Abstract:
A triac including on its front surface side an autonomous starting well of the first conductivity type containing a region of the second conductivity type arranged to divide it, in top view, into a first and a second well portion, the first portion being connected to a control terminal and the second portion being connected with said region to the main front surface terminal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a triac capable of operating in quadrants Q 1  and Q 4  and that can, in these quadrants, be turned on by a small gate current Igt, while having a low sensitivity to an unwanted turning-on by a voltage peak (dV/dt turning-on).  
         [0003]     2. Discussion of the Related Art  
         [0004]      FIG. 1  shows the equivalent diagram of a triac that comprises main terminals A 1  and A 2  and a gate terminal G referenced to terminal A 2 . By analogy with a thyristor, there is a tendency to call terminal A 1  the anode and terminal A 2  the cathode, but it should be noted that in a triac, terminal A 1  is intended to be connected to a load, itself connected to a positive or negative A.C. supply voltage with respect to terminal A 2 . Considering that terminal A 2  is grounded, the control quadrants of a triac are defined as follows: 
        first quadrant (Q 1 ): A 1  positive, G positive,     second quadrant (Q 2 ): A 1  positive, G negative,     third quadrant (Q 3 ): A 1  negative, G negative,     fourth quadrant (Q 4 ): A 1  negative, G positive.          
         [0009]      FIGS. 2 and 3  respectively show a simplified cross-section view and a simplified top view of an example of a conventional triac. This triac is formed in a lightly-doped N-type substrate  1  surrounded with a heavily-doped P-type insulating wall. The rear surface of the structure essentially comprises a P-type layer  3 . On the front surface side is formed a P-type well  5  in which are formed a main N-type region  6  and an auxiliary N-type region  7 . On the rear surface side, in layer  3 , is formed an N-type region  9  facing the P-type well, mainly in the regions unoccupied by main N-type region  6 . The entire rear surface is coated with a metallization M 1  intended to be connected to main terminal A 1  of the triac. On the front surface side, a main metallization M 2 , in contact with well  5  and region  6 , is intended to be connected to terminal A 2  of the triac, generally connected to ground GND. There thus exists a thyristor Th 1  formed of regions  3 - 1 - 5 - 6  that can be turned on in quadrants Q 1  and Q 2  and a thyristor Th 2  formed of regions  5 - 1 - 3 - 9  that can be turned on in quadrants Q 3  and Q 4 . Auxiliary N-type region  7  is coated with a metallization M 3  intended to be connected to gate terminal G.  
         [0010]     An N + -type channel stop ring substantially halfway between the limit of P well  5  and insulating wall  2  has further been shown in  FIG. 2 . Ring  11  is generally coated with a metallization  12  not connected to an external terminal. Similarly, the component periphery generally comprises a heavily-doped P-type ring  13 , formed in insulating wall  2 , coated with a metallization  14  unconnected to an external terminal. The portions of the upper silicon surface which are not in contact with a metallization are protected by an insulating layer  16 , generally made of silicon oxide, and the entire upper surface, except for the metallization portions to be contacted, is coated with an insulating layer  17 , for example, a PSG glass.  
         [0011]     In the top view of  FIG. 3 , the internal portion of channel stop ring  11  has been shown. A portion of substrate  1  appears between the outside of P well  5  and ring  11 . The limit of main N-type region  6  and the limit of auxiliary N-type region  7  have also been shown. On the one hand, the area in which metallization M 2  is in contact with wall  5  and region  6  and, on the other hand, the area in which metallization M 3  is in contact on the one hand with the auxiliary gate region, on the other hand with a portion of well  7  (this contact between metallization M 3  and a portion of P well  7  is not visible in the cross-section view of  FIG. 2 ), have been shown with dotted lines. It should be reminded that the starting of a triac, which will not be described in detail herein, starts with the flowing of a current between the gate and the cathode along the resistive path designated as R GK  in  FIG. 3 , which causes, after several intermediary steps, the turning-on of that of thyristors Th 1  and Th 2  which is properly biased.  
         [0012]     It is known that such triacs can be relatively easily controlled in quadrants Q 1 , Q 2 , and Q 3 , but operate poorly or not at all in quadrant Q 4 . Thus, it is not possible to control the gate with a positive signal, whatever the biasing of the main triac terminals. This thus leads to systematically controlling the triacs in quadrants Q 2  and Q 3 , that is, with a negative gate voltage with respect to ground, which makes control circuits more complex.  
         [0013]     Further, because of integrated resistor R GK  between the gate and the cathode, it is very difficult to obtain structures with a small gate-cathode control current further having good dynamic performance, especially as it relates to positive unwanted triggerings by abrupt voltage variations thereacross (dV/dt turning-on). Indeed, the smaller R GK , the higher the insensitivity to a dV/dt turning-on and, however, the higher the control currents must be. This is a disadvantage since it would be desired to be able to easily drive triacs with small currents to be able to, for example, control them directly with a microcontroller. On the other hand, the dV/dt strength is an important parameter to avoid triggerings by unwanted noise, especially in automobile applications.  
       SUMMARY OF THE INVENTION  
       [0014]     Thus, an object of the present invention is to provide a triac which is responsive in quadrants Q 1  and Q 4 , in which the compromise between the gate control current and the dV/dt strength can be optimized.  
         [0015]     Another object of the present invention is to provide such a triac which is easy to form with existing technologies.  
         [0016]     To achieve these and other objects, the present invention provides a triac comprising on its front surface side an autonomous starting well of the first conductivity type containing a region of the second conductivity type arranged to divide it, in top view, into a first and a second well portion, the first portion being connected to a control terminal and the second portion being connected with said region to the main front surface terminal.  
         [0017]     According to an embodiment of the present invention, the auxiliary well is arranged in the immediate vicinity of the main front surface well.  
         [0018]     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  
       [0019]      FIG. 1  shows the usual symbol of a triac;  
         [0020]      FIG. 2  is a simplified cross-section view of a conventional triac;  
         [0021]      FIG. 3  is a simplified top view of a conventional triac;  
         [0022]      FIG. 4  is a simplified cross-section view of a triac according to the present invention; and  
         [0023]      FIG. 5  is a simplified top view of a triac according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]     As usual in the representation of semiconductor components, the dimensions of the cross-section and top views are not to scale but have been arbitrarily expanded to simplify the drawings and make them more easily readable.  
         [0025]      FIGS. 4 and 5  show in cross-section view and in top view, in simplified fashion, an embodiment of a triac according to the present invention. In these views, same reference numerals as in  FIGS. 2 and 3  are used to designate similar elements which will not be described again. In particular, thyristors Th 1  ( 3 - 1 - 5 - 6 ) and Th 2  ( 5 - 1 - 3 - 9 ), connected between metallizations M 1  and M 2 , are present in  FIG. 4 .  
         [0026]     The main difference between the present invention and the state of the art described in relation with  FIGS. 2 and 3  lies in the triac starting area. The present invention provides an auxiliary P-type well  25  separate from main P-type well  5  in which main N-type region  6  is formed. In well  25  is formed an auxiliary N-type region  26  shallower than the well. As better shown by the top view of  FIG. 5 , region  26  extends into well  25  to divide it in top view into a portion  25 - 1  and a portion  25 - 2  on either side of region  26 .  
         [0027]     The upper surface of portion  25 - 1  of well  25  is coated with a metallization M 31 , and the upper surfaces of region  26  and of portion  25 - 2  of well  25  are coated with a metallization M 32 . Metallization M 32  is connected to metallization M 2  connected to main terminal A 2 , currently grounded. In practice, although this has not been shown, metallizations M 2  and M 32  altogether form one and the same metallization. Metallization M 31  is connected to a control terminal or gate terminal G.  
         [0028]     When a positive voltage is applied on terminal G with respect to terminal A 2 , a current flows from terminal G to terminal A 2 . This current flows through the P-type semiconductor forming P well  25  from portion  25 - 1 , under region  26  to reach portion  25 - 2 . When this current becomes sufficient for the voltage drop to be greater than approximately 0.6 volt, the PN junction between well  25  and region  26  becomes conductive, which results in an injection of electrons into substrate  1 . Then, in quadrant Q 1 , that is, when lower terminal A 1  is positive with respect to upper terminal A 2 , a pilot thyristor formed of regions and layers  3 - 1 - 25 - 26  is started; this starting causes the presence of a great density of carriers in substrate  1 , which causes the turning-on of power thyristor Th 1 . In quadrant Q 4 , that is, when lower terminal A 1  is negative with respect to upper terminal A 2 , the emission of electrons in the substrate causes the starting of a pilot thyristor formed of regions and layers  25 - 1 - 3 - 9 , followed by the starting of power thyristor Th 2 .  
         [0029]     An advantage of the present invention results from the fact that there no longer exists in the starting structure an integrated resistor equivalent to resistor R GK  described in relation with  FIGS. 2 and 3 . Thyristors Th 1 , Th 2  can thus be desensitized with respect to dV/dt startings, for example, by providing emitter short-circuit holes in N-type regions  6  and  9 , without for this to have an influence upon the triac turn-on characteristics.  
         [0030]     The present invention enables obtaining a significant increase in the turn-on sensitivity in quadrants Q 1 , Q 4 . First simulations, non refined, have shown that, all other things being equal, in quadrant Q 1 , a starting is obtained with the present invention for a 3.9-mA gate current Igt, while with an equivalent conventional structure a current on the order of 6.1 mA had to be provided. Similarly, in quadrant Q 4 , a 23.1-mA turn-on current Igt can be used instead of a 27-mA current Igt with a prior art structure. This difference is all the more significant as, in the case of a structure according to the present invention, an insensitivity to an unwanted dV/dt turning-on which is substantially twice as strong is obtained. Further, still better results can be expected by optimizing the structures. First simulations show that an improvement of more than 50% should be obtained.  
         [0031]     Of course, the present invention is likely to have various, alterations, improvements, and modifications which will readily occur to those skilled in the art. In particular, various modifications generally provided in conventional triacs may easily adapt to a triac according to the present invention.  
         [0032]     On the other hand, the topographies of the various layers may be modified. In particular, the rectangular shape of the turn-on area shown in  FIG. 5  is an example only of embodiment of the present invention. What matters is that the auxiliary P well intended for the turning-on comprises an N region which, in top view, substantially separates it in two portions so that the current from one to the other of the two portions must flow into the well under the N region and can turn on the PN junction between the N region and the P well.  
         [0033]     An advantage of the structure according to the present invention is that, main and auxiliary P wells  5  and  25  both being at the same potential, they can be arranged very close to each other without it being necessary to provide a large distance of N substrate therebetween. Indeed, since they are at the same potential, a self-shielding phenomenon occurs and the field lines are not likely, if these wells are close enough, to penetrate into the region interposed between the two wells. Thus, the structure according to the present invention causes no significant increase in the surface area dedicated to the gate region of the component. It does not require either a specific manufacturing step to form the gate area and/or to isolate other elements of the component.  
         [0034]     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.