An integrated device with a bipolar transistor and a MOSFET transistor in an emitter switching configuration

Abstract: A device integrated on a chip of a semiconductor material is disclosed which comprises an NPN bipolar transistor (T2) and an N-channel MOSFET transistor (T3) in an emitter switching configuration, both being vertical conduction types. The bipolar transistor (T2) has its base (13) and emitter (14) regions buried; the MOSFET transistor (T3) is formed with an N region (16) bounded by the base (13) and the emitter (14) regions and isolated by a deep base contact and isolation region (15). To improve the device performance, especially at large currents, an N+ region (17) is provided which extends from the front of the chip inwards of the isolated region (16) and around the MOSFET transistor (T3). In one embodiment of the invention, a MOSFET drive transistor (T1) is integrated which has its drain terminal in common with the collector (C) of the bipolar transistor (T2), its source terminal connected to the base of the bipolar transistor (T2), and its gate electrode connected to the gate electrode (G) of the MOSFET transistor (T3) in the emitter switching configuration.

Description: This invention relates to monolithic integrated semiconductor structures, in particular a device as defined in the preamble of claim 1. As is known, an emitter switching configuration is composed of a vertical bipolar transistor, usually a high-voltage power transistor, and an electronic switch in series with the emitter of the bipolar transistor. Expediently, the electronic switch is a low-voltage MOSFET power transistor connected with its drain terminal to the emitter terminal of the bipolar transistor. The electronic switch "break" causes the bipolar transistor to be cut off in an extremely fast manner, thereby such a configuration is used to advantage in those applications where the bipolar transistor is to be to switched between its conductive and non-conductive states at a fast rate. An integrated structure of a device including a bipolar power transistor and a MOSFET transistor in the above configuration is described in European Patent Application EP 88202899.6 filed on 16.12.88 by SGS-THOMSON MICROELECTRONICS and published as EP-A- 322041. Briefly, that structure, also shown in Figure 1 of the drawings which accompany this specification, is formed on a substrate 10 of a semiconductor material, e.g. a chip of monocrystalline silicon, of the N+ type, that is having a high concentration of impurities of the N type. (Notice that, in the drawing, the concentrations of impurities of the N type and the P type are identified as customary by the addition of a sign "-" or "+" to the characters N and P; the characters N and P, when added no "-" or "+" sign denoting concentrations of intermediate value.) Formed on the substrate 10 are two epitaxial layers 11 and 12 of the N- and N types, respectively. The layer 11 and the substrate 10, together contain the collector region of the bipolar transistor. A metallic layer 28, applied on the free surface of the substrate, forms the collector terminal C. A P- region, designated 13, formed between the epitaxial layers 11 and 12, and accordingly commonly referred to as a buried region, forms the base region of the transistor. A P+ isolation and deep base contact region 15 extends from the front surface of the chip, that is, the opposite surface from the collector terminal C to the edge of the base region 13, and encloses an isolated N region designated 16. A second buried region 14, of the N type with a high concentration of impurities and provided on the P- region 13 to form a junction therewith, constitutes the emitter region of the transistor. A P region 25 extends within the isolated region 16 which constitutes the body region of the MOSFET transistor and contains the transistor channel. Formed within the body region 25 is a region 26 constituting the source region of the MOSFET transistor. A strip 22 of an electrically conductive material which overlies the channel and is insulated from the chip surface constitutes the gate electrode, which also is a terminal, designated G, of the device. Electrically conductive surface contact strips 4 and 5 are formed on the source region 26 and the isolation region 15, respectively, to provide the source terminals S of the MOSFET transistor and the base terminal B of the bipolar transistor, respectively. The drain region of the MOSFET transistor is provided by that portion of the N layer 12 which is included between the buried emitter region 14 and the body region 25 and is unconnected to external electrodes. The structure just described can be implemented in an extremely compact form by virtue of the overlapped arrangement of the two transistors. However, it has been found that this beneficial feature cannot be fully exploited because the device performance rapidly deteriorates to become unacceptable as the distance of the body region 25 of the MOSFET transistor to the isolation and deep base contact region 15 drops below a certain limit value. Specifically, it has been observed that in such cases the bipolar transistor gain decreases drastically as the collector current increases. In addition, from the applicational standpoint, the device described hereinabove requires separate driver circuits for the two transistors because, as is well recognized, the bipolar transistor must be current driven, whereas the MOSFET transistor is to be voltage driven, and this also in those applications where the control signals from the two transistors must be simultaneous and with the same sign, such as in a switching power supply. In such cases, the need for separate control devices or circuits further reduces the benefits to be derived from the inherently compact design of the structure. From EP-A-247660 a vertically integrated emitter switch with a MOS device merged with a bipolar transistor is known, wherein a region having both the function of emitter of the bipolar transistor and the function of drain of the MOS device surrounds the body of the MOS device. From Pat. Abs. Jap. vol. 14 No. 102 (E-894) & JP-A-1305557 an emitter switch with a MOSFET and a bipolar transistor is known, wherein a dielectric isolation of the MOSFET is provided. In Pat. Abs. Jap. Vol. 14 No. 584 (E-1018) & JP-A-2254757 a lateral MOSFET is disclosed wherein a high-doped drain wall region connected to a high-doped buried region is provided to reduce parasitic current paths. It is an object of this invention to provide a device as defined in the preamble of claim 1 which has none of the structural limitations with which the conventional device is beset. Another object of the invention is to provide a compact integrated device as indicated, which can be used to advantage in applications providing for simultaneous commands with the same sign to the two transistors. These objects are achieved, according to the invention, by a device as defined and characterised in claim 1. The invention can be more clearly understood, and its advantages appreciated, from the following description of two embodiments thereof, given by way of example and not of limitation with reference to the accompanying drawings, in which: Figure 1, already described, is a sectional view through the structure of a conventional integrated device; Figure 2 is a sectional view through the structure of an integrated device according to one embodiment of the invention; Figure 3 shows an equivalent electric circuit of the structure in Figure 2; Figure 4 shows an equivalent electric circuit of the structure in Figure 2, to which certain electronic components, not to be seen in the structure, have been added; Figure 5 is a sectional view through the structure of an integrated device according to another embodiment of the invention; and Figure 6 shows an equivalent electric circuit of the structure in Figure 5, also with certain electronic components added which are not brought out in the structure. The structure of a device according to the invention -- shown in Figure 2, where similar or corresponding parts to those in Figure 1 are denoted by the same reference numerals or symbols -- is formed on a chip of a semiconductor material of the N type, starting with a substrate 10 of monocrystalline silicon which is heavily doped with impurities of the N type. On the substrate 10, there is formed, as by epitaxial growth, a first layer 11 having the same type of N conductivity, but at a much lower concentration of impurities, and accordingly denoted by N- in the drawing. Formed by implantation on the surface of the epitaxial layer 11 are P-type regions which have comparatively low concentrations of impurities, whereafter N-type regions with high concentrations of impurities are implanted substantially within the P-type regions. A second layer 12 of the N type and having a higher concentration of impurities than the first layer 11 is formed over the layer 11 by epitaxial growth. During this step, which is carried out at a high temperature, the implanted regions of the P and N types are allowed to expand by diffusion into the two epitaxial layers to provide the buried regions shown in the drawing at 13 and 14, respectively, and which together form a junction and are to provide the base and emitter regions, respectively, of the bipolar transistor. In the second epitaxial layer 12, there are then formed, using conventional masking and diffusing techniques, P+ regions, designated 15, which extend through the second epitaxial layer 12, merge with the regions 13, and provide isolation and deep contact base regions for the bipolar transistor. In plan view, the regions 13 and 14 are in the form of concentrically arranged squares or circles, and the regions 15 are each in the form of a square or circular frame, and accordingly, bound inwardly portions of the second epitaxial layer 12 which are closed at the bottom by the regions 13 and 14, thereby forming N-type wells, designated 16, which are isolated from one another. Shown in Figure 2 is a portion of a cellular structure formed by sundry isolated wells 16. Formed in each of the wells is a basic bipolar transistor, while outwards from each of the isolation regions 15 which bound the wells, there is formed a basic MOSFET transistor. A structure of this type is described in the Applicant's European Patent Application No. EP-A- 0 629 001 falling under Art 54(3)EPC. However, it will be appreciated that the inventive device may be implemented with individual transistors of discrete construction. Subsequently, additional regions 17 of the N type with a high concentration of impurities are formed, each inside one well 16 and also in the form of square or circular frames. In the example shown, such regions extend as far as their respective buried regions 14, although this is no strict requirement. As a matter of fact, the additional regions 17 could extend inside the walls 16 without merging with the buried regions 14. In this case, they could extend sideways to touch the deep contact and isolation regions 15. Within the area bounded by the additional region 17 of each well 16, there is formed a P+ region, designated 20, which constitutes the deep body of a MOSFET transistor. Similar P+ regions, designated 21, are formed along the outward edges of the contact and isolation regions 15, and these too constitute deep body regions. Then, a layer of polycrystalline silicon is formed which is insulated from the chip surface by a thin layer of a dielectric material and from which the gate electrodes, designated 22 and 23, of MOSFET transistors are formed. Such electrodes are electrically connected together and led to a common gate terminal which also is one of the device terminals, designated G in the drawings. P-type regions 24 and 25 are next formed with lower concentrations of impurities than the regions 20 and 21 to provide the body of the MOSFET transistors, which extend across their respective body regions 20 and 21. Formed within the body regions 24 and 25 then are N+ regions 26 and 27 which constitute the source regions of their respective MOSFET transistors. Thereafter, electric interconnection paths are formed across the front surface of the chip, which is coated with a layer of silicon dioxide, designated 9, using deposition, masking and etching techniques. Specifically, metallic strips, designated 4, 5 and 6, are formed. The strips 4 contact selected surface areas of the deep body 20, body 24, and source 26 regions, and constitute the source terminal of one of the MOSFET transistors which is led to one of the device terminals designated S. The strips 5 contact selected surface areas of the deep body 21, the body 25, and the source 27 of the other of the MOSFET transistors, and the strips 6 contact superficially the additional regions 17. Notice that the last-mentioned contact strips are not strictly needed for practicing the invention in its broadest form, but are useful to provide a specially advantageous circuit configuration to be explained. The strips 5, additionally to contacting the body and source regions superficially together, obviously provide contact with the base regions 13 through the deep contact and isolation regions 15. Finally, a metallic layer 28 is formed on the bottom of the substrate 10 which constitutes the common collector and drain terminal C of the device. In the example shown, the integrated structure includes a buried region of the N type with a high concentration of impurities, designated 30, within the drain region of the MOSFET transistor outside the isolated region 16, where the drain current is largest. The region 30 is provided, before the epitaxial layer 11 is grown, by forming an intermediate epitaxial layer and implanting impurities of the N type in those areas thereof where the drain regions are to be formed. In the course of the following thermal processes whereby the epitaxial layers 11 and 12 are formed, the implanted impurities will diffuse both into the intermediate epitaxial layer and the first epitaxial layer 11 to form the region 30. Inasmuch as the region 30 substitutes a high-conductivity material for part of the low-conductivity drain region, the resistance between the drain and the source of the MOSFET transistor will be decreased. The integrated structure described in the foregoing constitutes a device which may be represented by the equivalent electric circuit shown in Figure 3. Indicated at T2 is a bipolar transistor of the NPN type, and at T1 and T3, two N-channel MOSFET transistors. T3 and T2 are connected to each other into an emitter switching configuration, and T1 is connected to T2 into a Darlington type of configuration, of which it forms the driver stage, and has its gate electrode connected to the gate electrode of the transistor T3 in the emitter switching configuration. The gate electrode of T1, collector of T2, and source of T3, respectively designated G, C and S, constitute the terminals of the integrated device. As can be appreciated, the emitter and base regions of T2 are formed by the regions indicated at 14 and 13, respectively, in Figure 2, and the collector region of T2 is formed essentially by the areas of the epitaxial layer 11 and the substrate 10 which underlie the base region 13. The source region of T3 consists of the N+ region, designated 26, which is connected superficially to the body 24 and deep body 20 regions by the metallic strip 4 connected to the terminal S of the device. The drain region of T3 consists of the area of the epitaxial layer 12 which underlies the deep body region 20 and borders the emitter region 14. The source region of T1 consists of the N+ region 27 which is connected superficially to the deep body region 21 and the deep contact and isolation region 15 by the metallic strip 5. The drain region of T1 consists of the areas of the epitaxial layers 12 and 11 which underlie the body and deep body regions 25 and 21, as well as of the region 30 and the underlying substrate 10. The common collector terminal of T2 and drain terminal of T1 constitute the terminal C of the device, and the gate electrodes 22 and 23 of the MOSFET transistors T3 and T1, respectively, are joined together and form the terminal G of the device. With the device described hereinabove, the invention objects are fully achieved. In particular, it has been found that the gain of the bipolar transistor T2 can be kept virtually constant even at large currents. This is believed to be attributable to the additional N+ region 17 preventing the formation of parasitic components between the body region 24 and the isolation region 15, which would otherwise draw current from the base of T2. Furthermore, the MOSFET transistor T1 affords simultaneous voltage control, via the single terminal G, of the two components, T2 and T3, of the emitter switching configuration. In the embodiment illustrated, moreover, T1 and T2 are constructed and interconnected, as taught in the aforementioned European Application, to provide a device of the Darlington type wherein the series resistance of the driver stage is peculiarly low. The resultant three-terminal device is, in conclusion, extremely compact, has a substantially constant gain even at large currents, and can be used to advantage in those applications where a relatively large current requires to be switched rapidly. To further improve the switching speed of the device, various arrangements can be provided which are known per se. In Figure 4, where similar components to those in Figure 3 are denoted by the same reference symbols, the structure described hereinabove has been added some components which can be integrated using known techniques on the same silicon chip. Specifically, these are: a Zener diode connected between the source terminal of T3 and the base contact, designated A in both Figure 4 and Figure 2, of T2; T2; and two resistors R1 and R2 connected between the base of T2 and, respectively, the source of T3 and the emitter of T2. The use of the metallic layer 6 can now be fully appreciated: in fact, it functions to connect the emitter of T2, i.e. the N+ region 14, to the resistors R1 and R2 through the additional region 17. Notice that two diodes, designated D1 and D2, are also shown in Figure 4 which are connected between the source and the drain terminals of the MOSFET transistors T1 and T3, respectively, and result inherently from a MOSFET transistor structure of this kind. The circuit symbol for the two diodes is also shown in Figure 2 for convenient location of the regions that form their respective anodes and cathodes. A device which suits even faster switching applications is illustrated by the structure cross-section of Figure 5 and its equivalent electric circuit in Figure 6. The device comprises first and second transistor pairs, the one being bipolar and the other a MOSFET, which are connected together into the emitter switching configuration and quite similar to those shown in Figures 2 and 4, and a MOSFET drive transistor which is quite similar to the MOSFET drive transistor of Figures 2 and 4. In Figure 6, the components of the first and the second pairs are denoted by the same reference numerals and symbols as in Figure 4 with a "1" and "2" suffix added, respectively, and the equivalent components of those in Figure 4 by the same reference numerals and symbols primed. Shown in Figure 5 are just the reference symbols for the transistors, which only appear with their respective collector and drain regions, plus a few more references used in Figure 6. The two emitter switching configurations are cascade connected together. Specifically, the collector terminals of the two bipolar transistors T21 and T22 consist of the same metallic layer which extends across the surface of the substrate and form a terminal C' of the device; the source terminal S1 of the MOSFET transistor, designated T31, in the first pair is connected to the base terminal of the bipolar transistor T22 in the second pair. The source terminal S2 of the MOSFET transistor, designated T32, in the second pair also forms a terminal, designated S', of the device. The MOSFET drive transistor T1' has its drain terminal connected to the common collector terminal of the bipolar transistors in both pairs, its source terminal A' connected to the base terminal of the bipolar transistor T21 in the first pair, and -its gate electrode connected to the gate electrodes of the MOSFET transistors T31 and T32 in both pairs, as well as to a third terminal, designated G', of the device. The various components in the diagram of Figure 6 can be readily identified in Figure 5 from the previous description of Figure 2 and with the aid of the reference symbols entered.

Claims: A device comprising at least a pair of transistors, namely a bipolar one and a MOSFET one, connected in the emitter switching configuration and integrated on a chip of a semiconductor material, which device comprises: a substrate (10) of the N type having a high concentration of impurities; at least one layer (11, 12) of the N type overlying the substrate (10) and having a lower concentration of impurities than the substrate, such layer (11, 12) and substrate (10) including the collector region of the bipolar transistor (T2) in the pair; a metallic layer (28) which extends on the opposite surface of the substrate (10) from the layer (11, 12) and constitutes the collector terminal (C) of the bipolar transistor (T2); a region (13) of the P type having a low concentration of impurities which is buried in the N-type layer (11, 12) and includes the base region of the bipolar transistor (T2) in the pair; an isolation region (15) of the P type having a high concentration of impurities and extending from the front surface of the chip, i.e. the opposite surface of the chip from the collector terminal, as far as the edge of the base region (13), thereby bounding inwardly an isolated region (16) of the N type which includes the drain region of the MOSFET transistor (T3) in the pair; electrically conductive means (5) contacting the surface of said isolation region (15) which include the base terminal (B, A) of the bipolar transistor in the pair; a buried region (14) of the N type having a high concentration of impurities which forms a junction with the base region (13) and includes the emitter region of the bipolar transistor (T2) in the pair; a region (20, 24) of the P type which extends from the front surface of the chip into the isolated region (16) and includes the channel of the MOSFET transistor (T3) in the pair; a region (26) of the N type having a high concentration of impurities which extends from the front surface of the chip in said region (20, 24) of the P type including the channel of the MOSFET transistor (T3) and includes the source region of the MOSFET transistor (T3) in the pair; electrically conductive means (4) contacting the surface of said source region (26) which include the source terminal (S) of the MOSFET transistor; a strip (22) of an electrically conductive material overlying the channel, being insulated from the front surface of the chip by a layer of an insulating material, and including the gate electrode (G) of the MOSFET transistor; the collector terminal (C) of the bipolar transistor (T2), the source terminal (S) and the gate electrode (G) of the MOSFET transistor (T3) in the pair forming the terminals of the device; said device being characterized in that it comprises an additional region (17) of the N type having a high concentration of impurities and extending from the front surface of the chip into the isolated region (16) of the N type and surrounding the region (20, 24) of the P type which includes the channel; a MOSFET drive transistor (T1) integrated on the chip of semiconductor material and having its drain terminal connected to the collector terminal (C) of the bipolar transistor, its source terminal (S) connected to the base terminal (A) of the bipolar transistor (T2), and its gate electrode connected to the gate electrode of the MOSFET transistor (T3) in the emitter switching configuration; an outward region (21, 25) of the P type including the channel of the MOSFET drive transistor (T1) and extending from the front surface of the chip in the N-type layer (12) proximate to the outward edge of the isolation region (15); a region (27) of the N type having a high concentration of impurities and extending from the front surface of the chip in said outward region (21, 25) of the P type and including the source region of the MOSFET drive transistor (T1); a strip (23) of an electrically conductive material overlying the channel of the MOSFET drive transistor (T1), being insulated from the front surface of the chip by a layer of an insulating material, and including the gate electrode of the MOSFET drive transistor (T1); a region being part of the N-type layer (11, 12) overlying the substrate (10), being bounded by said outward region (21, 25) of the P type and the substrate (10), and forming with the latter the drain region of the MOSFET drive transistor (T1); and electrically conductive means of interconnection between the gate electrodes (22, 23) of the two MOSFET transistors (T1, T3) and between the source region (27) of the drive MOSFET transistor (T1) on the front surface of the chip and the base terminal (B,A) of the bipolar transistor (T2); the drain terminal of the drive MOSFET transistor (T1) being formed by the same metallic layer (28) forming the collector terminal (C) of the bipolar transistor (T2). A device according to Claim 1, characterized in that it comprises, within the region being part of the N-type layer (11, 12) which forms the drain region of the MOSFET drive transistor (T1), a buried region (30) of the N type having a high concentration of impurities. A device according to Claim 1 or 2, characterized in that the additional region (17) of the N type extends as far as the buried region (14) of the N type. A device according to Claim 1, 2 or 3 characterized in that it comprises a first (T21, T31) and a second (T22, T32) pair of transistors in the emitter switching configuration, and wherein the collector terminals of the bipolar transistors in both pairs are formed of the same metallic layer (28) which extends on said opposite surface of the substrate (10), the source terminal (S1) of the MOSFET transistor (T31) in the first pair is connected to the base terminal of the bipolar transistor (T22) in the second pair, the MOSFET drive transistor (T1') having its drain terminal connected to the collector terminals (C') of the bipolar transistors in both pairs, its source terminal connected to the base terminal (A') of the bipolar transistor (T21) in the first pair, and its gate electrode connected to the gate electrodes (G') of the MOSFET transistors in both pairs, the common collector terminal (C') of the bipolar tranistors in both pairs, the source terminal (S2) of the MOSFET transistor (T32) in the second pair, and the common gate electrodes (G1) of the MOSFET transistors forming the terminals of the device.

Inventors: PALARA SERGIO, PALARA, SERGIO
Applicants: CONS RIC MICROELETTRONICA, CO.RI.M.ME. CONSORZIO PER LA RICERCA SULLA MICROELETTRONICA NEL MEZZOGIORNO
Application year: 1993
Main classification: H01L27
IPC codes: H01L21, H01L27, H01L29