Monolithic semi-conductor device having a vertical structure with a deep-base and finger-emitter power transistor having a ballast resistance

Abstract: The device comprises a substrate (3; 20) suitable for operating as the collector of the power transistor, an epitaxial layer (2; 21) superimposed over the substrate (3; 20), a base region (1; 22) accomplished in the epitaxial layer (2; 21) at least one emitter region (6; 24; 54) in buried layer for each finger provided with at least one connection area (11; 14; 26; 32; 55) between it and an emitter surface metallization (8; 27), an emitter contact (15) connected to the emitter surface metallization (8; 27) of all the fingers and base and collector surface metallizations (5; 23). The at least one connection area (11; 14; 26; 32; 55) between the at least one emitter region (6; 24; 54) and the emitter surface metallization (8; 27) is made of a width such as to provide a ballast resistance (Rv) of an adequate value .

Description: The present invention relates to a monolithic semi-conductor device having a vertical structure with deep-base and finger-emitter power transistor having a ballast resistance. The use is known of the vertical structure technology, also known as VIPOWER, for the integration on the same silicon chip of a power transistor and of a control circuit. A process for the manufacture of a structure of thus type is, for example, described in EP-A-0322040. In its more general form a VIPOWER structure is formed by a semi-conductor substrate of a first type of conductivity over which there is superimposed an epitaxial layer of the same type of conductivity. Inside the epitaxial layer there are the power transistor and control transistors suitable for accomplishing the corresponding control circuit. The control transistors are contained entirely in insulation regions accomplished in the epitaxial layer. These regions have a second type of conductivity and are provided with regions of surface connection. Inside the insulation regions there are buried layers suitable for constituting collector regions of the control transistors. The buried layers are accomplished of the first type of conductivity and are provided with surface regions of enrichment. The collector region of each control transistor is thus in the shape of an L and contains base and emitter regions of the transistor itself accomplished one inside the other, of the first and of the second type of conductivity, respectively. Inside the epitaxial layer there is also the base region of the power transistor. This is accomplished of the second type of conductivity and is provided with regions of surface connection. The base region of the power transistor is suitable for circumscribing Laterally and for containing in depth the emitter region of the power transistor, which has the first type of conductivity and is provided with a region of surface enrichment. The collector of the power transistor coincides with the substrate. Instead of a single emitter region a power transistor of the type described above can comprise several regions or fingers connected together and wholly contained within the base region of the transistor itself. In both cases we have a deep-base power transistor. In its embodiment with a finger-emitter, the fingers of the emitter are in part buried in the base region and are provided with connection areas at the surface of the chip. The connection areas are regions with a low resistivity suitable for conveying the current toward the surface where it is collected through a metallization according to the known art described, for example, in the abovementioned European patent application. In order to provide the power transistor with a good safety area, the latter commonly known with the expression "safety operation area", the emitter fingers of the power transistor should be fairly wide, so that the ballast resistances that are formed in the connection areas between the emitter contact areas and the fingers themselves allow a good balance of the currents in the various fingers of the emitter. At high currents the current rating of the transistor is a function of the perimeter of the emitter facing the base region, not of the area of the emitter. In order to have high currents with transistors of the same size it is necessary to reduce the size of the fingers and to increase their number so as to increase the perimeter of the emitter itself. This is in contrast with a good safety area, since by decreasing the width of the connection areas of the fingers there is no more ballast, ie, a certain series resistance for each finger, which makes for a homogeneous distribution of current in the different fingers of the power transistor. JP-A-57034357 discloses another monolithic semiconductor device according to the prior art. The object of the present invention is to allow the accomplishment of a monolithic device with a deep-base and finger-emitter power transistor which, while still operating with high currents and having a limited size, has a homogeneous distribution of current in the various fingers of the emitter. According to the invention such object is attained through a monolithic semi-conductor device including the features defined in claim 1. In this way the advantage is obtained of being able to provide an emitter with a high perimeter and thus in a position of operating with high currents even with a limited size, without reducing the series resistance of each finger of the emitter, that is the ballast resistance, below the value required for making the current distribution homogeneous in the different fingers of the power transistor. The features of the presente invention shall be made more clear by its embodiments illustrated as a non-limiting example in the enclosed drawings wherein: Fig. 1 is view in a vertical cross-section of a power transistor having a vertical structure according to the known art; Fig. 2 is a similar view in a vertical cross-section of a first embodiment of a power transistor having a vertical structure according to the present invention; Fig. 3 is a similar view in a vertical cross-section of a variant of the embodiment of Fig. 2 of the power transistor having a vertical structure according to the invention; Fig. 4 is plan view from above of the emitter of the power transistor of Fig. 3; Fig. 5 is an equivalent circuit diagramme of portions of an emitter finger; Fig. 6 is a plan view from above of an emitter finger according to a variant with respect to Fig. 4; Fig. 7 is a vertical cross-setion of a power transistor having a vertical structure of the hollow emitter type according to the known art; Fig. 8 is a vertical cross-section of a hollow emitter power transistor with deep emitter regions according to the present invention; Fig. 9 is a vertical cross-section of another embodiment of the hollow emitter power transistor according to the invention; Fig. 10 is a vertical cross-section of a variant of the embodiment of Fig. 9 of the hollow emitter power transistor; Fig. 11 is a vertical cross-section of a further variant of the embodiment of Fig. 9 of the hollow emitter power transistor. With reference to Fig. 1, a semi-conductor power transistor having a vertical structure according to the known art comprises, on a monolithic chip 50, a substrate 3 of the type N+ suitable for operating as the collector of the transistor itself, an epitaxial layer 2 of the type N superimposed over the substrate and a base region 1 of the type P accomplished inside the epitaxial layer 2. The base region 1 extends along the surface with vertical connection regions 4 of the type P+ suitabl for being contacted by means of base contacts 5' making part of base metallizations 5. Inside the base region 1 there are emitter regions 6 of the type N+, each constituting a finger of a finger-emitter of the transistor, deeply buried in the base region 1, circumscribed laterally by a second epitaxial layer 30 of the type N, and provided with respective vertical areas 7 of the type N+ for their low resistivity connection to the surface of the chip 50. The connection areas 7 are suitable for being contacted by means of emitter surface contacts 8' making part of emitter surface metallizations 8. The metallizations 8 of the different areas 7 come together in a known way at a common contact suitable for constituting the contact of the finger-emitter of the power transistor. As already said in the introduction, with the object of having a power transistor of limited size that operates with high currents and with a balancing of the currents in the emitter fingers, it is necessary to increase as much as possible the perimeter of the emitter, and thus of each finger of the same emitter, that is, of the individual emitter regions 6 contained in the base region 1, without reducing the so-called ballast resistances which allow the balancing of the currents in the different emitter regions 6. This is obtained, according to the first embodiment of the invention illustrated in Fig. 2, by replacing each vertical connection portion 7 with a pair of vertical connection portion 11 connected at the surface by a horizontal connection portion 12 of the type N+ of a length such as to allow the opening of emitter contact. In this way a parallel resistive is created, constituting a ballast resistance Rv, which starting from the metallization 8 corresponding to a respective emitter region 6 runs parallel to the surface along a horizontal top connection portion 12 and then in depth along a corresponding vertical connection portion 11. With this structure, by having the vertical connection portions 11 of a sufficiently narrow size, it is possible to obtain a number of emitter fingers that is higher than standard by increasing the perimeter of the emitter, but maintaining the balance of the currents in the fingers. The maximum emitter perimeter is obtained, as is obvious, with fingers, and thus emitter regions 6, that are particularly narrow. For this purpose it is possible to use instead of the pair of vertical connection portions 11, corresponding to an emitter region 6, a single vertical connection portion 14 having a suitably-selected narrow width. A variant is thus obtained, illustrated in Fig. 3, of the embodiment of Fig. 2. Instead of the horizontal connection portion 12 between the pairs of vertical connection portions 11, related to an emitter region 6, there is here a contact region 13 of the type N+ whose width is such as to allow a sufficiently wide opening of emitter contact. According to this embodiment for each vertical connection portion 14 there is a ballast resistance Rv of an adequate value, function of the width of the portion 14 and of its doping, interposed between the metallization 8 of the corresponding emitter region 6 and the central area of the same emitter region 6. The presence of the contact region 13 is indispensable in the case wherein the corresponding vertical connection portion 14 has a very small width, of the order of 10 microns. In this case in fact the emitter contact would be narrower than 10 microns and too high a current density could damage the area of contact between the silicon and the metallization 8. A plan view of Fig. 3 from above is illustrated in Fig. 4. There is possible to see the structure with fingers, three in the drawing, represented by the different emitter regions 6, immersed in the base region 1, and by the corresponding metallizations 8 connected to the emitter contact indicated with dotted lines with the numerical reference 15. The connection areas 14 are also indicated with dotted lines. The same figure may be used as a plan view from above of Fig. 2 by replacing each connection portion 14 with a pair of connection portions 11. Considering just one emitter finger, say the middle one of Fig. 4, the corresponding emitter region 6 is divisible into adjacent portions that can be considered similar from a circuit viewpoint to transistors having a common base. Fig. 4 indicates portions 16, 17 which correspond in the circuit diagramme of Fig. 5 to respective transistors T16, T17. The generic portions 16, 17 have the same ballast resistance Rv, or vertical resistance, but different metallization resistance, since they are positioned at different points of the metallization 8 and in particular at a different distance, calculated in a direction parallel to the surface of the chip 50, from the emitter contact 15. For a thickness of 3 microns the metallization resistance can be calculated at some 10 milliohm per square. Indicating with Rm16 and Rm17 the metallization resistances of the transistors T16, T17, Rm16 is greater than Rm17. If we consider that the emitter current rises linearly from one extremity of a generic emitter finger toward the contact 15, it can be deduced that the voltage drop between the portions 16 and 17 becomes very substantial, of the order of hundreds of millivolts, thus de-biasing the portion 16 of the emitter region 6 with respect to the portion 17. As is obvious, this is true provided the ballast resistances Rv are not so large as to make the voltage drops negligible on the portions of metallization 8 interposed between the portions 16, 17 and the contact 15. In order to overcome this drawback it is possible to choose to accomplish the connection portion 14, in the case of Fig. 3, or in altogether similar manner the connection portions 11 in the case of Fig. 2, with a variable width as shown in Fig. 6 so as to have ballast resistances Rv that are differentiated for the different portions 16, 17 of a corresponding emitter region 6. In this way a greater metallization resistance Rm16, as in the case of the portion 16, shall correspond to a smaller ballast resistance Rv and thus a wider portion 18 of the connection area 14, and vice versa a smaller metallization resistance Rm17, as in the case of portion 17, shall correspond to a greater ballast resistance Rv, thus a narrower portion 19 of the connection area 14. A power transistor of the hollow type can also benefit from the introduction of connection areas of the emitter regions to the corresponding metallizations, which have balancing resistances of the currents flowing in the different fingers of the emitter. A vertical structure power transistor of the hollow emitter type according to the known art is shown in Fig. 7. According to such known art a hollow emitter power transistor with a standard structure comprises, in a monolithic chip 51, a substrate 20 of the type N+ suitable for operating as the collector of the power transistor, an epitaxial layer 21 of the type N, a base region 22 of the type P accomplished inside the epitaxial layer 21. The base region 22 extends along the surface with vertical connection areas 25 of the type P suitable for being contacted by means of base contacts 23' making part of base metallizations 23. Inside the base region 22 there are emitter regions 24 of the type N+, two for each emitter finger. The emitter regions 24 of each finger are suitable for being contacted at the surface by a respective emitter surface contact 27' making part of an emitter metallization 27. The set of metallizations 27 comes together in a known manner at a common contact suitable for constituting the contact of the finger-emitter of the hollow emitter power transistor. Starting with the standard structure of Fig. 7, a hollow emitter power transistor according to the invention may be accomplished in the deep-base structure of Fig. 8. The structure of Fig. 8 differs from that of Fig. 7 in that the pair of emitter regions 24 of each finger is buried in depth in the base region 22, circumscribed laterally by a second epitaxial layer 29 of the type N, and is provided with a respective pair of vertical areas 26 of the type N+ for the connection of each pair of emitter regions 24 to the corresponding metallization 27. This connection is accomplished with the interposition of a connection area 28 of the type N+ suitable for connecting at the surface the connection areas 26 and have a length such as to allow a suitable opening of emitter contact. Moreover, instead of the vertical connection areas 25 of the type P there are similar regions 31 of the type P+. The resulting structure is functionally similar to the first embodiment of a power transistor having a vertical structure and is structurally similar to its embodiment of Fig. 2 with the exception of the presence of a pair of emitter regions 24 for each finger instead of the single region 6. This fact, however, does not introduce any change to the considerations on the balancing of the currents in the emitter fingers. In a manner altogether similar to what has been seen for Fig. 2 a resistive parallel is created, constituting a ballast resistance Rv, which starting from the metallization 27 corresponding to the pair of emitter regions 24 of a finger runs parallel to the surface along a semi-region of the horizontal connection region 28 and thus in depth along the corresponding vertical connection area 26. In this case it is also possible to accomplish the connection regions 26 with a variable width so as to balance the different metallization resistances of portions of the emitter regions 24 positioned at a different surface distance with respect to the contact common to the different emitter metallizations 27. In the case of a hollow emitter power transistor it is also possible to provide for particularly narrow emitter regions 24. As illustrated in Fig. 9, in this case each of the emitter regions 24 of one finger is connected through a connection area 32 to the corresponding metallization 27. Instead of the connection region 28, contact regions 33 of the type N+ are provided for with a length such as to allow a sufficiently wide opening of emitter contact. In the case of Fig. 9 there is also a parallel resistive, constituting a ballast resistance Rv, which starting from the metallizzation 27 corresponding to the pair of emitter regions 24 of one finger crosses the connection area 32 vertically until it reaches the central area of the corresponding emitter region 24. The functional considerations and conclusions already illustrated previously with reference to the prior embodiments of the invention are still valid. A functional improvement of the embodiment of the invention of Fig. 9 is illustrated in Fig. 10. This variant provides for the displacement of the connection areas 32 of the emitter regions 24 of a finger in the proximity of the internal extremities of the regions 24 with respect to the vertical connection regions 31 of the base region 22. The emitter regions 24 of each finger are not activated in a uniform manner. For each of them the maximum activation takes place in the peripheral area next to the vertical region 31 from which the base current is flowing. With the progressive displacement away from the region 31 the base current meets a base pinch resistance of the region lying under the emitter region 24, due precisely to the pinching of the base region 22 caused by the diffusion of the type N+ corresponding to the emitter region 24. This causes a progressive de-biasing of the emitter region 24 which de-biases until it becomes a purely resistive region which makes no contribution to the increase of the power transistor's current rating. This purely resistive region combines advantageously with the corresponding connection area 32 contributing to the formation of the emitter's ballast resistance Rv. A further increase in the ballast resistance Rv may be obtained with the embodiment of the hollow emitter power transistor illustrated in Fig. 11. This differs from that of Fig. 10 due to the presence of contact region 34s which extend parallel to the surface of the chip 51 up to the proximity of the vertical regions 31 to be then contacted by the emitter metallization 27. In this way a resistive parallel is created, constituting a ballast resistance Rv, which starting from the metallization 27 corresponding to the pair of emitter regions 24 of a finger runs parallel to the surface of the chip 51 along the horizontal resistive region 34 and then in depth along the corresponding vertical connection area 32 until it reaches a portion of the biased emitter region 24, and thus made purely resistive, due to the effect of the pinch resistance of the base region 22 lying below the region 24 according to the above description with reference to Fig. 10.

Claims: Monolithic vertical-structure semiconductor device with a power transistor having a deep base and a finger-like emitter structure, comprising: a substrate (3;20) of a first type of conductivity having a bottom surface which forms a collector region of said power transistor, a first epitaxial layer (2; 21) of said first type of conductivity located on said substrate (3; 20), a base region (1; 22) of a second type of conductivity located on said first epitaxial layer (2; 21), at least one emitter region (6; 24) with high dopant concentration of said first type of conductivity formed in a buried layer for each finger of the emitter structure of the power transistor, located on said base region (1; 22), a second epitaxial layer (30; 29) of said first type of conductivity located on said base region (1; 22) and said emitter region (6; 24) and having a top surface forming part of a major surface of the power transistor, an emitter surface contact (8'; 27') for each emitter region (6; 24), located on said major surface and forming part of an emitter surface metallization (8; 27), an emitter contact (15) connected with the emitter surface metallizations (8; 27) of all the emitter regions (6, 24), base and collector surface contacts (5'; 23') electrically contacting said base (1; 22) and collector (20) regions and forming part of respective base and collector surface metallizations (5; 23), a connection region (11, 12; 14, 13; 26, 28; 32, 33, 34) with high dopant concentration of said first type of conductivity for each emitter region (6; 24), said connection region (11, 12; 14, 13; 26, 28; 32, 33, 34) being in physical contact with said emitter region (6; 24) and electrically connected with said emitter surface contact (8'; 27'), said connection region (11, 12; 14, 13; 26, 28; 32, 33, 34) comprising at least one vertical portion (11; 14; 26; 32) extending from said emitter region (6; 24) to said major surface of the power transistor and having a width in a direction transverse to the longitudinal axis of the emitter fingers such as to provide a ballast resistance (Rv) between each emitter region (6; 24) and the corresponding emitter surface contact (8'; 27'), said width varying along the longitudinal axis of the corresponding finger of the emitter structure and increasing with the distance from said emitter contact (15), said connection region (11, 12; 14, 13; 26, 28; 32, 33, 34) further comprising a top portion (12; 13; 28; 33; 34) with a function of ballast resistance, which extends in a direction parallel to said major surface and is electrically connected with said emitter surface contact (8'; 27'), said top portion (12; 13; 28; 33; 34) being wider than said vertical portion (11; 14; 26; 32) and said emitter surface contact (8'; 27'). Device according to claim 1, characterized in that each finger of the emitter structure has one emitter region (6) provided with a connection region (11, 12) formed by a pair of parallel vertical portions (11) as defined above connected together by said top portion (12). Device according to claim 1, characterized in that each finger of the emitter structure has one emitter region (6) provided with a connection region (14, 13) formed by said top portion (13) and by one vertical portion (14) extending between central areas of said emitter region and said top portion. Device according to claim 1, characterized in that each finger of the emitter structure has a pair of emitter regions (24) located on said base region (22) between a pair of vertical projections (31) of said base region (22) which extend toward, said major surface, each emitter region (24) being provided with a connection region (26, 28) formed by one vertical portion (26) as defined above by said top portion (28), the latter connecting said vertical portion (26) with a corresponding vertical portion of the other emitter region of the same finger of the emitter structure. Device according to claim 1, characterized in that each finger of the emitter structure has a pair of emitter regions (24) located on said base region (22) between a pair of vertical projections (31) of said base region (22) which extend towards said major surface, each emitter region (24) being provided with a connection region (26, 28) formed by said top portion (33; 34) and by one vertical portion (32) as defined above, extending between a lateral area of said emitter region and said top portion of the connection region, said lateral area being at a distal end of the emitter region with respect of said vertical projections (31) of the base region (22). Device according to claim 5, characterized in that said connection region has a top portion (34) which extends towards a corresponding vertical projection (31) of the base region (22).

Inventors: PALARA SERGIO, PALARA, SERGIO
Applicants: CONS RIC MICROELETTRONICA, CONSORZIO PER LA RICERCA SULLA
Application year: 1992
Main classification: H01L29
IPC codes: H01L21, H01L29, H01L27