplified cross-section views of alternatives to the second embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   As illustrated in  FIG. 1 , a bistable bidirectional switch according to the present invention includes a monolithic semiconductor component or circuit formed from a semiconductor substrate  1  and a capacitor C. 
   The vertical semiconductor component includes a central portion corresponding to a vertical triac TR, a left-hand portion corresponding to a first vertical thyristor Th 1 , and a right-hand portion corresponding to a second vertical thyristor Th 2 . 
   The lower surface or rear surface of the monolithic semiconductor component is coated with a metallization which is connected to a terminal A 2 . This rear surface corresponds to a main electrode of the triac and to the cathodes of the first and second thyristors. The second main electrode A 1  of the triac, the anodes of the first and second thyristors, and a gate terminal G 1  of the triac are located on the front surface side. The anode of the first thyristor is connected to a terminal  10  of a capacitor C having its other terminal connected to ground. The second main electrode A 1  of the triac is grounded. The first main electrode A 2  is connected to an A.C. voltage, for example the mains at 50 or 60 Hz, via a load which is desired to be controlled. The triac gate is accessible from a terminal G 1 . The anode of thyristor Th 2  is connected to terminal A 1 , that is, to ground. The anode region of thyristor Th 2  contains an additional N-type region which is connected to terminal  10 . A switch SW is connected in parallel with capacitor C. Switch SW is controllable from a terminal G 2 . 
   As indicated, the monolithic semiconductor component is formed from a substrate  1 . This substrate is lightly doped of type N. On the rear surface side of the substrate are formed a P-type layer  2  and N-type regions  3 . N-type regions  3  are conventionally absent in front of appropriate areas of triac TR and are present in front of anode region  4  of thyristor Th 1  and of anode region  5  of thyristor Th 2 . 
   On the front surface side, the additional region formed in anode  5  of thyristor Th 2  is designated with reference  6 . The triac includes a P-type area  7  in which is formed an N-type region  8 . Regions  7  and  8  and anode region  5  of thyristor Th 2  are coated with a metallization connected to terminal A 1 . A metallization covers additional region  6  and is connected to terminal  10 . Finally, a metallization covers anode region  4  and is also connected to terminal  10 . Gate G 1  is connected to a metallization which covers a portion of region  7  and an N-type region  9  formed therein. 
   Before explaining the operation of the device of  FIG. 1 , the conventional designations of the triggering modes of a bidirectional switch, for example, a triac, should be recalled. The triac is said to operate in one or the other of four quadrants Q 1 , Q 2 , Q 3 , Q 4 . A reference terminal, generally grounded, is chosen, and the biasings of the voltage on the other terminal, here terminal A 2 , and of the gate voltage on terminal G 1 , are considered. The triac is said to be triggered in first quadrant Q 1  when the voltages on terminals A 2  and G 1  are positive with respect to terminal A 1 . In second quadrant Q 2 , the voltage on terminal A 2  is positive and the voltage on terminal G 1  is negative. In third quadrant Q 3 , the voltages on terminals A 2  and G 1  are negative. Finally, in fourth quadrant Q 4 , the voltage on terminal A 2  is negative and the voltage on terminal G 1  is positive. 
   The operation of the component of  FIG. 1  after a pulse has been applied on gate terminal G 1 , to extract or insert a current in the gate, will now be studied. 
   It will first be considered that, at the time when the switch according to the present invention is controlled to turn on, electrode A 2  is positive with respect to electrode A 1 . In this case, the applying of a voltage on terminal G 1  triggers the triac in quadrant Q 1  or Q 2 . 
   Then, as illustrated in  FIG. 2 , a current flows from terminal A 2  to terminal A 1  through triac TR. A current also flows from terminal A 2  to terminal  10 , via a PNP-type transistor T 1  formed of P-type rear surface layer  2 , N-type substrate  1  and P-type region  4 . Capacitor C then charges to a voltage equal to the on-state voltage drop between terminals A 2  and A 1  minus the voltage drop at saturation (VCEsat) of transistor T 1 . As will be seen hereafter, the capacitor, during the on-state period of the triac in the considered halfwave, charges to a voltage level greater than 0.6 V. This level can be reached quite easily since the on-state voltage drop of a triac normally is on the order of 1.5 V while the saturation voltage of a PNP transistor normally is on the order of 0.3 V. The gain of the PNP transistor must however further be sufficient and the main current in the triac, which corresponds to the base current of the PNP transistor, must also be sufficient for the transistor to saturate. Further, the capacitance of capacitor C must be sufficient since, as will be seen hereafter, the charges accumulated in this capacitor will turn the triac on at the following halfwave. In a practical example, a capacitance on the order of 4.7 μF may be chosen for capacitor C. It should subsidiarily be noted that, during this operating phase in which terminal A 2  is positive with respect to terminal A 1 , thyristors Th 1  and Th 2  are reverse-biased and are not on. 
   It should be noted that there also exists a responsive thyristor Th 3  which includes, from its anode to its cathode, P-type layer  2  connected to terminal A 2 , N-type substrate  1 , P-type region  5 , and N-type region  6  (with no short-circuit holes). The cathode gate of this responsive thyristor corresponds to P-type region  5  and is grounded. Although this thyristor is properly biased between the anode and the cathode, it cannot turn on in this operating phase, its gate-cathode voltage being then negative or null (this gate-cathode voltage should be positive to turn thyristor Th 3  on). 
   At the end of the positive halfwave, once the current in the triac becomes smaller than hold current I H  of this triac, said triac tends not to be in a conductive state any more. However, terminal  10  is then more positive than terminal A 1  and, given that charges are present in substrate  1 , the lateral PNPN thyristor having as an anode P-type region  4  and as a cathode N-type region  8  connected to terminal A 1  turns on. It can also be said that the current injected from terminal  10  maintains the current in triac TR above the value of hold current I H  of this triac. Thus, at the end of the positive halfwave, there still exist charges in the substrate in the vicinity of the junction between this substrate and P-type region  7 . 
   Accordingly, as illustrated in  FIG. 3 , when the voltage inverts on terminal A 2  and this terminal becomes negative with respect to terminal A 1 , thyristors Th 1  and Th 2 , which are biased in the on direction, and triac TR, switch on due to the remaining of charges in the substrate at the time of the voltage inversion. Terminal  10  of capacitor C then negatively charges via thyristor Th 1 , which blocks as soon as this charge reaches a level close to the on-state voltage drop of the triac. However, thyristor Th 2  keeps on conducting and the current distributes between triac TR and thyristor Th 2 . 
   At the step illustrated in  FIG. 4 , it is assumed that the voltage on terminal A 2  becomes positive again. As soon as the voltage on terminal A 2  becomes sufficiently greater than the voltage on terminal A 1 , above-mentioned thyristor Th 3  turns on, a gate current being generated by the discharge of capacitor C and flowing from P-type region  5  to N-type region  6 . The turning-on of thyristor Th 3  generates charges in the substrate and enables turning-on of triac TR. The situation existing at the step illustrated in  FIG. 2  occurs again and capacitor C recharges again with a positive voltage on its terminal  10  to enable repeating the steps previously described in the progress of the successive halfwaves of the A.C. voltage applies to terminal A 2 . 
   It has thus been shown that the assembly of the semiconductor component shown in FIG.  1  and of capacitor C forms a bidirectional switch operating on A.C. current and that can be turned on by a pulse, and then indefinitely remain on under the effect of the charge and discharge of capacitor C. 
   To turn off this switch, capacitor C must be discharged to avoid for it to turn triac TR back on at the next halfwave. A switch SW controlled by a control terminal G 2  in parallel on capacitor C has been shown in  FIG. 1  as an example. Preferably, a discharge resistor will be arranged in series with switch SW. Thus, as soon as a turn-off pulse is applied on terminal G 2 , capacitor C discharges and the triac turns off at the end of the current halfwave. The device can operate with a mere pulse control on terminal G 2 , this pulse preferably having a non-negligible duration as compared to the duration of a halfwave. However, preferably, switch SW will be maintained off as long as triac TR is not desired to be turned back on, to avoid triggering of this triac under the effect of an unwanted pulse on terminal G 1 . Indeed, in the presence of an unwanted pulse, the triac could turn on and remain on under the effect of capacitor C which is not short-circuited. It should be clear for those skilled in the art that, if a device in which switch SW remains off during the off-state phases of the triac is chosen, switch SW should be immediately turned off immediately before applying a pulse on terminal G 1  when a turn-on phase is desired to be initiated. It should however be noted that switch SW can be left on if triac TR is desired to be conventionally operated under the sole effect of a control by gate G 1 . 
   The device according to the present invention can also operate in full-period control. If the portion of the semiconductor shown in  FIG. 1  corresponding to thyristor Th 2  is eliminated, when the triac is started in the first or in the second quadrant, while terminal A 2  is positive with respect to terminal A 1 , a single control pulse at the beginning of a positive halfwave will turn triac TR on for the entire positive halfwave. Then, the conduction will carry on during the following negative halfwave under the effect of the discharge of capacitor C, after which the triac will turn off. 
   Those skilled in the art should understand that the component of  FIG. 1  could also be modified so that it remains on for a complete period after a turning-on on a negative halfwave. For this purpose, one could, for example, roughly keeping the structure of  FIG. 1 , reduce the surface area of P-type region  4  forming the anode of thyristor Th 1 . Thus, transistor T 1  (see  Fig. 2 ) does not enable sufficient charging of capacitor C in a positive halfwave but thyristor Th 1  can do it during a negative halfwave. 
   The turn-on control of the switch according to the present invention in a quadrant Q 1  or Q 2 , that is, at the time when terminal A 2  is positive with respect to terminal A 1 , has previously been described. In a quadrant Q 3  or Q 4 , that is, at a time when terminal A 2  is negative with respect to terminal A 1 , the system actually amounts to that described in relation with FIG.  3 . The initial starting is caused by the action on gate G 1  and, then, thyristors Th 1  and Th 2  turn on and capacitor C charges to initiate an operation which continues as previously described. 
   According to an advantage of the present invention, the state (on or off) of the switch according to the present invention can be determined at any time. It is enough to measure the voltage across the capacitor. If, during a period (20 ms), this voltage exceeds a value of 0.2 V, the switch is known to be on. A comparison with a ±0.2 V threshold could also be performed every 10 ms. These values of 20 and 10 ms are given in the case of a periodic voltage of the 50-hertz mains. These values will be changed as appropriate if the A.C. voltage applied to the switch according to the present invention is at a frequency other than 50 hertz, for example, if it is a 60-hertz voltage. 
   The present invention is likely to have various alterations, modifications, and improvements which will occur to those skilled in the art. In particular, switch SW may be integrated in an isolated portion of substrate  1 . It should be noted that this switch may be a low-voltage switch since capacitor C sees across its terminals at most a voltage on the order of from 1 to 3 V. Thus, it is particularly simple to associate with the capacitor an easily assembled low-cost low-voltage switch. 
   According to an advantage of the present invention, gates G 1  and G 2  may both be grounded. The control signals on these gates are referenced with respect to the ground and thus are easily-implemented low-voltage signals. 
   The switch according to the present invention is easy to use since it can be started in any operating phase and in any of quadrants Q 1  to Q 4 . Similarly, the turning-off can be performed at any time. 
   The component may be adapted to specific requirements by optimizing PNP transistor T 1  (FIG.  2 ). 
   N-type regions  11  arranged on the upper surface side between the various P-type regions have been shown in FIG.  1 . These N-type regions are optional and have the usual function of channel stop regions intended for avoiding the occurrence of surface leakage currents. 
   On the other hand, the power triac surface area can be reduced, since auxiliary thyristor Th 2  completes the operation of this triac in one of its operating biasings (terminal A 2  negative with respect to terminal A 1 ). 
   According to an alternative of the present invention, a bidirectional switch other than a triac may be used, for example a bidirectional component having its reference electrode with respect to which the gate is controlled corresponding to the rear surface metallization. Such a component, which will be referred to as ACS (trade name registered by STMicroelectronics Company) is especially described in US patent application Ser. No. 6,034,381 (B3073), which is incorporated herein by reference. 
   The application of the present invention to such a component is illustrated in  FIG. 5  which shows the elements of a vertical bidirectional ACS component between terminals A 1  and A 2  at the center of the drawing. Thyristor Th 1  includes, from its anode to its cathode, P-type region  4 , substrate  1 , P-type region  7 , and N-type region  8 . Thyristor Th 2  includes, from its anode to its cathode, P-type region  5 , substrate  1 , P-type region  7 , and N-type region  8 . On either side of this component or more generally next to this component are formed lateral thyristors Th 1  and Th 2 . This time, A 1  designates the main rear surface terminal, which is grounded, and A 2  designates the main front surface terminal, which is connected to an A.C. voltage via a load. The starting of the vertical bidirectional component is ensured by an electrode G 1  connected to an N-type region  21  formed in a P-type extension  22  of an isolating wall  23  crossing the semiconductor wafer and in continuity with rear surface P-type layer  2 . As previously, a capacitor C keeps the component in the on-state upon each bias switching after the initial turning-on of the vertical component. A terminal of capacitor C is grounded. The other terminal of capacitor C is connected to the anode of thyristor Th 1  and to N-type region  6  formed in anode layer  5  of thyristor Th 2 . Those skilled in the art should understand that the operation of this system is similar to what has previously been described. Thyristors Th 1  and Th 2  now are lateral thyristors. The equivalent of vertical PNP transistor T 1  shown in  FIG. 2  is a horizontal PNP transistor T 2 , the emitter of which corresponds to P-type region  7  of the vertical bidirectional component, the base of which corresponds to N-type substrate  1 , and the collector of which corresponds to P-type anode region  4  of lateral thyristor Th 1 . 
   To determine the on or off state of the bidirectional switch of  FIG. 5 , it can as in the preceding case be determined whether capacitor C is charged or not during a halfwave or a period of the applied A.C. signal. In this case, a detection element may also be provided on the other side of an isolating wall which is crossed by carriers when a significant current is conducted by the power component as described in U.S. patent application Ser. No. 09,705,113 (B4438), which is incorporated herein by reference. 
     FIGS. 6 and 7  show alternative embodiments of the device of FIG.  5 . These alternatives essentially consist in modifications of the collector region of transistor T 2  (anode region of thyristor Th 1 ). In both cases, this anode region is extended by a P-type drive-in to increase the transistor gain. 
   In the case of  FIG. 6 , this drive-in region is designated by reference  25  and joins a drive-in  26  also of type P formed from the lower surface. The lower portion of drive-in  26  is covered with an oxide layer  27  to isolate P-type regions  4   25 - 26  from electrode A 1 , which is grounded. 
   In the case of  FIG. 7 , only drive-in  25  is formed. 
   Further,  FIGS. 6 and 7  show various detail alternatives with respect to the representation of FIG.  5 . Especially, gate area G 1  has been shown on the left-hand of the drawing rather than on the right-hand side to simplify the representation. 
   The present invention is likely to have various other alternatives and modifications which will readily occur to those skilled in the art, especially as concerns modifications of the main vertical bidirectional power component and alternative embodiments of auxiliary elements Th 1  and Th 2 . 
   The bistable bidirectional component according to the present invention could be formed individually in a silicon wafer or belong to a general structure incorporating other components of the same type, as described for example in U.S. Pat. No. 6,075,277 (B2578), which is incorporated herein by reference.