Integrated structure circuit for the protection of power devices against overvoltages

Abstract: An integrated structure protection circuit suitable for protecting a power device (M) against overvoltages comprises a plurality of serially connected junction diodes (D1-D5), each having a first electrode, represented by a highly doped region (1) of a first conductivity type, and a second electrode represented by a medium doped or low doped region (2) of a second conductivity type. A first diode (D1) of said plurality has its first electrode (1) connected to a gate layer (5) of said power device (M) and its second electrode (2) connected to the second electrode (2) of at least one second diode (D2-D5) of said plurality, and said at least one second diode has its first electrode (1) connected to a drain region of the power device (M). The doping level of the second electrode (2) of the diodes (D1-D5) of said plurality is suitable to achieve sufficiently high breakdown voltage values.

Description: The present invention relates to an integrated structure circuit for the protection of power devices, particularly MOSFETs and IGBTs (Insulated Gate Bipolar Transistors), against overvoltages. Power MOSFETs and IGBTs are frequently employed to drive an inductive load, such as an ignition coil. Due to the type of load, overvoltages appear across the device when it is switched off; said overvoltages can be very high and cause the device, which still drains the nominal current for a short time interval after it has been switched off, to be hardly stressed. Moreover, high overvoltages can lead to the breakdown of the device, and to its permanent damage if it is not able to dissipate the energy stored in the inductive load. To prevent such problems, power devices are generally protected with an external clamping diode having a breakdown voltage lower than that of the device to be protected. Stray inductances and operating times of the protection device, however, limit the effectiveness of such protections. In order to overcome such limits, the manufacturers of power MOS devices have developed integrated structure protection circuits, called "active clamps", comprising a resistive element, connected in series between an external gate electrode and the gate of the power device, and a clamping element between the gate and the drain of power device. When the power device is switched off, overvoltages give rise to a voltage drop across the terminals of the clamping element; if said voltage drop exceeds the breakdown voltage of the clamping element, current will flow through it and the resistive element, between the drain of the power device and the gate electrode. As soon as the voltage drop across the resistive element exceeds the turn-on voltage of the power device (which, in the case of power MOSFETs is represented by the threshold voltage), this will turn-on again and start draining current; thus the current that must be sustained by the active clamp is very low. Overvoltages across the power device will be clamped to a maximum value (Vc lamp) represented by the sum of the breakdown voltage of the clamping element and the turn-on voltage necessary to sustain the current flowing through the power device itself. It should be understood that a simple diode, whose anode and cathode are respectively connected to the gate and to the drain of a n-channel power MOSFET, is not suitable, since when the power MOSFET is in the on state said diode would be forward biased, and the current flowing from the gate to the drain through said diode would introduce an offset equal to the forward voltage of the diode in the output charactersistics of the device. A possible clamping element is represented by a pair of diodes back-connected by their anodes, while their cathodes are respectively connected to the gate and to the drain of the power MOSFET. It is furthermore possible to serially connect a plurality of such diodes, provided that the gate of the power MOSFET is not connected to any of their anodes: in this way it is possible to reach the desired value for Vclamp. In the prior art mentioned in US-A-5162966 an active clamp for a power MOSFET is described wherein the clamping element utilizes a chain of n + /p + junction diodes. The anode of each diode is made up of a p + region extending from the semiconductor surface into an n - epitaxial layer representing the drain of the power MOSFET, while the cathode is an n + diffusion into said p + region. Each diode of the chain has associated with it a parasitic diode whose anode and cathode are respectively represented by the p + region and by the n - epitaxial layer. It follows that the gate of the power MOSFET shall not be connected to any diode anode, in order to prevent one of said parasitic diodes from affecting the output characteristics of the device in the on state, because of the electrical connection between the gate and the drain. To this purpose, in the described structure the first diode of the chain is connected to the gate of the n-channel power MOSFET by its cathode, and has its anode connected to that of the second diode of the chain; all the other diodes have their cathode connected to the anode of the following diode, while the cathode of the last diode is connected to the drain of the power MOSFET. A problem arising from the use of p + /n + junction diodes is represented by their low breakdown voltage, tipically in the range of few Volts. When the power MOSFET is in the on state, the first diode of the chain is reverse biased; if the voltage across it exceeds its breakdown, current will flow through it and its associated parasitic diode, thus between the gate and the drain of the power MOSFET, affecting the output characteristics of the power device. Another problem associated with the low breakdown voltage of the p + /n + diodes of the chain is the necessity of a series connection of a large number of diodes in order to operate with high Vclamp value; this causes complications on the layout and increases the area occupied by the protection circuit. A further problem is represented by the impossibility to obtain active clamps with small thermal drift coefficient, since this parameter is given by the sum of the thermal coefficients of the breakdown voltage of the single diodes of the chain. Finally, it is impossible to obtain active clamps with Vclamp values different from the sum of the threshold voltage of the power MOSFET plus an integer multiple of the breakdown voltage of a p + /n + diode. In view of the state of the art described, the object of the present invention is to provide an integrated structure circuit for the protection of power devices, particularly MOSFETs and IGBTs, against overvoltages, which is not affected by the abovementioned problems. According to the present invention, such object is attained by means of an integrated structure protection circuit as set forth in claim 1. When an overvoltage occurs, each intermediate diode is forward biased, as well as the first diode of said plurality, while all the second diodes of the plurality are reverse biased; current will start flowing when they avalanche, and the intermediate diode are turned on. Furthermore, thanks to the structure according to the embodiment of the present invention, it is possible to reduce the thermal drift of Vclamp; the positive thermal coefficient of the diodes' breakdown voltage is in fact compensated by the negative thermal coefficient of the forward voltage Vf of the forward biased intermediate diodes. Moreover, since Vc lamp is given by the sum of the threshold voltage of the power device, the sum of breakdown voltages of the reverse biased second diodes of said plurality, and the sum of the forward voltage of the forward biased intermediate diodes, it can be varied almost continuously in steps corresponding to Vf (generally lower than 1V) by providing the desired number of such forward biased intermediate diodes. The present invention will now be described in greater detail in non-limiting way with reference to the annexed drawings, wherein: Figure 1 is a sectional view of an integrated structure protection circuit according to the present invention, with superimposed a corresponding equivalent electrical diagram; Figures 2-7 shows sectional views representing intermediate steps of the fabrication process of a power device provided with said integrated structure protection circuit. With reference to Figure 1, an integrated structure protection circuit according to the present invention comprises a plurality of junction diodes, for example five diodes D1-D5 connected in series. Each diode D1-D5 comprises an n + type cathode region 1, extending from a top surface of a semiconductor substrate 4 within a respective p type or p - type anode region 2 which in turn extends into an n - type epitaxial layer 3 grown on said semiconductor substrate 4, for example of the n + type; diodes D1, D4 and D5 are thus made up of p/n + junctions, while diodes D2 and D3 are made up of p - /n + junctions. The cathode region 1 of diode D1 is connected, by means of a superimposed conductive gate electrode layer G, to a polysilicon gate layer 5 of a power device (Fig. 7), while its anode region 2 is surrounded by an annular p type deep body region 6; one side of the latter is merged with a second annular p + type deep body region 7 which surrounds the p - type anode region 2 of diode D2; the anode regions 2 of diodes D1 and D2 are thus in electrical contact. The cathode region 1 of each of diodes D2-D4 is connected via separated strips of a superimposed conductive layer 8 (insulated from the top surface by an interposed insulating layer 11) to an n + type cathode region 9 of a forward biased serially connected intermediate diode belonging to a plurality of intermediate diodes, for example three diodes DF1-DF3; said n+ type cathode region 9 extends from the top surface within a respective p + type deep body region 10 representing an anode region of said intermediate diode DF1-DF3. Each of such intermediate diodes DF1-DF3 can be short-circuited by contacting its respective anode region 10 with the strip of conductive layer 8 which also contacts its cathode region 9 (as in the case of diodes DF1 and DF2 in Figure 1): this only requires to modify a single mask in the fabrication process, as will be explained later on. One side of the anode region 10 of each of the intermediate diodes DF1 and DF2 is merged with an annular p + type deep body region 12 surrounding the p type or p type anode region 2 of the following diode D3 and D4; this ensures the electrical contact between the anode regions 10 of intermediate diodes DF1 and DF2 and the anode regions of diodes D3 and D4 respectively. The anode region 10 of the intermediate diode DF3 is in contact with the anode region 2 of the diode D5, whose cathode region 1 extends laterally from inside the anode region 2 into the epitaxial layer 3, and is consequently in contact with a drain region of the power MOS device. As appears from the drawings, the cathode region 9 and the anode region 10 of intermediate diode DF3 are not short circuited, and said diode is effectively forward connected in series between diodes D4 and D5. A fabrication process suitable to obtain a power device M, for example MOSFET-type, provided with the integrated structure protection circuit of the present invention starts with the growth of the low-doped epitaxial layer 3, for example of the n type, over the highly-doped semiconductor substrate 4, in this example of the n + type. After a field oxide layer 18 has been grown, a selective implantation and subsequent diffusion of a high concentration of a p type dopant is performed, in order to form the p + type deep body regions 6, 7 and 10 of the protection circuit and a plurality of p + deep body regions 13 for the elementary cells 14 of the power MOSFET (Figure 2). After a masking step, active areas are defined on the surface of the epitaxial layer 3; a thin gate oxide layer 16 is then grown over said active areas, and a polysilicon gate layer 5 is subsequently deposited over said gate oxide layer 16, and doped to achieve low resistivity (Figure 3). The polysilicon gate layer 5 is then selectively etched outside said gate regions, and after a masking step, a low concentration of p type dopant is implanted and diffused under said gate regions to form p - type channel regions 17 of each cell 14 of the power MOSFET, as well as the p - type anode region 2 of the diode D2 of the protection circuit. If, as in the present example, some diodes of the protection circuit shall have medium doped p type anode regions 2, a masked implant and subsequent diffusion of a medium concentration of p type dopant is performed. In such a way the effects of a parasitic vertical npn transistor (whose emitter, base and collector are respectively represented by the cathode region 1 and the anode region 2 of the diodes D1-D5 and the drain of the power MOSFET) are reduced. The breakdown voltage of such diodes is lower than that of the n + /p - diodes, but higher than that of the n + /p + diodes; this allows an additional degree of freedom in the active clamp design. This implantation can as well be performed into the p + type deep body region 13 of the cells 14, to increase the device ruggedness. It is to be noted that this implantation and diffusion step can be performed simultaneously to the implantation and diffusion of the low concentration of p type dopant described above, or even preceed it; a further possibility is to perform this implantation and diffusion step before the definition of the gate regions. In any case, the condition must be respected that the dopant concentration is intermediate between those of the p + type regions and of the p - type regions. The following step is the selective implant and diffusion of a high concentration of n type dopant at the sides of said gate regions, to form source regions 19 of the power MOSFET M, and inside the p + , p and p - type regions 10 and 2 to form the cathode regions 1 and 9 of the diodes of the protection circuit. An insulating oxide layer 11 is then deposited over the polysilicon gate layer 5 (Figure 6). The final steps of the process involve the definition of contact areas, the deposition of a conductive layer needed to ensure the electrical interconnections among the various components, and the deposition of a passivating layer (Figure 7). By simply modifying the mask defining contact areas it is possible to selectively short-circuit those forward biased intermediate diodes, such as DF1 and DF2 in Figure 1, which are not deemed necessary in the particular application. The metallization of the bottom surface of the substrate 4 is also provided, in order to form a contact to the drain of the power MOSFET. It appears that, apart from the step of implantation of the medium concentration of p type dopant the described process is identical to a known process for the fabrication of a power MOS device. Such step is however optional, in that the diodes D1-D5 could all be made of p - /n + junctions. As it is clear to anyone skilled in the art, the structure according to the invention can also be used to protect Insulated Gate Bipolar Transistors (IGBTs), simply by starting the process flow with a p-type, heavily doped substrate, on which an n-type epilayer similar to the epilayer 3 described before has been grown. Also, the invention applies to p-channel devices; in this case it is necessary to substitute p-type regions for n-type regions and viceversa. Another possibility is to interpose a resistor (for example made of a portion of the polysilicon layer 5) between the gate of the power MOSFET and the cathode region 1 of the first diode, in order to increase the series resistance of the active clamp structure without increasing the series resistance between the gate region and the gate electrode of the power MOSFET.

Claims: Integrated structure protection circuit for a MOS power device (M), said circuit comprising at least one elementary cell (14) having a highly doped deep body region (13) of a second conductivity type extending from a top surface of a semiconductor material (3) of a first conductivity type thereinto, a low doped channel region (17) of the second conductivity type extending from said top surface adjacent to the deep body region, a highly doped source region (19) of the first conductivity type formed in the deep body region and the channel region, and a gate layer (5) substantially over said channel region, said circuit comprising a plurality of serially connected junction diodes (D1-D5) obtained in the semiconductor material (3), each junction diode comprising a first electrode made of a highly doped region (1) of the first conductivity type extending from said top surface within a doped region (2) of a second conductivity type which forms a second electrode of said junction diode and extends into the semiconductor material from said top surface, a first diode (D1) of the plurality having its first electrode (1) connected to said gate layer and its second electrode (2) connected to the second electrode (2) of at least one second diode (D2-D5) of the plurality, the first electrode (1) of said at least one second diode (D2-D5) connected to a drain region of the power device, characterized in that: said doped regions (2) have a doping level either equal to the doping level of the channel region, or intermediate between the doping level of the channel region and the doping level of the deep body region of the power device; said plurality of serially connected junction diodes (D1-D5) comprises a series of said second diodes (D2-D5), interposed between the second electrode (2) of said first diode (D1) and the drain region of the power device (M); and said plurality of serially connected junction diodes (D1-D5) comprises at least one intermediate diode (DF1,DF2,DF3) having a first electrode made of a highly doped region (9) of said first conductivity type which extends from said top surface of the semiconductor material (3) within a highly doped region (10) of said second conductivity type having a doping level equal to the doping level of the deep body region of the power device and representing a second electrode of the intermediate diode, said intermediate diode (DF1-DF3) being interposed between two of said second diodes (D2,D3,D4,D5) in such a way to have its first electrode (9) connected to the first electrode (1) of a preceding second diode (D2-D5) and its second electrode (10) connected to the second electrode of a following second diode (D2-D5). An integrated structure protection circuit according to claim 1, characterized in that said first electrode (1) of a last one of said series of second diodes (D5) of said plurality of diodes (D1-D5) laterally extends outsaid the respective doped region (2) to merge with said drain region of the power device (M). An integrated structure protection circuit according to any of the preceding claims, characterized in that it comprises a resistor interposed between said gate layer (5) of the power device (M) and the first electrode (1) of said first diode (D1). An integrated structure protection circuit according to claim 3, characterized in that said resistor is made of a portion of said gate layer (5). An integrated structure protection circuit according to any of the preceding claims, characterized in that said first conductivity type regions (3,1,9) are semiconductor regions doped with donor impurities, while said second conductivity type regions (2,6,7,10,12) are semiconductor regions doped with acceptor impurities. An integrated structure protection circuit according to claim 5, characterized in that said highly doped regions (1,9) of the first conductivity type are n + type semiconductor regions. An integrated structure protection circuit according to claim 5, characterized in that said highly doped regions (6,7,10,12) of the second conductivity type are p + type semiconductor regions. An integrated structure protection circuit according to claim 5, characterized in that said medium doped or low doped regions (2) are respectively p type or p - type semiconductor regions. An integrated structure protection circuit according to claim 8, characterized in that said medium doped or low doped regions (2) are surrounded by an heavily doped region (6,7,10,12) of the same conductivity type having annular shape. An integrated structure protection circuit according to claims 1 or 2, characterized in that said first conductivity type regions (3,1,9) are semiconductor regions doped with acceptor impurities, while said second conductivity type regions (2,6,7,10,12) are semiconductor regions doped with donor impurities. An integrated structure protection circuit according to claim 10, characterized in that said highly doped regions (1,9) of the first conductivity type are p + type semiconductor regions. An integrated structure protection circuit according to claim 10, characterized in that said highly doped regions (6,7,10,12) of the second conductivity type are n + type semiconductor regions. An integrated structure protection circuit according to claim 10, characterized in that said medium doped or low doped regions (2) are respectively n type or n - type semiconductor regions. An integrated structure protection circuit according to any of the preceding claims, characterized in that said semiconductor material (3) of the first conductivity type is an epitaxial layer (3) grown on a semiconductor substrate (4). An integrated structure protection circuit according to claim 14, characterized in that said substrate (4) is of said first conductivity type. An integrated structure protection circuit according to claim 14, characterized in that said substrate (4) is of said second conductivity type.

Inventors: ZAMBRANO RAFFAELE, ZAMBRANO, RAFFAELE
Applicants: CONS RIC MICROELETTRONICA, CONSORZIO PER LA RICERCA SULLA
Application year: 1993
Main classification: H01L27
IPC codes: H01L27, H01L21