Heterojunction bipolar transistor

Abstract: A vertical heterojunction equal area transistor is provided in which three epitaxial layers structure with wide band gap external layers (2, 4) and narrower band gap center layer (3) is provided with impurity concentrations outside a center portion (7, 8) such that the area around the wide gap electrodes is high resistivity and the area around the center narrower gap region is high conductivity. At least one ohmic contact (25) is formed to the area around the center narrower gap regions (3), said contact (25) being separated from the center portion (7, 8) by a portion of one of the external layers (4).

Description: The field of the invention is in heterojunction transistors wherein the emitter, base and collector regions are of different semiconductor materials. Such devices usually have at least one region of a compound semiconductor material. They have advantages in performance and in simplification in fabrication due to differences in response by the different materials. One type of heterojunction transistor known in the art has the compound semiconductor gallium arsenide (GaAs) for the emitter and collector regions and has the elemental semiconductor Ge for the base region. In that structure, described in the IBM Technical Disclosure Bulletin, Vol. 24, No. 7A, December 1981, pages 3229, 3230 and 3231, the wider band gap property of the GaAs provides hot electron carrier transport in the elemental Ge base region. A heterojunction bipolar transistor is disclosed in EP-A-0 197 424. The bipolar transistor comprises a first layer of a first semiconductor material of first condcutivity type, a second layer on the first layer, the second layer being of a second semiconductor material of a second conductivity type opposite to the first conductivity type, the second semiconductor material having an energy bandgap less than that of the first semiconductor material, a third layer on the second layer, the third layer being of the second semiconductor material and of the first conductivity type, a first doped region in the first layer, the first doped region being of the first semiconductor material and being doped to be semi-insulating regions, and a second doped region in the third layer, the second doped region being of the second semiconductor material and being doped to the second conductivity type. The second layer has a portion forming an active base region, the third layer has a portion forming a collector region in contact with the active base region, the first doped region defines in the first layer an active emitter region in contact with the active base region, and the second doped region forms a connecting base region electrically coupled with the active base region. Devices of that type, however, require careful procedures in fabrication to insure predictable and reproducible high speed response. Alignment techniques have been developed in the art in which a crystal body having a plurality of layers of different types of semiconductor materials parallel with a surface employ a material that serves as a bombardment mask placed in contact with that surface. In this type of alignment technique, a bombardment from a direction that is vertical with respect to the surface around the masked area produces material changes in the layers that are laterally positioned and self-aligned by the mask. Such a technique is shown in U.S. Patent 4,599,791. As the art has progressed in the direction of more precise electrical response, physical shape and size, and interconnectability specifications; electrical capacitance, interconnectability due to different region sizes and limitations on available materials due to process temperature excursions have limited ability in the art to meet those specifications. One embodiment of the invention is described in detail below with reference to drawings in which: Fig. 1 is a schematic illustration of an intermediate manufacturing product involving the invention. Figs. 2-7 are schematic illustrations of the structure at points in fabrication. In accordance with the invention, a vertical heterojunction transistor and a manufacturing process is provided as defined by the appended claims. The transistor structure is made up of three epitaxial layers of alternating wide band gap, narrower band gap and wide band gap semiconductor materials with the common property of being differently responsive to the presence of the same element as a conductivity type determining impurity. In other words, an impurity that produces, for example, p conductivity type in the wide band gap semiconductor material will produce n conductivity type in the narrower band gap semiconductor material. This structure is then employed with a vertical bombardment impurity implantation technology to provide, under a masking member, a vertically aligned transistor with all regions having the same cross sectional area. The interdependent combination of the different materials with their different properties and response to the conductivity type impurity, provides process constraint relaxation and electrical performance control. Referring to Fig. 1, in the monocrystalline structure 1 there is a layer 2 of a wider band gap semiconductor material epitaxial with a layer 3 of a narrower band gap semiconductor material which in turn is epitaxial with a layer 4 at the surface of a wider band gap semiconductor material each joining the adjacent layer at heterointerfaces 5 and 6 respectively. The layers 2 and 4 each contain a quantity of a conductivity type determining impurity providing one conductivity type whereas the layer 3 contains a quantity of the opposite conductivity type impurity. In the regions external to the region between lines 7 and 8, there is in addition a quantity of the same conductivity type determining impurity that produces different conductivity in the wider and narrower band gap. This condition can be produced as one example by introducing the impurity through the surface 9 when the region between lines 7 and 8 was masked. In order to facilitate explanation, the invention will be described in connection with the compound semiconductor material GaAs as the wider band gap material and the elemental semiconductor material Ge as the narrower band gap material using as the impurity a member of the group of boron, aluminum, gallium and proton (hydrogen). In the light of the principles set forth, many substitutions will be apparent to those skilled in the art. Since the impurity is of a type that produces low conductivity in the material selected for the wider band gap material and high conductivity in material selected for the narrower band gap, the interrelationship of the structure, materials and properties are such that the introduced impurity produces a high resistivity region surrounding the region between lines 7 and 8 in the wide band gap layers 2 and 4 which may later serve as equal area emitter and collector electrodes. In such electrodes, this reduces electrical capacitance and improves wirability. In the narrower band gap layer 3, the introduced impurity in the region external to lines 7 and 8 provides the high conductivity desirable for contacts to a base region. Since all emitter, base and collector areas are equal, the resulting device is completely reversible, providing radical simplication in wiring when used in high density integrated circuits. While the structure of Fig. 1 can be produced by a variety of techniques in standard semiconductor technology, it is particularly compatible with vertical bombardment type implantation processing, commonly referred to as ion implantation, in which the selected impurity is propelled through surface 9 around a precisely positioned mask that defines the device area, the adjacent region boundaries and depending on the material used may also serve as an external circuit terminal. The impurity is placed in the crystal 1 outside of the region between lines 7 and 8 through layers 4 and 3 and into or through 2. The region between lines 7 and 8 provides the same size regions in each layer, and each layer has device enhancing properties in the region outside of the lines 7 and 8. In layers 2 and 4 which when constructed to be the emitter and collector of a bipolar transistor the regions outside of lines 7 and 8 are high resistivity and in layer 3 when serving as the base, high conductivity is provided outside of lines 7 and 8. The production of a GaAs/Ge/GaAs heterojunction wide band gap emitter and collector bipolar transistor having equal area emitter, base and collector areas is illustrated in connection with the structures produced at various steps. Referring to Fig. 2, a schematic illustration is provided of the structure layers 2, 3 and 4 of Fig. 1 positioned on a supporting substrate 14. The substrate 14 is moderately atomic spacing compatible with GaAs, and may conveniently be high resistivity or semi-insulating GaAs thereby forming a moderately defect-free epitaxial interface 11 with the GaAs layer 2. An ion implantation mask member 12 is positioned on the surface 9 at the location of a device. The mask 12 if made of a metal can later be used as an ohmic contact to the GaAs layer 4, such as AuGe, or an alloy of W with In, is positioned on the surface 9. While functionally in the member 1, since the layers 2 and 4 are identical, the transistor being fabricated would be reversible, one skilled in the art at this state of the art should be aware that if the technique of epitaxial growth is employed in the fabrication of the layers 2, 3 and 4, Ge grown on GaAs is reported to have higher device quality than GaAs grown on Ge so that selection of a particular layer to serve the emitter of the transistor, usually the bottom layer, may be made when the three-layer member 1 is fabricated. Referring next to Fig. 3, the implantation operation is shown as arrows in which a selected impurity is implanted in the regions of the member 1 surrounding the mask 12. The implantation takes place essentially vertically with respect to the surface 9 in a technique in which the impurity element is propelled under sufficient energy to penetrate into the crystal in the regions outside of and to the general depth shown by lines 7 and 8. The implantation impurity is selected with respect to the material GaAs in layers 2 and 4 to make them high resistivity or semi-insulating whereas with respect to the material Ge in layer 3, high conductivity is produced. The elements boron, aluminum, gallium and protons (hydrogen) for the material GaAs satisfies these requirements with boron preferred because boron may be activated in the Ge layer 3 with a relatively low annealing temperature of about 425°C. This annealing temperature is not sufficient to restore the conductivity of the implanted GaAs regions which remain high resistivity. The structure at this point need only be provided with contacts to be a transistor that is symmetrical in that the GaAs regions 16 and 17 on each side of the base 18 under the mask 12 are approximately the same area which permits general interchangeability as emitter and collector, thereby simplifying later wiring of devices. It will be apparent to one skilled in the art that a fabrication technique has been described that produces a semiconductor device in a layered crystal body with equal area electrodes, the major current path of which is essentially perpendiular with respect to the surface by converting the region of the layered crystal surrounding an aligned region that extends through the layers and then providing access to the control electrode for the current path by removal of a portion of the surface layer around the major current path electrode. In Figs. 4 to 7, an illustration of the contacting technique of the invention is provided. Referring next to Fig. 4, a masking material 19 capable of resisting an erosion operation of the layer 4 is placed on the surface 9 and on the sides of the mask 12. A dielectric, such as Si₃N₄, is satisfactory for element 19. Referring next to Fig. 5, employing the dielectric element 19 as a mask, a vertical etching or erosion operation is performed through the surface 9 and layer 4 exposing the Ge layer 3 at the interface 6. This etch is facilitated by the nature of the structure of element 1 in that both reactive ion etching and chemical type etching may be readily performed down to a specific interface layer, in this case interface 6 wherein the fact that the etching properties of the GaAs and Ge materials are different is employed. The etching operation proceeds principally in the vertical direction perpendicular to surface 9. There will, however, be some slight horizontal etching, not shown, of the dielectric 19 and the remains of the semi-insulating portions of the layer 4. Dependent on the lateral travel of the etching, the width parallel to surface 9 of the dielectric 19 is selected to retain enough of the layer 4 under dielectric 19 to serve as an insulator when an electrode is applied. Referring next to Fig. 6, a contact metal 25, a portion of which will serve as the base contact in the final device, is deposited over the exposed layer 6, the remains of layer 4, the dielectric 19 and the metal mask 12. In Fig. 7, the final planarization and contact separation is accomplished by such techniques as ion milling or reactive ion etching of a portion of the metal 25. The metal 25 is removed from over the dielectric 19 and the metal mask 2 which then serves as an ohmic contact 12. In the practice of the invention, variations in processing become available. Should it be useful to protect element 12 in certain types of eroding or etching operations, a titanium cap, not shown, may be placed over the metal or any portion thereof. Where desirable for precise area delineation, ion milling operations with argon-oxygen mixtures may be employed which will retain the vertical profile of element 12 even adjacent the dielectric 19. This facilitates depositing device wiring lines later when fabricating an integrated circuit. Since the base contact metallurgy to the Ge does not require alloying, any convenient metal can be used, such as the AuGe used for element 12. An alloy type n+ GaAs ohmic contact metal member 26, again such as AuGe, is applied to the bottom of substrate 14 which then provides the third electrode, either the emitter or collector electrode, for the transistor. Should it be desirable, the region 14 can be uniformly or selectively doped to facilitate contact. Alternatively, contact to conductive element 2 can be made adjacent to other device electrodes on the transistor surface by suitable etching operations and a conductive region provided in element 2 or 14. The resulting transistor in Fig. 7 has the characteristics of equal area emitter, base and collector vetically oriented, electrically superior and reversible, and occupying the minimum area. What has been described is an intermediate manufacturing product, vertical transistor and manufacturing process that provides an equal area, reduced capacitance heterojunction bipolar transistor.

Claims: A heterojunction bipolar transistor comprising in combination a monocrystalline body (1) having first (2), second (3) and third (4) parallel layers of wider, narrower and wider band gaps respectively, an aligned portion (7, 8) through said body defined by an impurity implantation mask member (12) on said third layer (4), said portion defining equal area regions in said first, second and third layers, each with doping appropriate for transistor electrode performance, the portion of said body outside said aligned portion (7, 8) defining, in said first (2) and said third (4) layers, a region of high resistivity, and in said second layer (3) a region of high conductivity, at least one ohmic contact (25) to the portion of said second layer (3) in said portion outside said aligned portion (7, 8) separated from said aligned portion by a portion of said third layer (4), and defined by said impurity mask member (12) and by a dielectric layer (19) formed on the sidewall of said impurity mask member, an ohmic contact (26) to said first layer (2) in said aligned portion (7, 8), and an ohmic contact to said third layer (4), said contact comprising said impurity implantation mask member (12). The transistor of claim 1 wherein the same impurity element provides, in the region outside the aligned region (7, 8), high resistivity in said first (2) and third (4) layers and high conductivity in said second layer (3). The transistor of claim 1 or 2 wherein said first (2) and third (4) layers are GaAs and said second layer (3) is Ge. The transistor of claim 3 wherein said impurity element is taken from the group of boron, aluminum, gallium and proton (hydrogen). A process of fabricating a heterojunction bipolar transistor comprising in combination the steps of: providing a monocrystalline body (1) having first (2), second (3) and third (4) parallel layers of wider, narrower and wider band gap semiconductor material, positioning an impurity implantation resist member (12) having a device area on the exposed surface of said third layer (4), implanting an impurity through said exposed surface peripheral to said resist member (12) into each of said first, second and third layers (2, 3, 4), said impurity having the property of imparting high resistivity in said wide band gap semiconductor material and high conductivity in said narrower band gap semiconductor material whereby an aligned region (7, 8) through said body (1) is established, applying a dielectric (19) on the portion of said exposed surface of said third layer (4) adjacent said implantation resist member (12), removing said third layer (4) where not protected by said resist member (12) and said dielectric (19), thereby exposing a portion of said second layer (3), and applying ohmic contact means (25) to said first, second and third layers in said aligned region (7, 8), whereby the ohmic contact means to said third layer (4) comprises said resist member (12). The process of claim 5 where the semiconductor material of said first (2) and said third (4) layers are of GaAs and said second layer (3) is of Ge. The process of claim 6 where said implanted impurity is an element taken from the group consisting of boron, aluminum, gallium and hydrogen.

Inventors: JACKSON THOMAS N, JACKSON, THOMAS N.
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1988
Main classification: H01L29
IPC codes: H01L29, H01L21