Abstract:
A fast operating, electronic overvoltage protection device intended for a power transistor having at least one control terminal of the MOS type is disclosed. The device comprises a Zener diode associated with the power transistor and integrated together therewith in a semiconductor substrate, and a second transistor connected to the power transistor into a Darlington configuration and also connected to the Zener diode. The protection from overvoltages provided by the device is very fast in operation, and can be implemented in integrated form at reduced cost and without introducing parasitic elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to an electronic overvoltage protection device with a fast-operation feature, intended for a power transistor having a control terminal of the MOS type. More particularly, the invention concerns a device as above, intended for a power transistor having at least one control terminal of the MOS type, which device is of the type comprising a Zener diode associated with the power transistor and jointly integrated in a substrate. 
     2. Discussion of the Related Art 
     As those skilled in the art know well, a power transistor, such as a Power-MOS or IGBT transistor, must be protected during operation against overvoltages. In particular, such a transistor is to be given protection from any overvoltages presented to its drain terminal, or collector terminal for an IGBT transistor. An overvoltage occurring across the transistor, and specifically the presentation of a high potential to either the drain or the collector terminal thereof, can have different origins. For example, an overvoltage can be caused by an emf induced by current variations in the loads being driven from the power transistor, or by a voltage peak on the power supply distribution line. 
     In any case, upon the occurrence of an overvoltage, a large current may flow through the transistor which can force it to operate outside its safe range, and in some cases may result in the transistor being destroyed. Some proposals for providing power transistors with protection from overvoltages are known in the prior art. 
     A first approach is illustrated in FIG. 1, which shows at least one high-voltage Zener diode Z placed between the collector or drain terminal C and the control or gate terminal G of the power transistor. When the voltage at the collector reaches the threshold voltage level of the Zener diode, the resulting current flow through the diode charges the gate G of the transistor, which will be turned back on and clamp the voltage on the collector terminal C to a value equal to the combined values of the threshold voltage of the Zener diode Z, Vth and the voltage drop at the gate terminal G of the transistor, Vg. 
     During the time when the voltage remains clamped to the value Vc, the current flowing through the Zener diode obeys the following relationship: 
     
         Iz=Vg/Rd=(Vth+gm*Id)/R.sub.goff ; 
    
     where Vg is the voltage at the gate terminal, R goff  is the drive resistance connected to the drain terminal which will only be at work with the Zener diode in operation, gm is the transconductance of the power transistor, and Id is the drain current. 
     The current Iz through the Zener diode can attain very high values, especially when the drive resistance is selected low in favor of a high transistor switching speed. The protection device including the Zener diode is usually integrated in a semiconductor, to the same electronic circuit with which the power transistor is incorporated. 
     The prior art proposes a first way of integrating the Zener diode together with the power transistor. FIG. 2 shows schematically a vertical cross-section, drawn to an enlarged scale, of the structure of a high-voltage Zener diode integrated to an isolation well 2 which has been doped P-. Located over the well 2 is an N- doped buried region 3 which forms a PN junction with the well 2. The well 2 and region 3 are contacted by respective anode and cathode terminations. 
     This type of structure has, however, certain disadvantages. The power transistor structure with which this Zener diode protector is to be associated must be modified, which involves increased fabrication complexity and higher cost; the aforementioned PN junction involves the creation of a parasitic bipolar transistor of the npn type which may be triggered on during fast transients; and a Zener diode so constructed exhibits a large thermal drift about the value of the threshold voltage. 
     The prior art also proposes a second approach to providing a protector, which incorporates at least one Zener diode. FIG. 3 shows schematically a vertical cross-section, also to an enlarged scale, through the structure of a high-voltage Zener diode formed of a chain 5 of polycrystalline silicon diodes. This chain of diodes comprises basically a series of PN junctions of polycrystalline silicon deposited over an oxide layer 4 which is covering a semiconductor substrate 6. Respective anode and cathode terminations are provided in contact with the opposed ends of the diode chain 5. 
     This second prior art approach has certain advantages. Specifically, there is no need for additional processing steps, there is an absence of parasitic components and it is stable through temperature changes. 
     However, a Zener diode formed of a chain of polysilicon diodes has a series resistance of relatively high value. If the resistance between the gate and source terminals of the MOS power transistor is small, the Zener voltage combines with a large voltage drop across the series resistance. This causes a rise in the clamping voltage, which may then become as high as the breakdown voltage of the power transistor. 
     With the resistance between the gate and source terminals corresponding to the drive resistance on which the switching time of the power transistor is dependent, it may be appreciated that the polycrystalline silicon Zener diode does not suit fast-switching circuits. 
     In summary, the prior art approaches have been unable to provide a voltage-clamping protection device which can be effective with high switching speeds without introducing additional fabrication process steps and parasitic elements in the power transistor. 
     The underlying technical problem addressed by the present invention is the problem of providing a protection device, particularly for power transistors, which has constructional and functional features to ensure high switching speeds, but involves no additional process steps for its manufacture and introduces no parasitic elements in the power transistor. 
     SUMMARY OF INVENTION 
     The solution provided by the present invention is that of isolating a portion of the circuit area occupied by the power transistor and using it as a current amplifier for a polycrystalline silicon Zener diode. 
     According to one embodiment of the present invention, a fast operating, electronic overvoltage protection device intended for a power transistor is disclosed. The protection device includes a first Zener diode associated with the power transistor and integrated together with the power transistor in a semiconductor substrate, and a second transistor connected to the power transistor in a Darlington configuration and also connected to the Zener diode. The first Zener diode is connected between a control terminal and a conduction terminal of the second transistor. The device further comprises a second diode connected between one terminal of the second transistor and the control terminal of the power transistor and a resistor connected between a control terminal and a conduction terminal of the second transistor. The second diode is formed of a series of junctions in a polycrystalline silicon layer deposited over an insulating oxide layer overlying the substrate of the protection device and the resistor is formed in a layer of polycrystalline silicon forming a gate region of the control terminal of the second transistor. 
     According to another embodiment of the invention, voltage clamp is disclosed comprising first switch means having a first terminal, a second terminal and a control terminal and second switch means having a first terminal coupled to the control terminal of the first switch means, a second terminal connected to the second terminal of the first switch means and a control terminal coupled to the first terminal of the second transistor and connected to the second terminal of the second transistor through a high voltage clamping device. 
     The features and advantages of a device according to the invention will be apparent from the following detailed description of an embodiment thereof, given by way of illustration and not of limitation with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a diagrammatic view of a protection device against overvoltages as provided by the prior art for a power transistor; 
     FIG. 2 shows schematically, to an enlarged scale and in vertical cross-section, the structure of a semiconductor substrate in which a Zener diode, as incorporated in the circuit of FIG. 1, has been formed; 
     FIG. 3 shows schematically, to an enlarged scale and in vertical cross-section, the structure of another embodiment of a Zener diode as incorporated in the circuit of FIG. 1; 
     FIG. 4 is a diagrammatic view of a protection device against overvoltages as provided by the present invention for a power transistor; and 
     FIG. 5 shows schematically, to an enlarged scale and in vertical cross-section, the structure of a semiconductor substrate in which the device of FIG. 4 has been formed. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 4 and 5, schematically shown at 1 is an overvoltage protection device with a fast-operation feature, intended for a power transistor M1 having an input terminal 8 of the MOS type. The transistor M1 is formed in a semiconductor substrate 9. Specifically, the transistor M1 includes a body well 10 formed in an epitaxial layer 11 doped N-, as grown over the substrate 9. 
     The transistor M1 is a P-channel power transistor, and may be of the MOS type. However, the transistor M1 is preferably an insulated-gate bipolar transistor or IGBT, except that it is formed with MOS technology. The transistor M1 has drain, source and gate terminals, which are respectively denoted by the references D, S and G in FIGS. 4 and 5. The drain terminal D contacts the substrate 9 directly. The body well 10 has two active source areas 13 and 14 allowing duplication of the channel region perimeter. The source terminal S contacts the pair of active areas 13 and 14 directly by means of a metallization strip 19. The areas 13 and 14 are doped N+, whereas the well 10 is doped P+. 
     A thin layer of insulating oxide 15 is deposited over the epitaxial layer 11, and functions as gate oxide as well in the region which is to accommodate the gate G of the transistor M1. At a very close location to this gate region, the oxide layer 15 partly overlaps the well 10 and extends sidewards therefrom, over the epitaxial layer 11. A polysilicon region 16 is provided over that portion of the layer 15 which overlaps the well 10. This region 16 represents the gate of the power transistor M1. The region 16 is contacted by a metallization strip 22 which extends to the gate terminal G. The structure of the power transistor M1 is known per se, and accordingly, will not be further discussed. 
     Advantageously in this invention, the protection device 1 includes a second transistor M2 connected between the drain D and gate G terminals of the power transistor M1. It should be mentioned that the second transistor M2 is connected to the power transistor M1 in a Darlington configuration. The second transistor M2 also is a P-channel MOS type of power transistor. This transistor M2 could be an IGBT. The drain, source and gate terminals of the second transistor M2 are denoted by the references D2, S2 and G2, respectively, in FIG. 4. 
     The structure of the transistor M2 is substantially similar to that of the transistor M1, and has its drain terminal D2 in common with the drain terminal D of the transistor M1, and coincident with the substrate 9. The source region is formed in a well 20 having active areas 23 and 24 contacted by a metallization strip 21 which extends to the terminal S2. The gate region of the transistor M2 is formed laterally of the well 20 by a polysilicon layer 26 overlying the protective oxide layer 15 that partly overlaps the well 20 at the area 23. 
     The device 1 further includes at least one Zener diode Z1 placed between the drain terminal D2 and the gate terminal G2 of the second transistor M2. The diode Z1 can withstand relatively high voltages, and on this account, it will be referred to as the high-voltage diode hereinafter. 
     Preferably, the diode Z1 is implemented with a series of PN junctions in the polycrystalline silicon formed over the protective oxide layer 15. The series of PN junctions are formed in a polysilicon layer 17 overlying the oxide layer 15 and deposited concurrently with the polysilicon layers 16 and 26 which form the gate regions of the transistors M1 and M2, respectively. 
     A second Zener diode Z2 is provided between the source terminal S2 of the second transistor M2 and the gate terminal G of the power transistor M1. This second Zener diode Z2 is a low-voltage diode. This second diode Z2 is preferably implemented with a series of PN junctions in the polycrystalline silicon which are formed on top of the insulating oxide layer 15. 
     In addition, a resistor R is placed between the gate terminal G2 and the source terminal S2 of the second transistor M2. The resistor R is formed in the polysilicon, in the same layer 26 where the gate region of the second transistor M2 is formed. The resistor R provided avoids the potential at the gate terminal G2 of the second transistor M2 being indefinite and attaining thus the breakdown value. 
     On the other end, the presence of the second, low-voltage diode Z2 prevents the possibility of the body-drain (well 10-substrate 9) junction of the isolated cells being forward biased when the power transistor M1 is operated in the saturation phase. 
     The device 1 of this invention can be easily integrated by using just one additional mask to those required to form the power transistor M1 alone. Furthermore, the single metallization level of the power transistor M1 is adequate to permit formation of the various connections between the Zener diodes Z1, Z2, the resistor R and the transistors M1, M2. 
     The protection device 1 according to the invention does solve the previously described technical problem, and affords a number of advantages, foremost among which is the fact that the protection from overvoltages provided by the device 1 is quite fast in operation and suitable for implementation in the integrated form at a low cost and without introducing parasitic elements. 
     The high speed of operation is ensured by the high-voltage Zener diode Z1, and by that the power transistor M1 is formed with a small number of elementary cells and has, therefore, a very small intrinsic capacitance. Furthermore, the switching of the power transistor M1 can be driven within a very short time, since the clamp voltage remains quite stable as the drive resistance varies. Another major benefit is represented by the good stability of the Zener voltage with changing temperature, by virtue of the Zeners being formed in the polysilicon. 
     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 the spirit 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.