Field-effect transistor

Abstract: A field-effect transistor (FET) in which an InGaAs layer formed on a GaAs substrate is formed in such a manner that the In composition ratio on the gate electrode side on the substrate surface is made small and the In composition ratio on the GaAs substrate side is made large. Thereby, the FET does not cause a decline in the mutual conductance in the FET and a decline in the noise figure (NF) even if negative voltage is applied to a gate electrode.

Description: The present invention relates to a field-effect transistor (hereafter referred to as the FET) using a compound semiconductor. More particularly, the present invention concerns a so-called high electron mobility transistor of a pseudomorphic type in which an indium-gallium-arsenide (hereafter referred to as InGaAs) semiconductor crystal layer containing indium (hereafter referred to as In) and an aluminum-gallium-arsenide (hereafter referred to as AlGaAs) layer are formed on a gallium-arsenide (hereafter referred to as GaAs) semiconductor substrate, thereby constituting an FET. In recent years, the use of semiconductors in high-frequency regions has become active, FETs operating at high speed at high frequencies have been required, and high electron mobility transistors using compound semiconductors have been developed. To effect higher speed, the so-called pseudomorphic-type FETs in which InGaAs, whose electron mobility is greater than that of GaAs, is used in a channel layer have been developed. The basic structure of this pseudomorphic-type FET is such that In 0.15 Ga 0.85 As layer, whose In composition ratio is, for example, 15% is formed on a GaAs substrate, AlGaAs is formed thereon, and a drain electrode, a source electrode, and a gate electrode are formed on that surface. A similar structure is described in US-B 4,827,320: therein a semiconductor device with strained InGaAs-layers according to the preamble of claim 1 is shown. The known device shows the structure of a first semiconductor layer formed of semi-insulating GaAs, a pseudomorphic quantum well layer of In y Ga 1-y As, an undoped set-back layer and a doped layer, which are formed of AlGaAs, and these layers are covered with a GaAs-cap layer. Since the bandgap of In y Ga 1-y As is much smaller than that of GaAs, the structure described in this patent shows the advantage, that there is no need for a troublesome large mole fraction of Al in the In y Ga 1-y As-layer in order to maintain a large bandgap discontinuity. In the FET of such a structure, the channel layer through which electrons travel is formed in the InGaAs layer, its thickness is small, i.e. in the vicinity of about 15 nm (150 Å), and the substrate side thereof abuts against GaAs having small electron affinity, so that electrons are sealed in a quantum well in InGaAs. As a result, this type of FET has an outstanding characteristic that, as compared with a conventional GaAs-type FET, g m is larger, and the short channel effect is smaller. Energy band diagrams in cases where 0 V and negative voltage are applied to the gate electrode are respectively shown in Figs. 5 and 6. In these drawings, the abscissa represents the depth from the gate electrode side in the lower direction of the GaAs substrate, point A indicates an interface between the gate electrode and the AlGaAs layer, point B indicates an interface between the AlGaAs layer and the InGaAs layer, and point C indicates an interface between the InGaAs layer and the GaAs substrate, while the ordinate represents energy. In addition, E shows a distribution of electrons in the direction of the depth of the semiconductor substrate when the FET is operated. It can be appreciated from Figs. 5 and 6 that the distribution of electrons upon application of the negative voltage to the gate electrode becomes wider in the direction of the depth of the substrate, and a distance a from the surface (point A) of the substrate to the center (point D) of the distribution of electrons is large. Since the InGaAs layer containing In is formed in the channel layer as described above, the ability is improved. However, there still exists the problem that when the negative voltage is applied to the gate electrode, the mutual conductance g m of the transistor becomes small. Also, there is a problem in that the noise figure becomes aggravated in a region where the drain current is small. That is, g m is expressed by the following Formula (1): g m = µ · Wg · ε Lg · a ( Vg - Vth) where µ is the mobility of electrons, Lg is a gate length, Wg is a gate width, ε is a dielectric constant of AlGaAs, Vg is a gate voltage, Vth is a threshold voltage, and a is a distance (see Fig. 5 or 6) between the gate electrode and the channel. If the negative voltage is applied to the gate electrode, the distribution of electrons becomes farther from the substrate surface, and a becomes large. As a result, g m becomes small from Formula (1). In addition, the noise figure NF is expressed by the following Formula (2): NF = 1 + 2 π K f · f · Cgs Rs + Rg g m where K f is a fitting factor which is determined by the material, configuration, and the like of the semiconductor, f is a frequency during operation, Cgs is a gate capacitance, g m is mutual conductance, Rs is source resistance, and Rg is gate resistance. It can be appreciated from this Formula (2) that if g m becomes small, NF becomes large, and the noise figure becomes large. In view of the above-described circumstances, it is an object of the present invention to provide an FET, whose noise figure does not become deteriorated by preventing g m from becoming small, even when negative voltage is applied to the gate voltage. The FET in accordance with the present invention is a field-effect transistor as defined in Claim 1. Preferred embodiments of the FET in accordance with the present invention are the subject matter of the dependent claims. As a result, an increase in the distance a between the gate electrode and the channel is canceled by increasing the electron mobility µ. As described above, it is known that the InGaAs in which In is contained in GaAs has a large electron mobility µ, and that the greater the composition ratio of In up to the In composition ratio of 50% or thereabouts, the greater the mobility µ becomes. Meanwhile, the lattice constants of InGaAs and GaAs differ from each other. However, according to Matthews et al., it is shown that in a case where the difference in lattice constant is small and the thickness of the crystal layer is small, their respective crystalline lattices shrink and the lattices are in a distorted state, but growth is possible (Defects in Epitaxial Multilayers 1 Misfit Dislocation, Journal of Crystal Growth, Vol. 27, 1974, p. 118). That is, if the film thickness is within a critical film thickness, InGaAs can be grown on the GaAs substrate, and the critical thickness of the film which grows becomes small if the In composition ratio becomes large. In the present invention, therefore, the amount of In in the InGaAs layer is varied, and the In composition ratio is made small at the side of the gate electrode, and the In composition ratio is made large at the side of the GaAs substrate. As a result, when negative voltage is applied to the gate electrode, and the center of gravity in the distribution of electrons moves to the interior of the substrate (the distance a between the gate electrode and the distribution of electrons), then the In composition ratio becomes large and the electron mobility µ becomes large. Hence, an increased portion of a in the denominator can be canceled by an increase in the numerator µ, thereby making it possible to control a decline in g m . In addition, since the In composition ratio of the GaAs buffer layer-side InGaAs is large, a band offset, i.e., an energy gap between the InGaAs region and the GaAs buffer layer, becomes large, so that more electrons can be contained, and the short channel effect can be prevented. Fig. 1 is an explanatory diagram of the structure of a high electron mobility transistor of a pseudomorphic-type in accordance with an embodiment of the present invention; Fig. 2 is a diagram which shows changes in the In composition ratio of an InGaAs layer in an FET in accordance with one embodiment of the present invention; Fig. 3 is a diagram of another example which shows changes in the In composition ratio of the InGaAs layer in the FET in accordance with another embodiment of the present invention; Fig. 4 is a conceptual diagram of a molecular beam epitaxy apparatus; Fig. 5 is an energy band diagram below a gate electrode when the gate voltage in a conventional pseudomorphic-type FET is 0 V; and Fig. 6 is an energy band diagram below the gate electrode when the gate voltage in the conventional pseudomorphic-type FET is negative. Next, a detailed description will be given of the present invention. Fig. 1 is a structural diagram of a pseudomorphic-type FET which is an embodiment of the present invention. In Fig. 1, reference numeral 1 denotes a GaAs substrate, and numeral 2 denotes an undoped GaAs buffer layer formed with a thickness of approximately 500 nm (5000 Å), and this buffer layer is provided to smooth the irregularities in the GaAs substrate 1 prior to its growth. Numeral 3 denotes an undoped InGaAs layer formed with a thickness of approximately 10 nm (100 Å) such that the composition ratio of In becomes smaller toward the surface side (upper side in the drawing), and this layer forms a channel layer. Numeral 4 denotes an undoped Al 0.22 Ga 0.78 As layer, which is formed with a thickness of approximately 2 nm (20 Å) and functions to ensure that the distribution of electrons do not extend into an n⁺ AlGaAs layer 5 and the mobility does not decline. Numeral 5 denotes an n⁺-type Al 0.22 Ga 0.78 As layer, in which n-type impurities are doped with a concentration of approximately 2.5 x 10¹⁸cm⁻³, and which is formed with a thickness of approximately 30 nm (300 Å) and has the function of supplying electrons to the InGaAs layer 3 from the n-type impurities in the n+ AlGaAs layer through electron affinity between AlGaAs and InGaAs. Numeral 6 denotes an n⁺-type GaAs layer, which is formed with a thickness of approximately 70 nm (700) Å and is formed to establish a good ohmic contact between a drain electrode 7 and a source electrode 8 on the one hand, and the n⁺-AlGaAs layer 5 on the other. A portion of the n⁺-type GaAs layer 6 between the drain electrode 7 and the source electrode 8 is removed, and a gate electrode 9 is formed on the exposed portion of the AlGaAs layer 5. The drain electrode 7 and the source electrode 8 are formed of an alloy of AuGe, Ni or the like, while the gate electrode 9 is formed of Ti/Al, Al, Ti/Pt/Au, WSi/Au, or the like. In the FET of this structure, as a bias voltage is applied to the gate electrode 9, and a voltage is applied across the drain electrode 7 and the source electrode 8, electrons move in the InGaAs layer 3, and current flows in correspondence with a voltage applied to the gate electrode. This InGaAs layer 3 has a different composition ratio of In in the thicknesswise direction thereof, and the composition ratio of In close to the AlGaAs layer 4 is small, while the In composition ratio on the GaAs substrate side is large. This relationship is shown in Figs. 2 and 3. That is, in these drawings, the abscissa represents the thickness of the InGaAs layer 3, point B indicates an interface between the InGaAs layer 3 and the AlGaAs layer 4, while point C indicates an interface between the InGaAs layer 3 and the buffer layer 2. The example shown in Fig. 2 is an example in which the composition ratio of In in InGaAs is continuously varied, while the example shown in Fig. 3 is an example in which the composition ratio of In is varied in steps. Either case shows an example in which the farther (deeper) from the substrate surface, the more the composition ratio of In is increased up to 40%. If the above-described structure is adopted, if negative voltage is applied to the gate electrode as described above, the center of a distribution of electrons in the InGaAs layer 3 moves toward the rear surface side (toward the C side in Figs. 2 and 3) of the substrate. As a result, on the basis of the aforementioned Formula (1), a becomes large, so that g m becomes small. According to the present invention, however, the composition ratio of In becomes large and the mobility µ of electrons becomes large, with the result that g m becomes large and an increased portion of a is consequently canceled by an increased portion of µ thereby inhibiting g m from becoming small. Next, a description will be given of an example of the process of manufacturing the FET of this structure. To manufacture the FET of this structure, when the temperature of a crucible, in which a compound element to be formed by a usual molecular beam epitaxy apparatus is contained, is increased, and a shutter is then opened, a compound semiconductor is deposited on the substrate. A conceptual diagram of this molecular beam epitaxy apparatus is shown in Fig. 4. In the drawing, reference numeral 11 denotes a furnace which is formed such that the interior thereof can be set to a vacuum of 10⁻¹⁰ to 10⁻¹¹ Torr. Numeral 13 denotes a heater for increasing the temperature of the substrate, numeral 14 denotes a substrate mounting base, and numerals 15 - 19 denote crucibles in which respective elements are contained. A description will be given of the method of forming the InGaAs layer by changing the composition ratio of In, which is an important point in the present invention. The GaAs substrate 1 is mounted on the substrate mounting base 14, the temperature of the substrate is set to approximately 540°C by the heater 13, and the temperatures of the As crucible and the Ga crucible are fixed at approximately 290°C and approximately 900°C, respectively, thereby forming the GaAs buffer layer 2. To change the composition ratio of In in a range of 15 - 40%, since the higher the temperature of the In crucible, the greater the amount of In, in order to increase the In composition ratio of the GaAs substrate-side InGaAs layer 3, if the temperature during the early period of formation of the InGaAs layer 3 is set to 840°C, and the temperature is then lowered gradually, and the temperature when the formation is completed is set to 810°C, the InGaAs layer 3 whose composition ratio of In is made continuously small is formed. To form this InGaAs layer 3 in such a manner as that the composition ratio of In changes in steps as shown in Fig. 3, if the temperature is set in steps in the aforementioned temperature range, and if the process is repeated in which the shutter is opened by a predetermined several tens of angstroms, the temperature is lowered slightly and the shutter is opened by another several tens of angstroms, it is possible to form the InGaAs layer 3 whose In composition ratio is changed in steps. In addition, to form the Al 0.22 Ga 0.78 As layer 4 with an Al composition ratio of 22%, that layer can be obtained by setting the temperature of the Al crucible to approximately 1060°C. In addition, to form the n⁺-AlGaAs layer 5, the temperature of the Si crucible to approximately 1090°C, and to form the n⁺-type GaAs layer, the temperature of the crucible is set to approximately 1095°C, thereby allowing doping with impurities. It should be noted that the growth rate of the respective layers is substantially fixed and is 1.2 µm/time. Although in the above-described embodiment a description has been given of the example of Al 0.22 Ga 0.78 As having an Al composition ratio of 22%, the composition ratio is not restricted to the same, and it suffices if it is in the range of 15 - 40%. As described above, to obtain the InGaAs layer 3 in which the In composition ratio is changed, the layer can be formed easily only by the temperature control of crucibles when the compound semiconductor is formed by epitaxy. In addition, since the In composition ratio of the InGaAs layer is changed on the gate electrode side and the substrate side, a change in the mutual conductance g m of the FET is small even with respect to a change in the voltage applied to the gate electrode. Hence, as for its noise figure, a fixed value can be obtained within a large range of the voltage applied to the gate electrode. As a result, the device is capable of sufficiently coping with a high-frequency circuit.

Claims: A field-effect transistor comprising; a first semiconductor crystal layer (3) of an indium-gallium-arsenide (InGaAs) formed on a gallium arsenide (GaAs) semiconductor substrate (1, 2); a second semiconductor crystal layer (4, 5) of an aluminium-gallium-arsenide (AlGaAs) formed on said first semiconductor crystal layer (3); and a source electrode (8), a gate electrode (9) and a drain electrode (7), in electrical contact with the exposed portions of said second semiconductor layer (4, 5), characterised in that said first semiconductor layer (3) is formed such that the indium composition ratio of said first semiconductor layer (3) at the side of said second semiconductor layer (4, 5) is smaller than at the side of said substrate (1, 2). The field-effect transistor according to claim 1, wherein said indium composition ratio of said first semiconductor crystal layer is continuously increased towards said gallium-arsenide (GaAs) semiconductor substrate. The field-effect transistor according to any of claims 1 or 2, wherein said indium composition ratio of said first semiconductor crystal layer is increased in steps towards said gallium-arsenide (GaAs) semiconductor substrate. The field-effect transistor according to any of claims 1 to 3, wherein said indium composition ratio of said first semiconductor crystal layer changes from 15 % at the side of the AlGaAs semiconductor crystal layer to 40% at the side of the GaAs semiconductor substrate.

Inventors: NAKAGAWA YOSHIKAZU C O ROHM CO, NAKAGAWA, YOSHIKAZU, C/O ROHM CO., LTD.
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1992
Main classification: H01L29
IPC codes: H01L29, H01L21