NPN heterojunction bipolar transistor

Abstract: A heterojunction bipolar transistor (HBT) (10, 30) includes an indium-gallium-arsenide (InGaAs), indium-phosphide (InP) or aluminum-indium-arsenide (AlInAs) collector layer (14) formed over an indium-phosphide (InP) substrate (12). A base layer (16, 32) including gallium (Ga), arsenic (As) and antimony (Sb) is formed over the collector layer (14), and an AlInAs or InP emittor layer (18) is formed over the base layer (16, 32). The base layer may be ternary gallium-arsenide-antimonide (GaAsSb) doped with beryllium (Be) (16).

Description: The present invention relates to an NPN type heterojunction bipolar transistor (HBT) according to the preamble of claim 1. An NPN type heterojunction bipolar transistor (HBT) of the afore-mentioned kind is known from document "Patent Abstracts of Japan", Vol. 14, No. 302 (E-946), June 28, 1990, corresponding to document JP-A-2 97 026. HBTs provide substantial advantages over conventional homojunction bipolar transistors by enabling energy-gap variations in addition to electric fields as forces acting on electrons and holes. In an HBT, the emitter is designed to have a wider bandgap than the base, creating an energy barrier in the valence band at the emitter-base junction which inhibits unwanted flow of holes from the base to the emitter and substantially increases the emitter injection efficiency, current gain and operating frequency. The wide-bandgap emitter enables very high base doping, allowing low base resistance to be obtained even with small base widths. The emitter doping can be reduced to moderate levels, allowing reduction in base-emitter capacitance. The advantages of HBTs have been extensively demonstrated for high-speed aluminum gallium arsenide/gallium arsenide (AlGaAs/GaAs) HBTs. Moreover, alternative material systems for fabricating HBTs, including indium phosphide/indium gallium arsenide (InP/InGaAs) and aluminum indium arsenide/indium gallium arsenide (AlInAs/InGaAs) offer even higher performance as described in an article entitled "AlInAs/GaInAs HBT technology" by J. Jensen et al., in IEEE Journal of Solid-State Circuits, Vol. 26, No. 2, March 1991, pp. 415-421. However, HBTs fabricated using the latter material systems have maximum frequencies of oscillation (f max ) which are lower than their respective high cutoff frequencies (f T ) due to relatively high values of the product of base resistance and collector capacitance. This limits the potential speed and output power of circuits incorporating these HBTs because, as a desired design parameter, f max should be approximately twice f T . Moreover, the base layer of InP/InGaAs and AlInAs/-InGaAs/InGaAs HBTs is typically doped with beryllium (Be) as a p-type dopant. Be is highly diffusive, and migrates rapidly from the base layer into the emitter layer during growth and even during device operation, causing the P-N junction to be displaced from the emitter-based junction into the emitter layer. As a consequence, an energy barrier to electron flow is created and the barrier to hole flow is reduced, thus reducing the current gain. The HBT known from the document mentioned at the outset comprises an undoped InAlAs buffer layer, an Si doped In x Al 1-x As as the collector layer, a Be doped P-type GaAs y Sb 1-y as the base layer and an Si doped N-type In x Al 1-x As as the emitter layer, which are provided on a InP substrate. It is an object underlying the present invention to further improve an NPN type heterojunction bipolar transistor specified at the outset, that has enhanced performance characteristics, such as high F max , and is doped with a P-type dopant without conventional notorious effects. P According to the present invention, in the NPN type heterojunction bipolar transistor specified in the preamble of claim 1, as set out in the characterising part of claim 1 this object is achieved in that said base layer comprises a strained-layer superlattice (SLS) structure including alternating superlattice layers of gallium arsenide (GaAs) and gallium antimonide (GaSb), and said gallium antimonide (GaSb) superlattice layers are doped with silicon (Si). Accordingly, a heterojunction bipolar transistor (HBT) embodying the present invention having enhanced performance characteristics for example includes an indium gallium arsenide (InGaAs), indium phosphide (InP) or aluminum indium arsenide (AlInAs) N-type collector layer formed over an indium phosphide (InP) semi-insulating substrate. A P-type base layer including gallium (Ga), arsenic (As) and antimony (Sb) is formed over the collector layer, and an aluminum indium arsenide (AlInAs) or InP N-type emitter layer is formed over the base layer. The base layer is a strained-layer-superlattice (SLS) structure including alternating superlattice layers of undoped GaAs and P-typ e gallium antimonide (GaSb). The GaSb superlattice layers are doped with silicon (Si), which is much less diffusive than Be and enables the HBT to be fabricated with negligible displacement of the P-N junction from the emitter-base junction into the emitter layer. The use of Sb in the base layer rather than indium (In) and arsenic (As) as in conventional InP/InGaAs or AlInAs/InGaAs HBTs provides advantages including: 1. Higher valence band offset at the emitter-base junction, resulting in improved emitter injection efficiency. 2. Lower conduction band offset at the emitter-base junction, resulting in reduced power dissipation. 3. Reduced base resistance due to increased hole mobility, resulting in higher f max . 4. A potential barrier at the base-collector junction which reduces charge storage, also resulting in higher f max . These and other features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which like reference numerals refer to like parts. FIG. 1 is a simplified fragmentary sectional view illustrating an HBT; FIG. 2a is an energy band diagram illustrating a prior art HBT including an AlInAs emitter layer and an InGaAs base layer; FIG. 2b is similar to FIG. 2a, illustrating an HBT including an AlInAs emitter layer and an GaAsSb base layer; FIG. 3a is an energy band diagram illustrating a prior art HBT including an InP emitter layer and an InGaAs base layer; FIG. 3b is similar to FIG. 3a, illustrating an HBT including an InP emitter layer and an GaAsSb base layer; and FIG. 4 is a simplified fragmentary sectional view illustrating an HBT embodying the present invention. Referring now to FIG. 1 of the drawing, an NPN heterojunction bipolar transistor (HBT) is illustrated in simplified form and generally designated as 10. The HBT 10 includes a semi-insulating InP substrate 12, an indium gallium arsenide (In .53 Ga .47 As), indium phosphide (InP) or aluminum indium arsenide (Al .48 In .52 As) collector layer 14 which is formed over the substrate 12 and doped N-type, a ternary Ga y As 1-y Sb base layer 16 which formed over the collector layer 14 and doped P-type and an InP or Al .48 In .52 As emitter layer 18 which is formed over the base layer 16 and doped N-type. The value of y in the Ga y As 1-y Sb base layer 16 is preferably 0.5, but is variable between 0 and 1. The collector, base and emitter layers 14, 16 and 18 respectively are lattice matched as closely as possible to the substrate 12. Although the collector layer 14 is illustrated as being disposed between the base layer 16 and substrate 12 in the drawing, it is possible to reverse the relative positions of the collector layer 14 and emitter layer 18. Further illustrated are electrically conductive metal collector contacts 20, base contacts 22 and an emitter contact 24. The HBT 10 may be fabricated using conventional technology, preferably solid-source molecular-beam-epitaxy (MBE). The fabrication process per se is not the particular subject matter of the invention. The collector layer 14 has a thickness between approximately 250 - 500 nm, with the preferred value being 300 nm. The collector layer 14 is doped with Si to a free carrier concentration of approximately 1 - 5 x 10 16 electrons/cm 3 , with the preferred value being 1 x 10 16 . The emitter layer 18 has a thickness between approximately 150 - 250 nm, with the preferred value being 150 nm. The emitter layer 18 is doped with Si to a free carrier concentration of approximately 4 - 8 x 10 17 electrons/cm 3 , with the preferred value being 8 x 10 17 . The base layer 16 has a thickness between approximately 50 - 100 nm. The base layer 16 preferably includes a 50 nm thick main layer 16a doped with Be to a free carrier concentration of approximately 3 - 6 x 10 19 holes/cm 3 , with the preferred value being 5 x 10 19 , and a spacer layer 16b disposed between the main layer 16a and the collector layer 14. The spacer layer 16b is preferably 15 nm thick, and doped with Be to a free carrier concentration of 2 x 10 18 holes/cm 3 . In InP/InGaAs and AlInAs/InGaAs HBTs, the base layer is formed of InGaAs. In the HBT 10 the base layer 16 is formed of ternary GaAsSb doped P-type with Be. Be-doped GaAsSb in the base layer 16 provides improved performance over InGaAs base HBTs by increasing the hole mobilities and valence band offsets to confine the holes to the base region. FIG. 2a is an energy band diagram illustrating a prior art HBT including an AlInAs emitter layer and an InGaAs base layer, whereas FIG. 2b illustrates an HBT including an AlInAs emitter layer and a GaAsSb base layer. FIG. 3a is an energy band diagram illustrating a prior art HBT including an InP emitter layer and an InGaAs base layer, whereas FIG. 3b illustrates an HBT including an InP emitter layer and a GaAsSb base layer. The emitter, base and collector layers are designated as E, B and C respectively in the drawings. As described in an article entitled "Band-edge alignment in heterostructures", by F. Schuermeyer et al, in Applied Physics Letters, Vol. 55, No. 18, 30 Oct. 1989, pp. 1877-1878, the valence band offset ΔE v for a GaAsSb/AlInAs heterostructure is 0.43 eV, as compared to 0.2 eV for an InGaAs/AlInAs heterostructure. The increase in ΔE v is clearly evident from a comparison of FIGS. 2a and 2b, and provides a substantial improvement in emitter injection efficiency over the InGaAs base HBT by increasing the confinement of holes to the base layer 16. As illustrated in FIGs. 3a and 3b, the increase in ΔE v is even greater for the case of an InP emitter. The GaAsSb base provides ΔE v of 0.62 eV as compared to 0.33 for the InGaAs base. The lattice matched emitter/base structure of InP/GaAsSb with ΔE v = 0.62 eV provides especially efficient emitter injection efficiency. The conduction band offset ΔE c is also reduced. This enables a reduction in base-emitter turn-on voltage V SE , and a corresponding reduction in power dissipation. For the AlInAs emitter configuration illustrated in FIGs. 2a and 2b, the value of ΔE c is reduced from the value of 0.46 eV to 0.28 eV. For the InP emitter configuration illustrated in FIGs. 3a and 3b, the value of ΔE c is reduced from the value of 0.24 eV to 0.06 eV. The GaAsSb base 16 as lattice matched to the InP substrate 12 provides approximately twice the hole mobility than the InGaAs base, resulting in reduced base resistance and higher f max . More specifically, the hole mobility in GaSb is approximately 1,400 cm 2 /V-sec, as compared to 430 cm 2 /V-sec in InGaAs. As further illustrated in FIGs. 2b and 3b, the GaAsSb base configuration provides valence and conduction band offsets at the base-collector junction which are not present in the InGaAs base HBTs. The valence band offset ΔE v at the base-collector junction is 0.23 eV, whereas the corresponding conduction band offset ΔE c is 0.18 eV. These offsets produce a potential barrier which enhances hole confinement in the base layer 16, reduces charge storage and increases f max . FIG. 4 illustrates an HBT 30 embodying the present invention, in which like reference numerals are used to designate elements which are common to the HBT illustrated in FIG. 1. The HBT 30 differs from the HBT 10 in that the ternary GaAsSb base layer 16 is replaced by a base layer 32 in the form of a strained-layer-superlattice (SLS) structure including alternating superlattice layers 32a of gallium antimonide (GaSb) doped with Si, and undoped GaAs superlattice layers 32b. Although the SLS base layer 32 is illustrated in simplified form as including 2.5 periods (number of pairs of superlattice layers 32a and 32b), an actual HBT 30 fabricated in accordance with the invention will include an SLS base layer 32 having approximately 21 to 27 periods with a corresponding thickness range of approximately 50 to 100 nm. The preferred thickness value is 65 nm, corresponding to approximately 23 periods. The SLS base layer 32 is preferably fabricated such that the resulting free carrier concentration is approximately 2 x 10 18 holes/cm 3 . It will be understood that although Si is an N-type dopant in InGaAs, InP, AlInAs and GaAsSb, it is a P-type dopant in GaSb as described in an article entitled "p-type doping of gallium antimonide grown by molecular beam epitaxy using silicon", by T. Rossi et al, in Applied Physics Letters, Vol. 57, no. 21, 19 Nov. 1990, pp. 2256-2258. Si is much less diffusive than Be, enabling the HBT 30 to be fabricated with negligible displacement of the P-N junction from the emitter-base junction into the emitter layer 18. This also enables the fabrication process to be simplified since only one dopant material is required. The GaAs superlattice layers may be undoped or doped p-type by Be. The GaSb superlattice layers 32a have a compressive strain of approximately 3.5%, whereas the GaAs superlattice layers 32b have a tensile strain of approximately the same magnitude. Since the tensile and compressive strains complement each other, the SLS base layer 32 can be fabricated with an arbitrarily large number of periods if the mole fractions of Sb and As are made equal. Increasing the mole fraction of Sb in the SLS will improve the performance of the HBT 30 by increasing the hole mobility, reducing the conduction band offset ΔE t and increasing the valence band offset ΔE v . However, the practical upper limit for increasing the Sb mole fraction is believed to be approximately 67%, above which value the SLS will become overstressed and will relax resulting in the formation of misfit dislocations. The valence band offset ΔE v with this configuration is approximately 0.89 eV. EXAMPLE An HBT was fabricated as described with reference to FIG. 4, with equal mole fractions of Sb and As, and P-type doping of approximately 2 x 10 18 holes/cm 3 in the SLS base layer 32. The average hole nobility it room temperature was approximately 800 cm 2 /V-sec, four times that of a conventional HBT with an In .53 Ga .47 As base doped with Be.

Claims: An NPN type heterojunction bipolar transistor (30), comprising an emitter layer (18) being doped N-type and including a material selected from the group consisting of aluminum indium arsenide and indium phosphide; a base layer (32) being doped P-type and comprising gallium, arsenide and antimony; a collector layer (14) being doped N-type; and a substrate (12), said emitter, base and collector layers (18, 16, 14) being formed over said substrate (12), characterized in that said base layer (32) comprises a strained-layer superlattice structure including alternating superlattice layers (32b, 32a) of gallium arsenide and gallium antimonide ; and said gallium antimonide superlattice layers (32a) are doped with silicon. The transistor of claim 1, characterized in that said gallium arsenide superlattice layers (32b) are P-doped. The transistor of claim 2, characterized in that said gallium arsenide superlattice layers (32b) are doped with beryllium. The transistor of claim 1 , characterized in that said gallium arsenide superlattice layers (32b) are undoped. The transistor of any one of claims 1 through 4, characterized in that said superlattice structure has 21 to 27 periods. The transistor of any one of claims 1 through 5, characterized in that said collector layer (14) comprises a material selected from the group consisting of indium gallium arsenide indium phosphide and aluminum indium arsenide. The transistor of any of claims 1 through 6, characterized in that said substrate (12) is semi-insulating. The transistor of any of claims 1 through 7, characterized in that said substrate (12) comprises indium phosphide. The transistor of any of claims 1 through 8, characterized in that said collector layer (14) is disposed between said substrate (12) and said base layer ( 32).

Inventors: HASENBERG THOMAS, STANCHINA WILLIAM E, HASENBERG,THOMAS, STANCHINA,WILLIAM E.
Applicants: HUGHES ELECTRONICS CORP, HUGHES ELECTRONICS CORPORATION
Application year: 1993
Main classification: H01L29
IPC codes: H01L29, H01L21