MICROWAVE MONOLITHIC INTEGRATED CIRCUIT DEVICE

Abstract: A microwave monolithic integrated circuit (MMIC) device comprises a transistor (E,B,C) and a capacitor (C3) which are respectively formed on the same semiconductor substrate (41). The transistor has at least one high resistivity semicon­ ductor layer (42) which forms a heterojunction with the semiconductor substrate (41). The capacitor (C3) has a dielectric layer constituted by the high resistivity semicon­ ductor layer (42) of the transistor. The transistor and the capacitor can be built by the same production processes.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to microwave monolithic integrated circuit devices (hereinafter simply referred to as MMIC devices), and more particularly to an MMIC device which can be miniaturized and produced by simple processes. Conventionally, in an example of an MMIC device comprising a transistor and a capacitor for impedance matching, the capacitor comprises a lower electrode layer, a dielectric layer and an upper electrode layer which are successively formed on a substrate. The transistor is formed on the same substrate as the capacitor, but the transistor and the capacitor are respectively formed by independent processes. For this reason, there are problems in that a large number of processes are required to produce the MMIC device and the processes are complex. On the other hand, because it is difficult to accurately control the thickness of the dielectric layer, there is a problem in that it is difficult to accurately control the capacitance of the capacitor. Accordingly, in order to eliminate these problems, it is possible to form a first comb-shaped electrode layer and a second comb-shaped electrode layer on a substrate so that end surfaces, that is, teeth, of the first and second comb-shaped electrode layers mutually oppose each other with a predetermined separation. In this case, a capacitor is formed across a tooth of the first comb-shaped electrode layer and a corresponding tooth of the second comb-shaped electrode layer, and the capacitance of this capacitor can be controlled more accurately compared to the three-layer capacitor described before. In addition, processes for forming the capacitor itself are simple compared to those of the three-layer capacitor. However, in this case, the area occupied by the capacitor becomes extremely large, and there is a problem in that it is impossible to miniaturize the MMIC device. Furthermore, since the transistor and the capacitor of the MMIC device are respectively formed by independent processes, there is a problem in that it is impossible to simplify the processes of producing the MMIC device. On the other hand, a field-effect semiconductor device was previously proposed in an European Patent Application No.83303769.0 filed June 29, 1983 in which the applicant is the same as the applicant of the present application. The previously proposed semiconductor device comprises a substrate having a via-hole, a via-hole electrode formed within the via-hole and on the lower surface of the substrate, a dielectric layer formed on the upper surface of the substrate at a position where the via-hole electrode is exposed, a source electrode layer formed on the dielectric layer, and a gate electrode layer and a drain electrode layer respectively formed on the substrate. A capacitor is formed across the source electrode layer and the via-hole electrode. According to this previously proposed semiconductor device, the dielectric layer is first formed on the upper surface of the substrate, and the via-hole is formed by an etching process by use of such a reactive gas that the etching from the lower surface of the substrate is stopped at the dielectric layer. Then, the dielectric layer is patterned by an etching process in accordance with a desired capacitance, and the source electrode layer is formed on the patterned dielectric layer. Hence, in addition to processes for forming a field-effect transistor (FET) on the substrate, it is necessary to perform additional processes for forming the capacitor such as the processes of forming the dieletric layer which is not required by the FET and patterning the dielectric layer by the etching. As a result, there is a problem in that it is impossible to effectively simplify the processes of producing the semiconductor device. Moreover, when the dielectric layer is patterned by the etching, there is a problem in that surfaces of remaining parts on the substrate, e.g. the FET, are damaged by the etching. SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide a novel and useful MMIC device in which the problems described heretofore are eliminated. This object is attained by an MMIC device comprising a semiconductor substrate, a transistor and a capacitor formed on said substrate, and first and second electrodes constituting electrodes of said capacitor, in which, according to the present invention, the transistor has at least one high resistivity semiconductor layer which forms heterojunction with said semiconductor substrate or with an activating layer comprised of one or several semiconductor layers formed between said at least one high resistivity semiconductor layer and the second electrode of said capacitor and coupling them electrically, said capacitor using as a dielectric layer said at least one high resistivity semiconductor layer of said transistor. Thus, with the MMIC device of the present invention, it is unnecessary to perform additional steps exclusively for forming the capacitor because the high resistivity semiconductor layer of the transistor having the heterojunction is used as the dielectric layer of capacitor, and the MMIC device has a planar structure. For this reason, the processes of producing the MMIC device can be simplified. In addition, since the size of the capacitor can be reduced, it is possible to effectively miniaturize the MMIC device as a whole. Furthermore, the thickness of the high resistivity semiconductor layer can be accurately controlled by use of molecular beam epitaxy (MBE), for example, and it is possible to accurately control the capacitance of the capacitor. Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view showing a capacitor portion of an example of a conventional MMIC device; Fig.2 is a plan view showing a capacitor portion of another example of a conventional MMIC device; Fig.3 is a cross-sectional view taken along a line III-III in Fig. 2; Fig. 4 is a plan view showing an essential part of a first embodiment of the MMIC device according to the present invention; Fig.5 is a cross-sectional view along a line V-V in Fig. 4 showing an essential part of the first embodiment of the MMIC device according to the invention; and Fig. 6 is a cross-sectional view showing an essential part of a second embodiment of the MMIC device according to the present invention. DETAILED DESCRIPTION FIG.1 is a cross-sectional view showing a capacitor portion of an example of the conventional MMIC device. A lower electrode layer 12, a dielectric layer 13 and an upper electrode layer 14 are successively formed on a substrate 11. A capacitor is constituted by the dielectric layer 13 which is formed between the upper and lower electrode layers 14 and 12. A transistor (not shown) is also formed on the substrate 11, but the transistor and the capacitor are respectively formed by independent processes. In other words, in addition to the processes for forming the transistor, it is necessary to perform additional processes, e.g. forming of a SiO₂ layer as the dielectric layer 13, for forming the capacitor. For this reason, there are problems in that a large number of processes are required to produce the MMIC device and the processes are complex. On the other hand, because it is difficult to accurately control the thickness of the dielectric layer 13, there is a problem in that it is difficult to accurately control the capacitance of the capacitor. For convenience sake, in FIG.1 and the drawings which will be described hereinafter, the thickness of the substrate is shown on a small scale compared to the thickness of other layers. FIG.2 is a plan view showing a capacitor portion of another example of the conventional MMIC device, and FIG.3 is a cross-sectional view showing the conventional MMIC device along a line III-III in FIG. 2. In FIGS.2 and 3, a first comb-shaped electrode layer 15 and a second comb-shaped electrode layer 16 are formed on the substrate 11 so that end surfaces, that is, teeth, of the first and second comb-shaped electrode layers 15 and 16 mutually oppose each other with a predetermined separation. In this case, a capacitor is formed across a tooth of the first comb-shaped electrode layer 15 and a corresponding tooth of the second comb-shaped electrode layer 16 as indicated by Ca in FIG.3, and the capacitance of this capacitor can be controlled more accurately compared to the three-layer capacitor shown in FIG.1. In addition, processes for forming the capacitor itself are simple compared to those of the three-layer capacitor shown in FIG.1. However, in this case, the area occupied by the capacitor becomes extremely large, and there is a problem in that it is impossible to miniaturize the MMIC device. Furthermore, although a transistor (not shown) is also formed on the substrate 11, the transistor and the capacitor of the MMIC device are respectively formed by independent processes. Thus, as in the case of the example shown in FIG.1, there are problems in that a large number of processes are required to produce the MMIC device and the processes are complex. Accordingly, the MMIC device according to the present invention comprises a transistor having at least one high resistivity semiconductor layer which forms a heterojunction with an activating layer or a semiconductor substrate, and a capacitor formed on the same semiconductor substrate as the transistor and having a dielectric layer thereof constituted by the high resistivity semiconductor layer of the transistor, so that the problems of the conventional MMIC devices are eliminated. FIG.4 is a plan view showing an essential part of a first embodiment of the MMIC device according to the present invention. In FIG.4, a source electrode, a drain electrode and a gate electrode of a heterojunction field-effect transistor (hereinafter simply referred to as an HFET) are respectively denoted by S,D and G. Electrodes 20 and 25 constitute upper electrodes of capacitors, and via-holes 21 and 26 are indicated by one-dot chain lines. The drain electrode D and the electrode 20 are connected via a lead wire 23. Portions on the outer side of regions 22 and 27 indicated by phantom lines are implanted with oxygen ions, for example, so as to have a high resistivity separation region. Fig. 5 is a cross-sectional view showing an essential part of the MMIC device along a line V-V in Fig.4. In Fig.5, those parts which are the same as those corresponding parts in Fig. 4 are designated by the same reference numerals, and description thereof will be omitted. In Fig.5, the MMIC device generally comprises a semiconductor substrate 31 made of GaAs or the like, a high resistivity dielectric layer 32 made of intrinsic AlGaAs or the like and having a thickness in the order of a few tenths of µm, a high resistivity dielectric layer 33 as a channel layer made of intrinsic GaAs or the like and having a thickness in the order of a few tenths of µm and an electron supplying layer 34 made of n-type AlGaAs or the like. The via-hole 21 is formed in the substrate 31, and an electrode 36 which constitutes a lower electrode of the capacitor is formed on the lower surface of the substrate 31 and within the via-hole 21. The gate electrode G is formed within a recess 37. An insulating separation region 38 is formed around a capacitor region in which capacitors C1 and C2 are formed. These capacitors are of the kind described in APPLIED PHYSICS LETTERS, vol. 42 N o 9, 1st May 1983, pages 821-923, American Institute of Physics, New York, US; P.M. SOLOMON et al.: "Electrical measurements on n-GaAs-undoped Ga 0.6 Al 0.4 As-n-GaAs capacitors". Such capacitors consist of a heterostructure which can be treated as a semiconductor - insulator - semiconductor structure. The insulating separation region 38 surrounding the capacitor region and the HFET region are formed by implanting into the layers 34,33 and 32 and the substrate 31 oxygen ions, for example, and has a high resistivity. The via-hole 21 is formed by a dry etching process which is performed from the lower surface of the GaAs substrate 31 by use of CCl₂F₂ as the etchant, for example. In this case, since the etching rate of the intrinsic AlGaAs high resistivity layer 32 is extremely low compared to that of the GaAs substrate 31, the dry etching is accurately stopped at the intrinsic AlGaAs high resistivity dielectric layer 32, that is, at the heterojunction between a spacer layer and the substrate. The electrode 36 is formed within the via-hole 21 and on the lower surface of the GaAs substrate 31 by depositing a metal material by a process such as vapor deposition or sputtering. As may be seen from FIG.5, the capacitor C1 uses the high resistivity dielectric layer 33 of the channel layer of the HFET as a dielectric layer thereof, and the capacitor C2 uses the high resistivity dielectric layer 32 of the spacer layer of the HFET as a dielectric layer thereof. Accordingly, it is unnecessary to perform additional processes other than the processes required to form the HFET, that is, it is unnecessary to perform processes exclusively for forming the capacitors C1 and C2 such as forming the dielectric layers of the capacitors C1 and C2. Furthermore, the MMIC device has a planar structure. As a result, the processes of producing the MMIC device can be simplified compared to the conventional devices. In addition, it is possible to effectively miniaturize the MMIC device as a whole because the size of the capacitors C1 and C2 can be reduced. The capacitances of the capacitors C1 and C2 are determined by the dielectric constants of the respective high resistivity dielectric layers 33 and 32, the thicknesses of the respective high resistivity dielectric layers 33 and 32 and the areas at the respective junctions between the dielectric layers and the capacitor electrodes. When the high resistivity dielectric layers 32 and 33 are respectively formed by the molecular beam epitaxy (MBE), it is possible to control the thicknesses of these layers 32 and 33 with an extremely high accuracy, and the capacitances of the capacitors C2 and C1 can thus be controlled with an extremely high accuracy. For example, in the case where the areas at the respective junctions between the dielectric layers of the capacitors C1 and C2 and the capacitor electrodes are both equal to 100 µm² and the thicknesses of the dielectric layers are both equal to 1000 0.1 nm, capacitance C T of the capacitors C1 and C2 as a whole can be described by the following. C T = 8.854x10⁻¹⁴x12.5x(100x10⁻⁴)²/(0.2x10⁻⁴) = 5.5 pF In this case, the capacitance C T is suited for an MMIC device which is used at a frequency in the range of 30 GHz. In the first embodiment described heretofore, the high resistivity dielectric layers of the HFET, i.e. the channel layer 33 and the spacer layer 32 are used as the dielectric layer of the capacitor. However, the transistor which is formed on the same substrate as the capacitor is not limited to the HFET. Next, description will be given with respect to a second embodiment of the MMIC device according to the present invention, by referring to FIG. 6. FIG.6 is a cross-sectional view similar to FIG.5, showing an essential part of the second embodiment of the MMIC device according to the present invention. In the present embodiment, a capacitor is formed on the same substrate as a heterojunction bipolar transistor (hereinafter simply referred to as an HBT). The MMIC device generally comprises a semiconductor substrate 41 which is made of GaAs or the like and has a thickness in the order of 20 µm, a high resistivity dielectric layer 42 as a spacer layer which is made of intrinsic AlGaAs or the like and has a thickness in the range of 0.1 to 1.0 µm, a subcollector layer 43 which is made of n⁺-type GaAs or the like and has a thickness in the order of 1.0 µm, a collector layer 44 which is made of n-type GaAs or the like and has a thickness in the order of 0.3 µm, a base layer 45 which is made of p-type GaAs or the like and has a thickness in the order of 0.1 µm and an emitter layer 46 which is made of n-type AlGaAs or the like and has a thickness in the order of 0.2 µm. A via-hole 48 is formed in the substrate 41, and an electrode 49 which constitutes a lower electrode of the capacitor is formed within the via-hole 48 and on the lower surface of the substrate 41. A base electrode B and a collector electrode C of the HBT and an electrode 50 which constitutes an upper electrode of the capacitor are respectively formed on the upper surface of the substrate 41. A contact layer 51 which is made of n⁺-type GaAs or the like and has a thickness in the order of 0.2 µm is formed on the emitter layer 46, and an emitter electrode E of the HBT is formed on the contact layer 51. As in the case of the first embodiment described before, an insulating separation region 53 surrounds an HBT region and a capacitor region in which a capacitor C3 is formed. The insulating separation region 53 is formed by implanting into the layers 46, 45, 44, 43 and 42 and the substrate 41 oxygen ions, for example, and has a high resistivity. A p⁺-type region 55 is formed under each of the base electrodes B, and this p⁺-type region 55 is formed by implanting p-type impurity ions into the layers 46,45 and 44. An n⁺-type region 56 is formed under the collector electrode C, and an n⁺-type region 57 is formed under the electrode 50. The n⁺-type regions 56 and 57 are respectively formed by implanting n-type impurity ions into the layers 46, 45, 44 and 43. For example, the layers 43 through 46 are respectively doped with appropriate ions at concentrations of 6x10¹⁸/cm³, 1x10¹⁷/cm³, 1x10¹⁸/cm³, 5x10¹⁷/cm³ and 6x10¹⁸/cm³. In the present embodiment, the capacitor C3 uses the high resistivity dielectric layer 42 of the spacer layer of the HBT as a dielectric layer thereof. Accordingly, it is unnecessary to perform additional processes other than the processes required to form the HBT, that is, it is unnecessary to perform processes exclusively for forming the capacitor C3 such as forming the dielectric layer of the capacitor C3. As a result, it is possible to obtain effects similar to those obtainable in the first embodiment described before. According to the present invention, the MMIC device comprises a transistor having at least one high resistivity semiconductor layer which forms a heterojunction with a semiconductor substrate or with an activating layer out of a plurality of semiconductor layers formed on a semiconductor substrate, and a capacitor formed on the same semiconductor substrate as the transistor and having a dielectric layer thereof constituted by the high resistivity semiconductor layer of the transistor. Hence, it is unnecessary to perform additional processes exclusively for forming the capacitor, and the MMIC device has a planar structure. Therefore, the processes of producing the MMIC device are simple. Furthermore, because the size of the capacitor can be reduced, it is possible to effectively miniaturize the MMIC device as a whole. When the high resistivity semiconductor layer of the transistor is formed by the molecular beam epitaxy (MBE), for example, it is possible to control the capacitance of the capacitor with an extremely high accuracy.

Claims: A microwave monolithic integrated circuit device comprising a semiconductor substrate (31;41), a transistor (S,G,D; E, B,C) and a capacitor (C1,C2;C3) formed on said substrate, and first (36;49) and second (20;50) electrodes constituting electrodes of said capacitor, characterized in that the transistor (S,G,D; E,B,C) has at least one high resistivity semiconductor layer (32,33;42) which forms heterojunction with said semiconductor substrate (31;41) or with an activating layer comprised of one or several semiconductor layers (34;43-46) formed between said at least one high resistivity semiconductor layer (32,33; 42) and the second electrode (20;50) of said capacitor (C1,C2;C3) and coupling them electrically, and in that said capacitor uses as a dielectric layer said at least one high resistivity semiconductor layer of said transistor. A microwave monolithic integrated circuit device as claimed in claim 1, in which said semiconductor substrate (31;41) comprises a via-hole (21;48), said first electrode (36;49) being formed on the lower surface of said semiconductor substrate and within said via-hole. A microwave monolithic integrated circuit device as claimed in claim 1 or 2, which further comprises an insulating separation region (38;53) surrounding the region in which said capacitor is formed, said insulating separation region having a high resistivity and being in contact with said first (36;49) and second (20;50) electrodes, said insulating separation region being formed by implanting oxygen ions or the like into each of a plurality of semiconductor layers (32-34;42-46) formed on said semiconductor substrate and including said at least one high resistivity semiconductor layer (32,33;42) said activating layer (34;43-46) and said semiconductor substrate (31;41). A microwave monolithic integrated circuit device as claimed in any one of claims 1 to 3, in which said second electrode (20;50) constitutes an electrode (D;B) of said transistor (S,G,D; E,B,C). A microwave monolithic integrated circuit device as claimed in any one of claims 1 to 4, in which said one or several semiconductor layers (34; 43-46) on said at least one high resistivity semiconductor layer (32,33;42) are commonly implanted with impurity ions in the region of said capacitor (C1,C2;C3). A microwave monolithic integrated circuit device as claimed in any of claims 1 to 5, in which said semiconductor substrate (31;41) is made of GaAs and said at least one high resistivity semiconductor layer (32,33;42) is made of intrinsic AlGaAs or GaAs. A microwave monolithic integrated circuit device as claimed in any one of claims 1 to 6, in which said transistor is a heterojunction bipolar transistor (E,B,C) comprising a spacer layer (42), a collector layer (44), a base layer (45) and an emitter layer (46) formed on the semiconductor substrate (41), said spacer layer being said at least one high resistivity semiconductor layer (42). A microwave monolithic integrated circuit device as claimed in claim 7, in which said second electrode (50) of said capacitor (C3) constitutes the base electrode (B) of said bipolar transistor. A microwave monolithic integrated circuit device as claimed in any one of claims 1 to 6, in which said transistor is a heterojunction field-effect transistor (S,G,D) comprising a spacer layer (32), a channel layer (33) and an electron supplying layer (34) formed on the semiconductor substrate (31), said spacer layer (32) and said channel layer (33) being said at least one high resistivity semiconductor layer (32,33). A microwave monolithic integrated circuit device as claimed in claim 9, in which said second electrode (20) of said capacitor (C1,C2) constitutes the drain electrode (D) of said field-effect transistor.

Inventors: HIRACHI, YASUTAKE
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: H01L27
IPC codes: H01L21, H01L29, H01L27