HETEROJUNCTION BIPOLAR TRANSISTOR

Abstract: A collector lead-out portion (22) and a base lead-out portion (14) are formed so as to oppose each other in such a manner that active regions including a collector (3), a base (2) and an emitter (1) are sandwiched therebetween. The collector lead-out portion (22) and the base lead-out portion (14) are formed by ion implantation or selective epitaxial growth. Thus, a planar type heterojunction bipolar transistor capable of a high density of integration is formed.

Description: BACKGROUND OF THE INVENTION: The present invention relates to a heterojunction bipolar transistor for processing high-speed signals, and more particularly to a device structure well suited to raise the integration density and enhance the performance of the transistor and a method of producing the same. The heterojunction bipolar transistor is characterized in that a base-emitter junction is formed using semiconductors with which the band gap of an emitter becomes wider than that of a base, thereby to reduce the injection of minority carriers from the base into the emitter. Thus, the efficiency of the injection of majority carriers from the emitter into the base is enhanced, and the impurity concentration of the base can be raised, so that a transistor of high current gain and low base resistance becomes possible. The idea of the heterojunction bipolar transistor is old, and has been realized using a heterojunction of Ga x-1 Al x As-GaAs (Proceedings of the 12th Conf on Solid State Devices, 1980, p. 1). Figs. 1(a) and 1(b) show the sectional structure and energy band structure of the active regions of the transistor, respectively. This transistor is constructed of an emitter 1 made of an n-type Ga 0.7 Al 0.3 As layer, a base 2 made of a p-type GaAs layer, and a collector made of an n-type GaAs layer 3 and an n⁺-type GaAs layer 4. The band gap (5) of Ga 0.7 Al 0.3 As is 1.79 eV, which is approximately 0.37 eV wider than the band gap (6) of GaAs. Approximately 0.05 eV of the difference is allotted to a valence band and lowers the energy level of the emitter by this component (7), to suppress the injection (8) of holes. At a hetero-interface on a conduction band side, a notch (9) of about 0.3 eV arises in the band gap and similarly reduces the injection (10) of electrons. As an expedient for eliminating the notch (9), it has been considered to gently change the mixed ratio in the vicinity of the hetero-interface (for example, Solid State Electron., Vol. 15, No. 12, p. 1339, ('72)). In addition, a structure in which the base 2 is also formed of Ga 1-x Al x As having a mixed ratio gradient has been considered as an expedient wherein electrons in the base 2 are caused to reach the collector region 3 by drift, not by diffusion, thereby to achieve an enhanced operating speed (JP-A-49-43583). However, both of the known examples mentioned above concern the active regions of the transistor, and the way how a base electrode and a collector electrode are led out becomes important for enhancing the performance. As an example whose portions for leading out the electrodes are most advanced, a transistor of a sectional structure shown in Fig. 2 has been developed ('84 ISSCC, Digest of Tech. Papers, p. 51 ('84)). In this transistor, an n⁺-type GaAs layer 12 is disposed on active layers in order to improve the property of the ohmic contact between an emitter electrode 13 and the emitter 1. Portions accessory to the active regions are principally formed of base lead-out portions 14 as well as base electrodes 15 and a collector electrode 16. The base lead-out portions 14 are made of p-type layers which are formed on both the sides of the active layers by ion implantation. The collector electrode 16 is formed directly on the exposed collector layer 4 after the stacked layers have been windowed down to the collector layer 4 by selective etching as indicated by numeral 17. Numerals 18, 19 and 20 designate insulating layers which are all formed by ion implantation. The layers 18 isolate the collector and the base, the layers 19 isolate the emitter and the base, and the layers 20 isolate the individual elements (numeral 11 indicates a substrate). In the heterojunction bipolar transistor of this structure, the collector electrode 16 is formed at a position about 0.3 µm lower than the surface 21 of the device, to basically construct a mesa type structure, so that a high density of integration is difficult. Moreover, since the base lead-out portions are formed around the active regions, there are the problems that the area of the device enlarges and that the distance between the active regions and the collector electrode 16 lengthens to increase the series resistance. A heterojunction bipolar transistor with the features set forth in the first part of claim 1 is known from Japanese Journal of Applied Physics, Supplements, Proceedings of the 16th International Conference on Solid State Devices and Materials, 30th August to 1st September 1984, Kobe (Japan) Final Program and Late News Abstracts, Pages 26 to 27. The transistor known from this document is largely similar to that shown in Fig. 2 and described above, except that a collector lead-out portion is formed by a burried metal contact. JP-A-58-135 671 (Patent Abstracts of Japan, Vol. 7, No. 247 (E-208) 2nd November 1985, Page 125) discloses another heterojunction bipolar transistor were the base and collector lead-out portions are disposed on opposite sides of the active layers. SUMMARY OF THE INVENTION It is an object of the present invention to provide a planar-type heterojunction bipolar transistor of high integration density, high performance and low parasitic capacitance. This object is met by the transistor defined in claim 1, preferred developments of the invention being set forth in the dependent claims. Since the lead-out portion for the base region or the collector region is formed on only one side of the active regions, the surface area of the element can be made small, and the integration of the elements at a high density is permitted. Further, the surface portion of a semiconductor body which is a parent material constructing the heterojunction bipolar transistor is rendered substantially flat, and an emitter electrode, a base electrode and a collector electrode are formed on the flat portion. In forming the base electrode and the collector electrode on the flat portion of the semiconductor body, the base region lead-out portion and the collector region lead-out portion are formed by ion implantation or selective epitaxial growth. The base electrode and the collector electrode are formed on the surfaces of the base region lead-out portion and the collector region lead-out portion thus formed, whereby the fabrication of the planar element becomes possible. In addition, when the base lead-out portion and the collector lead-out portion are arranged in opposition and are formed so as to flatten the surface of the element, they can be self-aligned to the emitter. In this case, the area can be decreased, and uniformity in the sectional structures of the elements, in turn, uniformity in the characteristics of the elements can be achieved. Further, the active regions are isolated from the collector and base lead-out regions by damaged layers which are formed by impurity ion implantation. Thus, the reduction of parasitic capacitances becomes possible, and a heterojunction bipolar transistor of high performance is obtained. It is to be understood that the heterojunction bipolar transistor of the present invention is applicable to both the n-p-n type and the p-n-p type. Further, it is possible to invert the role of the emitter and collector of the heterojunction bipolar transistor of the pesent invention each other as they are formed symmetrically on both side of the base. In this case, an emitter lead-out portion and a base lead-out portion are formed so as to sandwich the collector region therebetween. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1A is a diagram for explaining the active regions of a heterojunction bipolar transistor, while Fig. 1B is a diagram showing the energy band structure thereof. Fig. 2 is a sectional view showing a prior-art heterojunction bipolar transistor. Figs. 3A to 3G are sectional views of an element showing the manufacturing steps of a heterojunction bipolar transistor. Figs. 4A to 4I are sectional views of an element showing the manufacturing steps of the heterojunction bipolar transistor which is a first embodiment of the present invention. Figs. 5A and 5B are a sectional view and a plan view of the heterojunction bipolar transistor according to the first embodiment of the present invention, respectively. Figs. 6A to 6F are sectional views of an element showing the manufacturing steps of the heterojunction bipolar transistor which is a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION: Embodiments of the present invention will be described with reference to Fig. 3A - Fig. 6F. While the embodiments exemplify bipolar transistors employing heterojunctions of Ga x-1 Al x As-GaAs, the invention is not restricted to the cases of the heterojunctions to be concretely mentioned below. It is needless to say that the invention is applicable to, for example, InGaAs -AlInAs, InGaAs-InGaAsP and Si-SiGe. Of course, in constructing the bipolar transistors, the respective combinations of semiconductors shall be of semiconductors which can form p-n junctions and whose lattice constants are close. A basic manufacturing process will be described with reference to Figs. 3A thru 3G. In fabricating elements, on a GaAs substrate 11, there are epitaxially grown in succession an n⁺-GaAs layer (concentration: 1 x 10¹⁸ cm⁻³, thickness: 300 nm) 4 and an n⁻-GaAs layer (concentration: 5 x 10¹⁶ cm⁻³, thickness: 200 nm) 3 which constitute a collector layer, a p⁺-GaAs layer (concentration: 2 x 10¹⁸ cm⁻³, thickness: 50 nm) 2 which is a base layer, an n-Ga 1-x Al x As layer (concentration: 1.5 x 10¹⁷ cm⁻³, thickness: 200 nm, mixed ratio x : approximately 0.2 - 0.4) 1 which is an emitter layer, and an n-GaAs layer (concentration: 2 x 10¹⁷ cm⁻³, thickness: 100 nm) 12 which is a protective film for the n-Ga 1-x Al x As layer 1. The n-GaAs layer 12 prevents the oxidation of the n-Ga 1-x Al x As layer 1 and facilitates to establish an ohmic contact. Although the growth has been performed by the molecular beam epitaxy (MBE) of good film thickness controllability, it may well be performed by the MO-CVD (OMVPE: organo-metal vapor phase epitaxy). Further, the surface of the grown structure is covered with a CVD-SiO₂ film (thickness: 200 nm) 24 (Fig. 3(a)). The resulting structure is subsequently coated with a photoresist film 25 (thickness: approximately 1 µm), whereupon the pattern of each collector lead-out portion is formed using the ordinary photolithographic process. After the SiO₂ film 24 has been windowed at each predetermined part 26 by utilizing the photoresist film 25, the collector lead-out portion 22 of the n⁺-type is formed by ion implantation 27 and annealing (Fig. 3B). This ion implantation is carried out so that the peak depth of implanted ions may substantially agree with the depth (∼ 300 nm) of the base layer 2, thereby to invert the base layer into the n-type. In case of using Si⁺ as an n-type impurity, the implantation energy is set at about 250 keV, and the dosage at 5 x 10¹³ cm⁻². S, Se, Sn or the like may well be used instead of Si⁺. In this case, the implantation energy needs to be increased in proportion to the mass of the corresponding ion species. Thereafter, the resulting substrate is subjected to annealing for activating the implanted ions. The annealing is recommended to be performed at 800 °C for 15 minutes (in AsH₃ atmosphere), but annealing for a short time, for example, flash lamp annealing may well be adopted. Next, similarly to the above process for forming the collector lead-out portion 22, p-type ion implantation 28 for forming each base lead-out portion 14 is carried out (Fig. 3C). Shown at numeral 29 is a photoresist mask for the ion implantation. Mg⁺ or Be⁺ is employed as ions to be implanted. The quantity of p-type carriers should desirably correspond to a concentration of at least 1 x 10¹⁸ cm⁻³ at the lower surface of the base layer. It is desirable to set the dosage at 2 x 10¹³ cm⁻² and the implantation energy at approximately 100 keV for Mg⁺ and approximately 30 keV for Be⁺. Next, ion implantation 30 for the isolation between the emitter 1 and the base lead-out portion 14 is carried out (Fig. 3D). The purpose of this ion implantation is to reduce a parasitic capacitance by forming a damaged layer 19. Accordingly, the depth of the implantation may be such that damages reach a depletion layer 50 on the emitter side. The thickness of this depletion layer is several ten nm in case of the aforementioned emitter concentration, so that the depth of the damaged layer may be approximately 250 nm. The ion species may be any as long as the coefficient of diffusion in the crystal is small, and it may be one often used ordinarily, such as C⁺, O⁺, Ar⁺ or Si⁺. This ion implantation is performed so as to surround the active regions as shown in a plan view of Fig. 5(b) to be referred to later. When using C⁺, the optimum values of the dosage and the implantation energy are 1 x 10¹³ cm⁻² and approximately 100 keV respectively. Shown at numeral 31 is a photoresist film for a mask. Subsequently, ion implantation for the isolation 23 between the active regions and the collector lead-out portion 22 is carried out. This ion implantation 32 is similar to the ion implantation in the case of the isolation of the base lead-out portion 14, and the same species of ions can be used. The depth needs to reach the lower side of the base layer 2, and in case of using C⁺, the ion implantation is performed under conditions of 200 keV and 1 x 10¹³ cm⁻² . Numeral 33 designates a mask of a photoresist film (Fig. 3E). Lastly, electrodes are formed on the surface of the element by the lift-off technique. First, an emitter electrode 13 and a collector electrode 16 are respectively formed on the n-type GaAs layers of the emitter portion 12, and the collector lead-out portion 22 (Fig. 3F). A multilayer metal of Au/Ni/AuGe is used as an electrode material, and after patterning it by the use of the lift-off technique, ohmic contacts are formed by a heat treatment at 450 °C for 5 minutes. Subsequently, a base electrode 15 is formed on the base lead-out portion 14 by a similar process (Fig. 3G). In/Ag (Ag: 75 %) is used as an electrode material. The conditions of a heat treatment for forming an ohmic contact are 450 °C and 5 minutes as in the case of the n-type layers. As thus far described, a planar type heterojunction bipolar transistor can be formed, so that a high density of integration becomes possible, and the active regions are electrically isolated from the base lead-out portion as well as the collector lead-out portion, so that the element which can operate at high speed owing to a low parasitic capacitance can be formed. The first embodiment of the present invention will be described with reference to Figs. 4A to 4I. The present embodiment reduces a transistor area in such a way that the base lead-out portion 14, collector lead-out portion 22 and damaged layers 19, 23 of the heterojunction bipolar transistor described above are formed in self-alignment to an emitter electrode. Accordingly, all of a semiconductor substrate 11 and epitaxially grown layers 1, 2, 3, 4 and 12 may be similar to those described above (Fig. 4A). The present embodiment will therefore be described in detail as to only points not included in the above structure. To the end of forming the base and collector lead-out portions 14, 22 in self-alignment to the emitter electrode, annealing for activation (800 °C) after ion implantation is needed, and refractoriness is required also of the emitter electrode. MoGe or WGe is used as a refractory ohmic electrode material 35 therefor. The electrode material is deposited on the surface of the epitaxial layer 12 to a thickness of 200 - 300 nm by sputtering or evaporation. Numeral 35 designates the deposited layer. In order to lower the resistance, Au/W, Al/W or Au/Pt/W may well be further deposited on the upper surface. By the way, in case of a p-n-p type heterojunction bipolar transistor, MoZn, MoMg or the like is employed as the refractory ohmic electrode material. Subsequently, a CVD-SiO₂ film 36 is deposited on the whole surface to a thickness of 300 nm. Using a photoresist film 37 in an emitter electrode pattern as a mask, the SiO₂ film 36 and the electrode film 35 are successively etched to form a T-shaped pattern at 38 and 13' by the two films (Fig. 4B). The etching of the SiO₂ film 36 is executed by reactive ion etching with CF₄ + H₂ gas. The electrode film 35 is worked by first etching it anisotropically by the use of reactive ion etching with NF₃ gas and thereafter etching it laterally by the use of isotropic plasma etching. Thus, the T-shaped pattern is formed. Next, using the SiO₂ film 38 of the T-shaped pattern as a mask, ion implantation operations 39 and 40 for forming the collector lead-out portion 22 and the base lead-out portion 14 respectively are carried out (Figs. 4C and 4D). Numerals 41 and 42 designate photoresist films for masking the outer parts of the collector and emitter regions, respectively. Ions to be implanted and annealing therefor are similar to those in the above description. The SiO₂ film 38 is subsequently removed, whereupon ion implantation 43 for the electrical isolation between the collector portion 3, 4 and the base 2 is performed using the emitter electrode 13' as a mask (Fig. 4E). O⁺ is used as ions. Numeral 44 designates a mask of photoresist. The conditions of the ion implantation are set at 200 keV and 2 x 10¹² cm⁻² so as to bring the position of a peak concentration into the collector layer 3. After the ion implantation, a heat treatment at 700 °C for 20 minutes for annealing damaged layers is carried out, while at the same time a layer 45 having deep traps owing to O⁺ is formed. Next, as above, ion implantation operations 29 and 32 are performed for isolating the active regions from the base lead-out portion 14 and the collector lead-out portion 22 (Figs. 4F and 4G). In the present embodiment, the conditions of the ion implantation operations are the same as above except that the emitter electrode 13' and photoresist films 31', 33' are used as masks. Next, a collector electrode 16 and a base electrode 15 are formed as above (Fig. 4H). Lastly, ion implantation 46 for isolating elements is carried out to finish up the elements. Ion species may be any capable of forming damages. For example, when the ions O⁺ are used, they are implanted by 1 x 10¹³ cm⁻² at 150 keV and 250 keV respectively. Shown at numeral 47 is a photoresist mask for covering the element portion. Figs. 5A and 5B show a sectional view and a plan view of the heterojunction bipolar transistor of the present invention fabricated by the steps explained above, respectively. According to the present embodiment, the collector lead-out portion 22, the base lead-out portion 14 and the isolation regions 23, 19 between them and the active regions (1, 2, 3) are all formed in self-alignment to the emitter electrode 13', so that the area of the element is reduced to permit a raised density of integration, along with a high-speed operation. Moreover, since the respective regions can be formed at high positional accuracies, the dispersion among the elements can be diminished. The self-alignment of the collector lead-out portion 22 as well as the base lead-out portion 14 in the present embodiment has been performed by the ion implantation employing the SiO₂ film 38 of the T-shaped pattern as the mask. However, this SiO₂ film 38 is not always necessary, but the ion implantation may well be performed using the emitter electrode 13' as a mask. Also in this case, the active regions and the lead-out portions 14, 22 can be isolated by the damaged layers 19 and 23. While, in the present embodiment, the base lead-out portion 14 is disposed on the side A of the active regions as shown in Fig. 5B it may well be disposed on the side B or the side C or so as to surround the sides A, B and C. This applies also to the collector lead-out portion 22. In addition, these facts hold true also for the embodiment. described below. The second embodiment of the present invention will be described with reference to Figs. 6A to 6F. The present embodiment consists in that the collector-base insulation layer 45 in the first embodiment is formed before the epitaxial growth of the base layer 2, thereby to prevent the base lead-out portion 14 from being damaged into a high resistance by the ion implantation for forming the insulating layer 45. Accordingly, this embodiment differs from the first embodiment merely in the process for forming the insulating layer 45 and some processing steps and is quite the same as the first embodiment in points of the other individual processes. Only the features not included in the first embodiment will be described in detail. A semiconductor substrate 11 and epitaxially grown layers 1, 2, 3, 4 and 12 to be used may be made of materials similar to those in the first embodiment. In the present embodiment, the collector layer 3, 4 is grown on the semiconductor substrate 11 as shown in Fig. 6A, whereupon a collector-base insulation layer 45' is formed by ion implantation 43 (Fig. 6B). O⁺, Cr⁺ or the like capable of forming deep acceptors is used as an ion species. In case of O⁺, implantation conditions are 100 keV and 3 x 10¹³ cm⁻² . In case of forming the respective epitaxial layers by the use of the MBE (molecular beam epitaxy), the ion implantation 43 should desirably be continuously executed within an identical vacuum vessel. In that case, a selective ion implantation system which can scan a focused ion beam is employed. After the formation of the insulating layer, the base layer 2, the emitter layer 1 and an n⁺-GaAs layer 12 are epitaxially grown (Fig. 6C). The formation of the insulating layer 45' can also be executed using a photoresist film as a mask as in the first embodiment (Fig. 4E). In this case, the substrate is once taken out of the vacuum vessel, and the surface of the epitaxial layer is exposed to the atmosphere, so that the surface is contaminated. On this occasion, the growth is carried out after the epitaxial layer 3 has been etched by 20 - 30 nm with an etchant of an NH₄OH + H₂O₂ system to expose a clean surface. After forming the epitaxial layers 2, 1 and 12, the formation of the emitter electrode 13', collector lead-out portion 22 and base lead-out portion 14 (Fig. 6D), the formation of the collector electrode 16 and base electrode 15 (Fig. 6E), and the formation of the damaged layers 19, 23 and 20 for insulation are executed in quite the same way as in the first embodiment. Thus, the heterojunction bipolar transistor is fabricated. Although, in the present embodiment, the collector lead-out portion 22 as well as the base lead-out portion 14 and the insulating layers 19, 23 have been self-aligned to the emitter electrode 13', the self-alignment is not always necessary, but they may well be formed by the individual photolithographic processes as initially described. As thus far described, according to the present embodiment, the ion implantation for forming the base-collector insulation layer 45' can be executed without the intervention of the base lead-out portion 14, so that the base and collector can be isolated without raising the resistance of the base lead-out portion 14. Therefore, both a parasitic resistance and a parasitic capacitance can be reduced, and the fabrication of a heterojunction bipolar transistor of high performance becomes possible.

Claims: A heterojunction bipolar transistor comprising, on a semiconductor substrate, collector, base, and emitter layers (1....4) formed in sequence one on top of the other, said layers including active layer portions and forming at least one heterojunction, a base lead-out portion (14) and a collector lead-out portion (22) disposed in said layers (1....4) and, laterally of said active layer portions, and insulating regions (19, 23) interposed between said active layer portions and each of said lead-out portions (14, 22), characterized in that said base and collector lead-out portions (14, 22) are disposed on opposite sides of said active layer portions, that said base and collector lead-out portions (14, 22) have the same layer structure as said active layer portions and are respectively doped with an impurity of the same conductivity type as said base and collector layers (2, 3), and that a collector-base insulating layer (45) having deep traps is provided under said base lead-out portion (14) in said collector layer (3). The transistor of claim 1 wherein the upper surface of said transistor is substantially flat, and an emitter electrode (13), a base electrode (15) and a collector electrode (16) are formed on the flat upper surface. The transistor of claims 1 or 2, wherein the impurity concentration of said collector lead-out portion (22) has its peak at a depth corresponding substantially to said base layer (2). The transistor of claim 3, wherein said impurity is of the n-conductivity type. The transistor of any one of claims 1 to 4, wherein said insulating regions (19, 23) are damaged regions (19, 23) formed by impurity ion implantation. The transistor of claim 5 wherein the insulating region (23) between said active layer portions and said collector lead-out portion (22) extends deeper than the base layer (2). The transistor of any one of claims 1 to 6, wherein the isolation region (19) between said active layer portions and said base lead-out portion (14) extends to a depletion layer (50) formed at an interface between said emitter and base layers (1, 2).

Inventors: TAKAHASHI, SUSUMU, TSUKADA, TOSHIHISA, UEYANAGI, KIICHI, UMEMOTO, YASUNARI, USAGAWA, TOSHIYUKI
Applicants: HITACHI, LTD.
Application year: 1985
Main classification: H01L29
IPC codes: H01L29, H01L21