Abstract:
Presented is an integrated circuit structure having a power transistor in a first well and control circuitry in another well. Between the power and control regions is an intermediate region including a biaging circuit secured to prevent flow of parasitic current from the wells into the substrate by biasing the intermediate region at a value of potential which is tied to the value of potential of the first well. The biasing circuit can include a bipolar transistor.

Description:
TECHNICAL FIELD 
     This invention relates to an integrated circuit structure which comprises a power circuit portion and a control circuit portion and is free from parasitic currents. 
     BACKGROUND OF THE INVENTION 
     In an integrated electronic structure including a power circuit portion, specifically a bipolar type of power device with a vertical current flow, and a control circuit portion, such as a P-type region, a parasitic PNP transistor is created between the base of the power device and the P region. 
     FIG. 1 shows a conventional integrated circuit  1 ′ comprising a semiconductor substrate  2 ′ of the N-type which is formed with a first well  3 ′ of the P type provided for forming a power device, and a second well  4 ′, also of the P type, comprising a control region. 
     In particular, where a bipolar power transistor Power is to be provided, a third well  5 ′ of the N type is formed inside the first well  1 ′, in which the emitter terminal for the bipolar power transistor Power can be formed. The power transistor Power will have its collector terminal in the semiconductor substrate  2 ′ and its base terminal in the first well  3 ′. 
     A fourth wall  6 ′, also of the N type, is likewise formed inside the second well  4 ′ and may be connected to a supply voltage reference Vcc, for example. 
     Consequently, the integrated circuit  1 ′ will include a first parasitic transistor P 1 , whose emitter terminal is coincident with the base terminal of the bipolar transistor Power, i.e., with the first well  3 ′. The base terminal of this parasitic transistor is coincident with the collector terminal of the bipolar power transistor Power, i.e., with the semiconductor substrate  2 ′, and its collector terminal is coincident with the second well  4 ′ facing the bipolar power transistor Power. 
     In order for the first parasitic transistor P 1  to be turned off, the value of the potential applied to its base terminal must be at least equal to, or higher than, the value of the potential applied to its emitter terminal. 
     This condition is always met when the power device Power is operated in the linear range. But in most applications, the power device Power will be operated actually in the saturation range, since it is to serve a switching function. 
     Under this operating condition, the base-collector junction of the bipolar power transistor Power, and hence the emitter-base junction of the first parasitic transistor P 1 , would be forward biased. 
     There practically occurs a current injection from the first well  3 ′, where the power transistor is formed, to the second well  4 ′. The value of this current is a function of the gain of the first parasitic transistor P 1 . 
     The appearance of this current in the second well  4 ′ causes malfunctioning, of the control logic of the integrated circuit  1 ′. For the integrated circuit  1 ′ to operate correctly, it is necessary that the potential at the second well  4 ′ be lower than, or equal to, the potential at the semiconductor substrate  2 ′ in which the bipolar power transistor Power and control circuit portion are both formed. It is only by meeting this restriction on potentials that the turning on of a second parasitic transistor P 2 , having its base terminal coincident with the second well  4 ′, emitter terminal coincident with the semiconductor substrate  2 ′, and collector terminal coincident with the fourth well  6 ′, can be prevented. 
     It should be considered, in particular, that the second well  4 ′ is essentially biased to a reference value Visas through a resistive path which is represented by a resistive element R 1 . The injected current from the turning on of the first parasitic transistor P 1  obeys the following relation; 
     
       
         Vbias+R 1 *I−V(2)=VbeP 2   (1) 
       
     
     where, 
     Vbias is the bias voltage of the second well  4 ′; 
     R 1  is the resistance of the resistive path inside the second well  4 ′; 
     I is the current injected into the second well  4 ′ from the turning on of the first parasitic transistor P 1 ; 
     V(2) is the value of the potential applied to the semiconductor substrate  2 ′; and 
     VbeP 2  is the base-emitter voltage of the second parasitic transistor P 2 ; 
     the parasitic transistor P 2  is turned on, causing malfunction to occur in the control region of the integrated circuit. 
     A first known technical solution to the problem posed by the presence of parasitic transistors provides for that area of the semiconductor substrate  2 ′ that lies intermediate between the first well  3 ′ and the second  4 ′ to be doped more heavily. In this way, the gain of the first parasitic transistor P 1  is reduced. 
     However, this doping must not be carried too far, if the integrated circuit  1 ′ is to be held at the correct voltage. In addition, the flow of current between the wells of the P type is reduced but not eliminated, with this solution. 
     A second solution provides for increased spacing of the P-wells. However, not even this solution ig effective to suppress the flow of current brought about by the turning on of parasitic transistors. 
     A third solution provides for an intermediate region  7 ′, also of the P type, to be included between the aforementioned P-wells, as shown schematically in FIG.  2 . 
     Unfortunately, this solution also has several drawbacks: 
     First, where the intermediate region  7 ′ is a floating region, a self-biasing of the intermediate region  7 ′ to the value of potential of the first well  3 ′ is precipitated upon a parasitic PNP transistor P 1 ′, associated with the well  3 ′ and the intermediate region  7 ′, entering its saturation range. As a result, an additional parasitic transistor P 1 ″, associated with the second well  4 ′ and tho region  7 ′, is caused to move into its conduction range. The net effect of providing this intermediate region  7 ′ is one of lowering the current gain of the parasitic elements as a whole: the net effect of the parasitic components is split between the two transistors, P 1 ′ and P 1 ″, but one (P 1 ′) of them will be in its saturation range. 
     Second, when the intermediate reio  7 ′ is connected to a voltage reference, e.g., to ground, and by reason of the application involved the second well  4 ′ is biased to a potential level below ground, the parasitc transistor P 1 ″ will move into its conduction range and draw current from the ground reference terminal to the second well  4 ′. 
     Therefore the prior art does not adequately solve the problems of parasitic flow of current in these types of integrated circuits due to parasitic components. 
     SUMMARY OF THE INVENTION 
     Presented is an integrated circuit comprising a power device and a control region that has structural and functional features that eliminates the parasitic flow of current. The circuit is formed on a semiconductor substrate with conductivity of a first type, and incorporates a first circuit portion incorporated in a first well that includes at least one power transistor. The circuit also has a control circuit portion incorporated in a second well, and an intermediate region located between the first and second circuit portions. The conductivity of the first well, second well, and intermediate region are all of a second type. The intermediate region between the wells is biased as a function of the potential of the well wherein the power device is formed. 
     The features and advantages of an integrated circuit structure according to the invention will become apparent from the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a cross section of an integrated circuit that includes at least one power device and a control region, according to the prior art. 
     FIG. 2 is a diagram showing a cross section of an alternative embodiment of the integrated circuit of FIG.  1 . 
     FIG. 3 is a diagram showing a cross section of an integrated circuit according to an embodiment of this invention, which includes at least one power circuit portion and a control circuit portion. 
     FIG. 4 shows a schematic diagram of the integrated circuit created by the structure of FIG.  3 . 
     FIG. 5 is a diagram showing a cross section of an integrated circuit according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawing views, specifically to the example of FIG. 3, an integrated electronic circuit is shown generally and schematically at  1  which includes a power circuit portion comprising at least one power device  2 , and which includes a control circuit portion  3 . A region  4  is provided intermediate the first and second circuit portions. 
     In particular, the integrated circuit  1  comprises a semiconductor substrate  5  of the N-type which has a first well  6  of the P type and a second well  7  of the P type formed therein for respectively accommodating the power device  2  and the control circuit portion or region  3 . 
     Formed respectively inside the first  6  and second  7  wells are additional wells  8  and  9  of the N type. In particular, the well  9  inside the second well  7  will be connected, illustratively, to a supply voltage reference Vcc to create the control region  3 . Further, the second well  7  is essentially biased to a bias voltage reference Vbias through a resistive path represented by a resistive element R 1 . 
     The bipolar power device PW is as arranged to have all emitter terminal within the well  8 , a collector terminal in the semiconductor substrate  5 , and a base terminal within the first well  6 . 
     The presence of NPN junctions at the wells  6  and  7  of the P type, combined with a semiconductor substrate  5  of the N type, originates parasitic components. In particular, the integrated circuit  1  has a first parasitic transistor P 2  whose emitter terminal is coincident with the semiconductor substrate  5 , collector terminal is coincident with the well  9 , and base terminal is coincident with the second well  7 . Its base terminal will, therefore, be biased to the bias voltage reference Vbias via the resistive element R 1 . 
     As previously mentioned in connection with the conventional integrated circuit, an intermediate region  4 , also of the P type, is provided between the first well  6  and the second well  7  and substantially splits, between second P 3 ′ and third P 3 ″ parasitic transistors, the parasitic effect of the PNP junction linked to said P-wells  6  and  7  associated with the semiconductor substrate  5  of the N type. 
     In particular, the second parasitic transistor P 3 ′ has its emitter terminal coincident with the first well  6 , collector terminal coincident with the intermediate region  4 , and base terminal coincident with the semiconductor substrate  5 . Likewise, the third parasitic transistor P 3 ″ has its emitter terminal coincident with the intermediate region  4 , as collector terminal coincident with the second well  7 , and base terminal coincident with the semiconductor substrate  5 . 
     Advantageously in this embodiment, a circuit is arranged to bias the intermediate region  4 , differently, according to the potential applied to the first well  6 . This circuit includes a biasing circuit  10  connected to the intermediate region  4  and the first well  6 . 
     More particularly, the biasing circuit  10  comprises a bipolar transistor T having its emitter terminal connected to a voltage reference such as ground GND, collector terminal connected to the intermediate region  4 , and base terminal connected to an output terminal OUT of the biasing circuit  10  through a series of first R 2  and second R 3  resistive elements. 
     In addition, a circuit node X intermediate the first R 2  and second R 3  resistive elements is connected to the first well  6 . 
     Any suitable controlled switch could be substituted for the transistor T. 
     The use of a bipolar transistor T makes for a simple overall construction of the integrated circuit. In fact, a bipolar transistor T can be readily formed either inside the second well  7 , i.e., in the control circuit portion of the integrated circuit, or within the intermediate region  4  itself. In particular, efficiency largely benefits from the transistor T being formed in the intermediate region  4 . 
     To make the operation of the integrated circuit containing a biasing circuit  10  according to embodiments of the invention more easily understood, its equivalent circuit, shown in FIG. 4 will be discussed first. 
     In particular, the equivalent circuit  100  of FIG. 4 comprises a power transistor PW which has an emitter terminal connected to the ground reference GND, a base terminal connected to a first node  51 , corresponding to the first well  6 , and a collector terminal connected to a second node  52 , corresponding to the semiconductor substrate  5 . 
     The equivalent circuit  100  also includes a parasitic transistor P 2  having a collector terminal connected to the supply voltage reference Vcc, an emitter terminal connected to the second node  52 , and a base terminal connected to a third node  53  corresponding to the second well  7 . 
     As previously explained, the third node  53  is connected to a bias voltage reference Vbias through a resistive element R 1 . 
     Finally, the equivalent circuit  100  includes a parasitic transistor P 3 ′ and a parasitic transistor P 3 ″, which are connected in series with each other between the first node  51  and the third node  53  and have their base terminals connected to the second node  52 . 
     The parasitic transistors P 3 ′ and P 3 ″ are connected together at a fourth node  54 , corresponding to the intermediate region  4  and itself connected to the collector terminal of the transistor T in the biasing circuit  10 . 
     The transistor T also has its emitter terminal connected to the ground reference GND, and its base terminal connected to the intermediate circuit node X via the resistive element R 2 . The intermediate circuit node X is connected to the first node  51 , and connected to the output terminal OUT of the biasing circuit  10  via the resistive element R 3 . 
     The combination of the transistor T and the resistive element R 2  operates like a switch connected between the fourth node  54  and ground GND and controlled by the potential at the first node  51  that is at the base terminal of the power device PW. Thus, the resistive element R 2  functions as a decoupling element. 
     Against the background of the equivalent circuit  100  just described, the circuit action under different conditions of operation follows. 
     When the power device PW is in the ON state, the collector terminal can either have a value of potential close to the ground reference GND value (“saturated device”) or a value of a few Volts (“unsaturated device”), while the base terminal is set at the value of the base-emitter voltage (Vbe) in the conduction range; 
     When the power device PW is in the OFF state, the collector terminal goes to a high voltage value which will be specific to a particular application. 
     It should be noted that, with the power device PW in the ON state, its collector terminal may also go to a negative potential. In this case, a current would be caused to flow from the ground reference to the semiconductor substrate  5 ; therefore, the integrated circuit is normally provided with a diode which has its anode connected to the ground reference and its cathode connected to the semiconductor substrate  5 . 
     In the instance of an unsaturated device being ON, its base-emitter junction is always reverse biased, and the parasitic transistors P 3 ′, P 3 ″ would be OFF regardless of the value of the potential at which the intermediate region  4  is biased. 
     In this case, the transistor T of the biasing circuit  10  would be ON, and the intermediate region  4  biased all the same at the ground reference value. 
     In the instance of a saturated device being ON, its base-emitter junction is forward biased, and the parasitic transistor P 3 ′ would be ON, whereby a current would be injected into the intermediate region  4 . 
     Advantageously in this embodiment of the invention, the transistor T of the biasing circuit  10  would be ON in this case, and drains this current back to the ground reference, thereby preventing the current from reaching the second well  7  through the parasitic transistor P 3 ″. 
     Furthermore, the biasing circuit  10  keeps the potential applied to the intermediate region  4  at a lower value than, or the same value as, that of the semiconductor substrate  5 , thereby ensuring that the parasitic transistor P 3 ″ is turned off. In fact, this parasitic transistor P 3 ″ has its base-emitter voltage Vbe near zero and, accordingly, will be “off” regardless of the value of potential at the second well  7 . 
     In the event of the device being ON in a “below ground” condition, that is for high values of the potential at its external base terminal or contact, the value of the potential at the collector terminal would be normally clamped at 0.7V through a diode in parallel with the power device itself. 
     When the semiconductor substrate  5  is at a value of potential equal to −0.7V, the first well  6 , intermediate region  4 , and second well  7  will all have a value of potential of approximately 0V, and the PN junctions (i.e., the diodes) formed by the above regions and the semiconductor substrate  5  will be forward biased. 
     Advantageously, the transistor T of the biasing circuit  10  would be OFF in this case, since its base and collector terminals are at the same value of potential, which. value is lower than, or equal to, the value of the potential at the emitter terminal, which is at the ground reference value. 
     Thus, the intermediate region  4  is “disjoined” from the biasing circuit  10 , ensuring isolation of the voltage at the collector terminal of the power device from the regions  6  and  7 . 
     In this way, the conduction of the parasitic transistor P 3 ″ and consequent injection of current into the second well  7  are prevented. 
     Lastly, with a device in the OFF state, the value of potential of the semiconductor substrate  5  will always be higher than the value of potential of the P regions, and no parasitic PNP transistors would be ON between said regions and the semiconductor substrate  5 . 
     FIG. 5 is a diagram showing a practical embodiment of the integrated circuit with biasing circuit  10 , wherein the circuit  10  comprises a bipolar transistor T formed in the intermediate region  4 . 
     In particular, a first interior well  11  of the N type, providing the collector terminal for the bipolar transistor T, is formed in the intermediate region  4 . 
     Inside this first interior well  11  there are diffused a second interior well  12  of the P type, providing the base terminal for the bipolar transistor T, and a third interior well  13  of the N type which provides the emitter terminal for the transistor T and is diffused inside the second interior well  12 . 
     Advantageously, the first interior well  11  is connected to the intermediate region  4  by an external short circuit  16 . In particular, this external short circuit  16  is produced in an area of the intermediate region which is facing the first well  6  that accommodates the power device PW. 
     Further, the second interior well  12  is connected to the first well  6  through the resistive element R 2 , and connected to the output terminal OUT of the biasing circuit  10  through the additional resistive element R 3 . 
     Finally, the third interior well  13  is connected to the ground reference GND. 
     To summarize, with an integrated circuit incorporating embodiments of this invention, all current that flows due to the presence of parasitic PNP transistors between the P-type wells and the N-type substrate can be cut off, through the intermediate region  4  of the P type being connected between the power circuit portion and the control portion and biased by means of the biasing circuit  10  to suit varying conditions of the power device, operation. 
     Changes can be made to the invention in light of the above detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all methods and devices that are in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined by the following claims.