Field-effect transistor

Abstract: There is disclosed a field-effect transistor comprising a channel layer (3), an electron supply layer (5) and a semiconductor layer (4) formed between the channel layer (3) and the electron supply layer (5), the semiconductor layer (4) having a thickness for spatially separating two-dimension electron gas from donor ions in the electron supply layer (5) and forming two-dimension electron gas in the channel layer (3) by a Coulomb's force of the donor ions and having a better high frequency characteristic than that of the electron supply layer (5).

Description: Background of the Invention The present invention relates to a high speed field effect transistor (FET) primarily used in a microwave band. An excellent high frequency characteristic is required for a semiconductor material of a FET used in a microwave band. Such a material includes an InGaAs semiconductor which exhibits a good electron transport characteristic. Among others, a high electron mobility transistor (HEMT, MODFET) which uses In 0.52 Al 0.48 As and In 0.53 Ga 0.47 As which match to InP in terms of lattice as an electron supply layer and a channel layer exhibits an excellent high frequency characteristic. Detail of the HEMT is described in IEEE Electron Device Letters, Vol. 11, No. 1, January 1990, pages 56 to 62. An AlGaAs/GaAs-HEMT which uses AlGaAs and GaAs semiconductor materials as the electron supply layer and the channel layer has been put into practice. The semiconductor material for the electron supply layer is selected to have a larger energy gap and a smaller electron affinity than those of the semiconductor material for the channel layer. When the channel layer is made of GaAs, AlGaAs is selected as the material for the electron supply layer, and when the channel layer is made of InGaAs, AlInAs is selected as the material for the electron supply layer. Two-dimensional electron gas is generated in the channel layer by forming the epitaxial structure of the HEMT by those materials. Usually, a gate length of the FET is very short, for example less than 1 µm. Thus, the electrons which run therealong have a large energy. Accordingly, when a high voltage is applied to a gate electrode, the electrons do not stay in the two-dimensional channel but they may jump over an energy barrier which is present on a junction plane of the channel layer and the electron supply layer and are moved to the electron supply layer. This phenomenon is explained as follows with reference to an energy band chart of Fig. 1. Fig. 1A shows an energy band on a hetero-junction plane of the AlGaAs/GaAs-HEMT, two-dimensional electron gas shown by hatching is generated in the GaAs semiconductor near the junction interface. When a high voltage is applied to the gate electrode and a high electric field is applied to the electrons, the electrons e in the GaAs semiconductor jump over the energy barrier which is present on the hetero-junction interface and are moved to the AlGaAs semiconductor. The same phenomenon takes place in the AlInAs/InGaAs-HEMT shown in Fig. 1B. When a high electric field is applied to the two-dimensional electron gas generated in the InGaAs semiconductor and a high energy is imparted to the electrons e⁻, the electrons e⁻ jump over the energy barrier on the hetero-junction interface and are moved to the AlInAs semiconductor. The AlGaAs or AlInAs semiconductor material which forms the electron supply layer has a smaller electron mobility and a lower electron saturation speed than those of the GaAs or InGaAs semiconductor material which forms the channel layer. As a result, when a portion of electrons moves from the channel layer to the electron supply layer, the electron mobility of the entire current channel decreases and the electron saturation speed reduces. As a result, a high frequency characteristic of the FET is deteriorated. The deterioration of the characteristic is also seen in a transfer conductance g m which is an index to indicate a transfer function of the device. Namely, in a characteristic of transfer conductance g m vs gate voltage, the transfer conductance g m materially decreases as the gate voltage increases in a positive direction. This problem is pointed out in IEEE Transactions on Electron Devices, Vol. ED-31, No. 1, January 1984, p.29 and it is noticed as a technical problem in the high speed FET. Furthermore, the document APPLIED PHYSICS LETTERS, Vol. 53 No. 25, 19. Dec. 1989, New York, USA, pages 2534 - 2536 discloses a field effect transistor including inter alia a channel layer, an electron supply layer comprising donor ions and an undoped InP setback layer provided between the channel layer and the electron supply layer. The setback layer has a thickness of 5 nm for spatially separating a two-dimensional electron gas in the channel and for forming a two-dimensional electron gas in the channel by the Coulomb's force of the donor ions. However, when a high electric field is applied to the two-dimensional electron gas, some electrons of the gas are attracted to the electron supply layer by the Coulomb's force of the donor ions. As a result, the high frequency characteristic of the field effect transistor is deteriorated. PATENT ABSTRACTS OF JAPAN Vol.13, no. 468 (E-834) (3816) 23 October 1989 disclose a heterojunction field-effect transistor in which an undoped InP layer, an undoped InGaAs layer, an n-InP layer, an undoped InP layer and an n-AlInAs layer are formed successively onto a semi-insulating InP substrate. A source electrode and a drain electrode brought into ohmiccontact onto the n-AlInAs layer are formed, and a gate electrode Schottky-joining between the source-drain electrodes is formed. Consequently, a first channel of a first two-dimensional electron gas is formed into the undoped InP layer by electrons fed from the N-AlInAs layer , and a second channel of a second two-dimensional electron gas is formed into the undoped InGaAs layer by electrons supplied from the n-InP layer. Accordingly, electrons dominantly flow in the channel of the second two-dimensional electron gas having high electron mobility in a low electric field, and electrons flow in the channel of the first two-dimensional electron gas having high electron saturation speed in a high electric field. This structure, however, is provided to obtain large drain currents with a low source resistance. Summary of the Invention In the light of the above, it is an object of the present invention to provide a field effect transistor having an excellent high frequency characteristic. According to the present invention, this object is solved by the features of claim 1. Particular embodiments of the invention are set out in the dependent claims 2 to 4. In accordance with the present invention, when a high electric field is applied to the two-dimensional electron gas and the energy of the electrons increases so that the electrons jump out of the channel layer, the electrons are moved to the semiconductor layer of the predetermined thickness having the good high frequency characteristic, which is adjacent to the channel layer. Furthermore, an energy barrier caused by the heterojunction between the semiconductor layer and the electron supply layer is provided such that electrons jumped out from the channel are caused to stay in the semiconductor layer when high voltages are applied. As a result, even if the high electric field is applied to the channel of the FET, the transport characteristic of the electrons is not deteriorated and the high frequency characteristic of the device is assured. The present invention is particularly effective when it is applied to a basic transistor structure operated in a microwave band or a millimeter wave band and having a gate length of less than 0.25 µm. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, Brief Description of the Drawings Fig. 1 shows an energy band chart for explaining a problem encountered in a prior art HEMT, Fig. 2 shows a sectional view of a structure of one embodiment of a HEMT of the present invention, Fig. 3 shows sectional views in respective steps in manufacturing the HEMT shown in Fig. 2, Fig. 4 shows an energy band of the HEMT shown in Fig. 2, and Fig. 5 shows a graph of a gate voltage V g vs transfer conductance g m of the HEMT shown in Fig. 2. Detailed Description of the Preferred Embodiment Fig. 2 shows a sectional view of a structure of one embodiment of the HEMT of the present invention. A method for manufacturing the HEMT is explained below with reference to a manufacturing step sectional view shown in Fig. 3. A buffer la er 2, a channel layer 3, a spacer layer semiconductor layer 4, an electron supply layer 5 and a contact layer 6 are sequentially and continuously epitaxially grown on an InP semiconductor substrate 1 (see Fig. 3A). The epitaxial growth may be carried out by a molecule beam epitaxy (MBE) method or an organic metal vapor phase epitaxy (OMVPE) method. The buffer layer 2 is made of undoped AlInAs and has a thickness of 1 µm, the channel layer 3 is made of undoped InGaAs and has a thickness of 15 nm (150Å), and the spacer layer 4 is made of undoped InP and has a thickness of 3 nm (30Å). The electron supply layer 5 is made of n-type AlInAs and has a thickness of 40nm (400 Å), and the contact layer 6 is made of n-type InGaAs and has a thickness of 10nm (100 Å). The electron supply layer 5 has a donor impurity concentration of 2 x 10¹⁸/cm³, and the contact layer 8 has a donor impurity concentration of 5 x 10¹⁸/cm³. The AlInAs material which forms the buffer layer 2 and the electron supply layer 5, and the InGaAs material which forms the channel layer 3 and the contact layer 6 have compositions of Al 0.48 In 0.52 As and In 0.53 Ga 0.47 As, respectively, so that the respective layers match to the InP semiconductor substrate 1 in terms of lattice. The thickness of the spacer layer 4,3nm (30 Å), is thick enough for the electrons supplied from the electron supply layer 5 to stay in the channel layer 3. Namely, it is thick enough for the Coulomb force of the donor ions in the electron supply layer 5 to be applied to the two-dimensional electron gas in the channel layer 3, Further, the spacer layer 4 is thick enough to secure a sufficient spatial distance between donor ions of the electron supply layer 5 and the two-dimensional electron gas in the channel layer 3. The InP material which forms the spacer layer 4 has a higher electron saturation speed and a better high frequency characteristic than those of the AlInAs material which forms the electron supply layer 5. It has a lower electron mobility and a higher electron saturation speed than those of the InGaAs material which forms the channel layer 3. The semiconductor layers in a transistor formation area are selectively etched away by mesa-etching and devices are electrically isolated (see Fig, 3B). Source and drain electrode patterns are patterned on the contact layer 6 by a conventional photolithography method. Afterward, an AuGe/Ni metal is vapor-deposited and the patterns are lifted off. The electrode metals left after the lift-off are alloyed at 400 °C for one minute to make ohmic contacts to the contact layer 6 to form a source electrode 7 and a drain electrode 8 (see Fig. 3C). Then a gate electrode is patterned by an electron beam lithography method, and a recess is formed in the gate electrode formation area by using the pattern as a mask (see Fig. 3D). The depth of the recess is controlled such that a predetermined drain current is produced, and a Ti/Pt/Au metal is vapor-deposited. After the vapor-deposition, the electrode pattern is lifted off to form a gate electrode 9. In this manner, the HEMT having the structure shown in Fig. 2 is formed. In Fig. 2, the like elements to those shown in Fig. 3 are designated by the like numerals. The energy band of the HEMT having such an epitaxial structure is shown in Fig. 4. The areas of Fig. 4 correspond to the electron supply layer (n-AlInAs) 5, the spacer layer (InP) 4, the channel layer (undoped InGaAs) 3 and the buffer layer (undoped AlInAs) 2, from left to right. An energy gap is formed in the channel layer 3 near the junction of the spacer layer 4 and the channel layer 3, and the two-dimensional electron gas shown by hatching is generated in the energy gap. An energy band which supports the accumulation of the two-dimensional electron gas is present in the spacer layer 4 and a higher energy barrier than the energy band is formed between the electron supply layer 5 and the spacer layer 4. When a high voltage is applied to the gate electrode 9 and a high electric field is applied to the two-dimensional electron gas, the electrons in the channel bear a high energy. As a result, some of the electrons in the two-dimensional electron gas jump out of the energy gap formed in the channel layer 3. The Jumped-out electrons are attracted to the electron supply layer 5 by the Coulomb's force of the doner ions in the electron supply layer 5 but they are caused to stay in the spacer layer 4 by the energy barrier between the electron supply layer 5 and the spacer layer 4. As described above, the InP material which forms the spacer layer 4 has a better high frequency characteristic and a higher electron saturation speed than those of the AlInAs material which forms the electron supply layer 5. Also, the InP material has a lower electron mobility and a higher electron saturation speed than those of the InGaAs material which forms the channel layer 3. As a result, even if the electrons jump out of the channel layer 3 when the high electric field is applied to the two-dimension electron gas, the jumped-out electrons travel in the spacer layer 4 having the high electron saturation speed. Accordingly, the high frequency characteristic of the device is not deteriorated unlike the prior art device, even if the high electric field is applied to the two-dimension electron gas. A gate voltage vs transfer conductance g m characteristic of the HEMT is shown in Fig. 5, in which an abscissa represents the gate voltage V g , an ordinate represents the transfer conductance g m , a solid line curve 11 shows a characteristic of the HEMT of the present embodiment, and a broken line curve 12 shows a characteristic of a prior art HEMT. As seen from Fig. 5, in the characteristic curve 11 for the present embodiment, the decrease of the transfer conductance g m is suppressed when the gate voltage V g increases in the positive direction. On the other hand, in the characteristic curve 12 for the prior art, the decrease of the transfer conductance g m is remarkable as the gate voltage V g increases. In accordance with the HEMT of the present embodiment, a high transfer conductance g m is assured over a wide range of gate voltage.

Claims: A field-effect transistor comprising: a channel layer (3); an electron supply layer (5) comprising donor ions for supplying electrons in said channel layer (3); and a semiconductor layer (4) between said channel layer (3) and said electron supply layer (5) made of undoped JnP material, and located having a thickness for spatially separating two-dimensional electron gas in the channel and for forming two-dimensional electron gas in said channel layer (3) by a Coulomb's force of the donor ions, characterized in that the JnP material forming the semiconductor layer (4) has a higher electron saturation speed than an AlJn As material forming said electron supply layer (5); and that an energy barrier caused by the heterojunction between the semiconductor layer (4) and the electron supply layer (5) is provided such that electrons jumped out from the channel are caused to stay in the semiconductor layer (4) when high voltages are applied. A field-effect transistor according to claim 1, wherein said channel layer (3) is made of undoped InGaAs. A field-effect transistor according to claim 2, comprising: an InP semiconductor substrate (1); a buffer layer (2) formed on said semiconductor substrate (1) and made of undoped AlInAs; said channel layer (3) formed on said buffer layer (2); said semiconductor layer (4) formed on said channel layer (3); said electron supply layer (5) formed on said semiconductor layer (4) and made of n-type AlInAs; a contact layer (6) formed on said electron supply layer (5) and made of n-type InGaAs; ohmic electrodes (7, 8) formed on said contact layer (6); and a gate electrode (9) formed in a recess formed in said contact layer (6) and said electron supply layer (5). A field-effect transistor according to claim 3, wherein said buffer layer (2) and said electron supply layer (5) are made of Al 0.48 In 0.52 As, and said channel layer (3) and said contact layer (6) are made of In 0.53 Ga 0.47 As.

Inventors: NAKAJIMA SIGERU, NAKAJIMA, SIGERU, NAKAJIMA, SIGERU C/O YOKOHAMA WORKS OF SUMITOMO EL
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1991
Main classification: H01L29
IPC codes: H01L21, H01L29