Bipolar transistor and method of making the same

Abstract: A bipolar transistor includes a substrate (12) of semiconductor material having an epitaxial body (16) of the semiconductor material on a surface thereof. A collector region (24) of one conductivity type (N) is in the body at the major surface (18) and a base region (26) of the opposite conductivity type (P) is in the collector region at the major surface (18) and forms with the collector region a collector/base junction (28) which extends to the surface (18). A plurality of emitter regions (30) of the one conductivity type (N) are in the base region (26) and form with the base region emitter/base junctions (32) which extend to the surface (18). At least some of the emitter/base junctions are adjacent to but spaced from the collector/base junction at the major surface. A layer (38) of insulating silicon oxide is on the major surface and a layer (42) of conductive polysilicon is on the insulating layer. The polysilicon layer includes strips (42) which extend only over the area of the surface between the collector/base junction and the adjacent emitter/base junctions. Resistor areas (44) are connected to the strips. The emltter regions (30) are connected to the polysilicon layer. The bipolar transistor may be part of an integrated circuit which includes MOS transistors and the polysilicon layer also forms the gates of the MOS transistors.

Description: The present invention relates to a bipolar transistor and to a method of forming the same. In the field of MOS transistor integrated circuits, it has become the practice to form the gates of the transistors of polysilicon which is doped to make it conductive. Also, the polysilicon gates are generally used to mask the channel region of the transistors while forming the source and drain regions on opposite sides of the gates so that the source and drain regions are aligned with their respective gate. Recently there has been developed integrated circuits which include both MOS transistors and bipolar devices so that the circuits can perform both analog and digital functions. In such circuits, a layer of doped polysilicon is provided for making the MOS transistors. EP-A-190 423 discloses a bipolar transistor comprising a body of semiconductor material having a collector region, a base region and a single emitter region. A conductive polysilicon layer is formed on an insulating layer on a major surface of the body and extends over an area of the surface between the single emitter/base junction and the collector/base junction and over the collector region. According to one aspect of the present invention, there is provided a bipolar transistor comprising: a body of a semiconductor material having a major surface; a collector region of one conductivity type in said body and at said major surface; a base region of the opposite conductivity type in said collector region at said major surface with a first junction between the base region and the collector region; a plurality of spaced emitter regions of the one conductivity type in said base region with a plurality of second junctions between the emitter regions and the base region, some of said emitter regions having a portion of their junction with the base region adjacent to but spaced from the first junction; a layer of conductive polycrystalline silicon over and insulated from said major surface of the body and having strip portions extending only over the area of the surface between said first and second junctions and extending along the first junction over the entire area of the major surface located between said first junction and said portions of the second junctions; base and collector conductors electrically connected to said base and collector regions; and an emitter conductor electrically connecting said layer of conductive polycrystalline silicon and said emitter regions, said layer of conductive polycrystalline silicon thereby serving as a field shield. The present invention further relates to a method of forming such a bipolar transistor, as set out in claim 10. For a better understanding of the present invention, reference will now be made by way of example to the accompanying drawings in which: Figure 1 is a sectional view of an illustrative bipolar transistor incorporating the present invention. Figure 2 is a sectional view taken along line 2-2 of Figure 1. Figure 3 is a sectional view taken along line 3-3 of Figure 1. Referring initially to Figure 1, a bipolar transistor, generally designated as 10, is formed on a substrate 12 of a semiconductor material, such as single crystalline silicon, of one conductivity type, such as P-type. The substrate 12 may have formed therein or thereon other components, such as MOS transistors and other bipolar devices, which form a desired electrical circuit. On the surface 14 of the substrate 12 is a body 16 of the semiconductor material of the opposite conductivity type, such as N-type, having a major surface 18. The body 16 may be a layer of the semiconductor material which was epitaxially deposited on the substrate surface 14. The body 16 includes a thin region 20 along the substrate surface 14 which is highly conductive, i.e. an N+ type region, and highly conductive, N+ type, contact regions 22 extending through the body 16 to the highly conductive region 20. This forms a well region 24 of N-type conductivity in the body 16 which is the collector of the bipolar transistor 10. Within the well region 24 and at the surface 18 is a region 26 of the one conductivity type, i.e. P-type, which forms the base of the bipolar transistor 10 with the junction 28 between the well region 24 and the base region 26 extending to the surface 18. Within the base region 26 and at the surface 18 are a plurality of regions 30 of the opposite conductivity type, i.e. N-type, which form the emitters of the bipolar transistor 10. As shown in FIGURE 2, the emitter regions 30 are arranged in rows and columns. Each of the emitter regions 30 forms a junction 32 with the base region 26 which extends to the surface 18. The emitter/base junctions 32 of the emitter regions 30 which are adjacent the base/collector junction 28 are spaced from the base/collector junction 28. Also within the base region 26 at the surface 18 and between the columns of the emitter regions 30 are highly conductive regions 34 of the one conductivity type, i.e. P+ type, which serve as contacts to the base region 26. At the surface 18 and over the contact region 22 is a highly conductive collector contact region 36 of the opposite conductivity type, ie. N+ type. On the body surface 18 is a layer 38 of an insulating material, such as silicon oxide. On the silicon oxide layer 38 is a layer 40 of conductive polysilicon. As shown in FIGURES 1 AND 2, the polysilicon layer 40 includes a pair of narrow strips 42 which extend over and along the area of the surface 18 between the collector/base junction 28 and the emitter/base junctions 32 of the emitter regions 26 which are adjacent the collector/base junction 28. As will be explained, in forming the bipolar transistor 10, the polysilicon strips 42 are used as masks to define the space between the collector/base junction 28 and the adjacent emitter/base junctions 32 and thereby define an electrical characteristic of the transistor 10. In addition, the polysilicon strips 40 serve as an emitter/base field shield. Each of the polysilicon strips 40 extends to one end of a substantially rectangular area 44 of the polysilicon which is at one end of the emitter regions 30. The polysilicon areas 44 serve as emitter ballast resistors and have openings 46 therethrough which define the resistance of the resistors. A layer 48 of an insulating material, such as silicon oxide, is over the polysilicon layer 40 and the uncovered portions of the silicon oxide layer 38. A first level metalization is on the silicon oxide layer 48. The first level metalization will usually be a layer of a conductive metal, such as aluminum, but may be a layer of doped polysilicon. The conductive layer is defined to form conductors extending between the various portions of the transistor 10. As shown in FIGURE 3, the first level metalization includes emitter conductors 50 which extend between the emitter regions 30 and through openings in the silicon oxide layers 48 and 38 to make ohmic contact to the emitter regions 30. The emitter conductors 50 also extend over one end of the resistors 44 and extend through openings in the silicon oxide layer 48 to make ohmic contact to the resistors 44. Thus, the emitter regions 30 are electrically connected to the resistors 44. Base conductors 52 extend between the base contact regions 34 and through openings in the silicon oxide layers 48 and 38 to make ohmic contact to the base contact regions 34. The conductors 50 and 52 extend to separate terminal pads, not shown, at the edges of the transistor 10 where conductors from other components in the circuit can be connected to the emitter and base of the transistor 10. A layer 54 of an insulating material, such as silicon oxide, is over the first level metalization and any uncovered area of the silicon oxide layer 48. A second level metalization is on the silicon oxide layer 54. The second level metalization is also usually a layer of a conductive metal, such as aluminum, but may be a layer of doped polysilicon which is defined to form conductors. The second level metalization is formed into collector conductors 56 which extend through openings in the silicon oxide layers 54, 48 and 38 to make ohmic contact to the collector contact regions 36. The second level metalization also includes conductors, not shown, which extend through openings in the silicon oxide layers 54 and 48 to make ohmic contact to the other ends of the resistors 44. In making the bipolar transistor 10, after the collector region 24 and its contact regions 20 and 22 are formed in the body 16, and the base region 26 is formed in the collector region 24, such as by diffusion or ion implantation, the silicon oxide insulating layer 38 is formed on the body surface 18. A layer of doped polysilicon is then deposited on the silicon oxide layer 38 and defined to form the strips 42 and resistors 44. If the bipolar transistor 10 is being made as part of an integrated circuit which also includes MOS transistors, the polysilicon layer is also defined to form the gates of the MOS transistors. The emitter regions 30 are then formed in the base region 26 such as by diffusion or ion implantation using the strips 42 as a mask to define the spacing between the collector/base junction 28 and the emitter/base junctions 32 which are adjacent the collector base junction 28 at the surface 18. Since this spacing determines certain electrical characteristics of the bipolar transistor 10, by using the strips 42 to define this spacing, the spacing can be easily and accurately controlled to provide the desired characteristics for the bipolar transistor 10. Thus, there is provided by the present embodiment a bipolar transistor 10 having a layer of conductive polysilicon, a portion of which serves as a mask to define the spacing between the collector/base junction and the emitter/base junction so as to define certain electrical characteristics of the bipolar transistor 10. In addition, if the bipolar transistor 10 is part of an integrated circuit containing MOS transistors, the same conductive polysilicon layer can be used to form the gates of the MOS transistors. Also, the same portions of the doped polysilicon layer which serve as the masking shield also serve as an emitter/base field shield which is connected to the emitters 30 through the emitter conductors 50 which contact the polysilicon layer at the resistors 44 so as to be at the emitter potential. Furthermore, the doped polysilicon layer includes resistor regions 44 which are connected to the emitters 30 to serve as ballast resistors for the bipolar transistor 10. Thus, the doped polysilicon layer 40 serves a number of different functions in the bipolar transistor 10 both in the making of the transistor and its operation. Although the bipolar transistor 10 has been described as having N-type conductivity collector and emitter regions and a P-type conductivity base region to form a NPN transistor, the conductivity type of the regions can be reversed to form a PNP transistor. The bipolar transistor may be formed in an integrated circuit also including MOS transistors, the polysilicon layer being used for both types of transistor. For example the polysilicon layer may form the gates of the MOS transistors. The polysilicon layer ballasts and shields the bipolar transistor in the foregoing example.

Claims: A bipolar transistor (10) comprising: a body (16) of a semiconductor material having a major surface (18); a collector region (24) of one conductivity type in said body and at said major surface (18); a base region (26) of the opposite conductivity type in said collector region at said major surface (18) with a first junction (28) between the base region and the collector region; a plurality of spaced emitter regions (30) of the one conductivity type in said base region with a plurality of second junctions (32) between the emitter regions and the base region, some of said emitter regions having a portion of their junction with the base region adjacent to but spaced from the first junction; a layer (40) of conductive polycrystalline silicon over and insulated from said major surface of the body and having strip portions (42) extending only over the area of the major surface between said first and second junctions and extending along the first junction over the entire area of the major surface located between said first junction and said portions of the second junctions; base and collector conductors (52,56) electrically connected to said base and collector regions; and an emitter conductor (50) electrically connecting said layer of conductive polycrystalline silicon and said emitter regions, said layer of conductive polycrystalline silicon thereby serving as a field shield. A bipolar transistor in accordance with Claim 1, wherein the layer of conductive polycrystalline silicon includes an area (44) forming a resistor. A bipolar transistor in accordance with Claim 2 including a layer of insulating silicon oxide (38) over said major surface (18) of the body and wherein the polycrystalline silicon layer is on the said layer of insulating silicon oxide (38). A bipolar transistor in accordance with Claim 3 in which said spaced emitter regions are arranged in rows and columns. A bipolar transistor in accordance with Claim 4 including contact regions (34) of the same conductivity type as the base region but more highly conductive than the base region, said contact regions being in the base region between the columns of the emitter regions. A bipolar transistor in accordance with Claim 5 in which the said resistor areas are substantially rectangular areas of the polycrystalline silicon layer with the said strip portions being connected to an end of the resistor areas. A bipolar transistor in accordance with Claim 6 in which the said resistor areas, include openings (46) therethrough whereby the resistance of the resistors is adjusted. A bipolar transistor in accordance with Claim 7 including a layer of insulating silicon oxide (48,54) over the polycrystalline silicon layer, said emitter, base and collector conductors (50, 52, 56) being over the silicon oxide layer and extending through openings in the silicon oxide layers, the emitter conductors (50) making ohmic contact with the emitter regions and the base conductors (54) making ohmic contact with the said contact regions (34) of the base region. A bipolar transistor in accordance with Claim 8 in which the contacts which make ohmic contact with the emitter regions also make ohmic contact with the resistor areas. A method of forming the bipolar transistor of any of the preceding claims comprising the steps of: providing a body (16) of a semiconductor material having a major surface (18); forming a collector region (24) of one conductivity type in the body (16); forming a base region (26) of the opposite conductivity type in the collector region (24) and thereby defining a first junction (28) between the collector region (24) and the base region (26); forming a silicon oxide layer (38) on the major surface (18) of the body (16); forming a layer (40) of conductive polycrystalline silicon on the insulating layer (38), which conductive layer (40) has strip portions (42) extending along the first junction (28); forming a plurality of spaced emitter regions (30) of the one conductivity type in the base region (26) by diffusion or ion implantation using the said strip portions (42) as a mask and thereby defining a plurality of second junctions (32) between the emitter regions (30) and the base region (26), some of the emitter regions (30) having a portion of their junction (32) with the base region (26) adjacent to but spaced from the first junction (28) and defined by means of the said mask.

Inventors: SCHADE OTTO HEINRICH JR, SCHADE, OTTO HEINRICH, JR.
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1989
Main classification: H01L29
IPC codes: H01L27, H01L29, H01L21