Field effect transistor

Abstract: An active layer of a field effect transistor disposed on an InP substrate (101) comprises at least an InAs layer (105) and two InGaAs layers (104, 106). The InGaAs layer (104) is In x Ga 1-x As (wherein 0.55 < x < 1) and the InGaAs layer (106) is In y Ga 1-y As (wherein 0.55 < y < 1). The active layer comprises, for example, In 0. 53 Ga 0. 47 As layer (103) / In 0. 8 Ga 0. 2 As layer (104) / InAs layer (105) / In 0. 8 Ga 0. 2 As layer (106) / In 0. 53 Ga 0. 47 As layer (107). Electrons which have been leached out of the InAs layer (105) are confined into the InGaAs layers (104, 106), and about 90 % of the active electrons are accumulated in the layers (104, 105, 106) to achieve an excellent electron transport performance, so that an excellent high frequency characteristic can be obtained exhibiting a high cut-off frequency and an improved transconductance.

Description: FIELD OF THE INVENTION The present invention relates to a field effect transistor and, in particular, to a field effect transistor excellent in high frequency characteristic so as to exhibit a high cut-off frequency and an improved transconductance. BACKGROUND OF THE INVENTION Conventionally, in an InAlAs/InGaAs field effect transistor provided on an InP substrate, in order to improve its high frequency characteristic, a structure in which InAs composition ratio of the active layer is increased has been used. As a typical example thereof, there is a structure using an InAs layer in the active layer. This structure was reported in, for example, IEEE ELECTRON DEVICE LETTERS, VOL. 13, NO. 6, JUNE 1992, PP. 325-327, by Tatsushi Akazaki et al. Fig. 1 illustrates a cross-sectional view of the structure of the field effect transistor which is shown in the foregoing document. As shown in Fig. 1, an InAlAs buffer layer 202 is formed on an InP (100) substrate 201. On the buffer layer 202, an active layer or channel layer is formed. The active layer comprises an InGaAs layer 203 having the InAs composition ratio of 0.53, an InAs layer 204 and an InGaAs layer 205 having the InAs composition ratio of 0.53. On the active layer, an InAlAs spacer layer 206, an n-InAlAs electron supply layer 207, InAlAs schottky layer 208, an n-InAlAs cap layer 209 and an n-InGaAs cap layer 210 are formed. Incidentally, in Fig. 1, reference numeral 211 denotes a source electrode, 212 a gate electrode and 213 a drain electrode. In this structure, more electrons can be confined than with the structure comprising only the InGaAs layer having the InAs composition ratio of 0.53 as the active layer because a conduction band discontinuity between the InAlAs layer and the InAs layer is larger than that between the InAlAs layer and the InGaAs layer having the InAs composition ratio of 0.53. Further, the InAs layer can transport electrons faster than the InGaAs layer with the InAs composition ratio of 0.53. As a consequence, this structure exhibit a higher cut-off frequency and higher transconductance than the structure utilizing only the InGaAs layer having the InAs composition ratio of 0.53 for the active layer, so that the high frequency characteristic of the transistor is improved. However, in the foregoing conventional technique, with the increase of the InAs composition ratio in the active layer, lattice mismatching with the InP substrate becomes large and the critical layer thickness becomes small, it is necessary to thin the thickness of the active layer having a higher InAs composition ratio in order to maintain excellent crystallinity. In particular, when the InAs layer is used for the active layer, it is necessary to thin the thickness of the InAs layer down to, for example, about 4 nm. If, as in this conventional example, the about 4 nm thick InAs layer is used for the active layer, electrons are leached out of or seep from the InAs layer into the InGaAs layers, and only about 50 % of the electrons are accumulated within the InAs layer, of which an excellent electron transport characteristic can be expected. Since the electrons which have been leached out exist in the InGaAs layer with the InAs composition ratio of 0.53 which is far inferior to InAs layer in electron transport performance, the excellent electron transport performance which the InAs layer has is difficult to be reflected on the transistor characteristic, and the improvement of the cut-off frequency and the transconductance is also small. US-A-5373168 discloses an two dimensional electron Gas field effect transistor on a GaAs-semiconductor substrate. A ternary compound InGaAs plannar channel layer serving as a quantum well and having a variation of an In fraction in a perpendicular direction to a heterojunction interphase is provided. The variation has a step-graded profil with taking a maximum value at or in the vicinity of a portion where the two dimensional electron gas takes a maximum density. Such a multilayered structure provides a great electron mobility and a strong electron confinement to major electrons at a high electron density portion. Additionally, the multilayered structure provides a large effective electron mobility and the large sheet electron density to the two dimensional electron gas without a gain of the average of the Infractions, thereby suppressing the enlargement of the lattice mismatch, which causes misfit dislocations in crystals. The proposed In fractions of the In y Ga 1-y As-layers are 0.06 < y < 0.5. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to remove the above-mentioned conventional defects and to provide a field effect transistor excellent in high frequency characteristic showing a far higher cut-off frequency and transconductance than that of the conventional technique. In order to achieve the foregoing object, the present invention provides a field effect transistor according to claim 1. The dependent claim is related to a further advantageous aspect of the invention. In the present invention, the active layer includes an In x Ga 1-x As (wherein 0.65 < x < 0.85) / InAs / In y Ga 1-y As (wherein 0.65 < y < 0.85) structure. The In x Ga 1-x As (wherein 0.65 < x < 0.85) layer and the In y Ga 1-y As (wherein 0.65 < y < 0.85) layer less in strain than the InAs layer are each inserted between the InAs layer high in strain and the InGaAs layer with the InAs composition ratio of 0.53 which is lattice matched to the InP substrate, so that the heterointerface between the adjacent layers can be smoothly formed. By the smooth heterointerface being formed, the degradation of the electron transport performance, which is caused by scattering due to the heterointerface roughness, can be suppressed. Further, since the In x Ga 1-x As (wherein 0.65 < x < 0.85) layer and the In y Ga 1-y As (wherein 0.65 < y < 0.85) layer are positioned adjacent to the InAs layer, which InGaAs layers have conduction band energy more separated from that of the InAlAs layer as compared with the InGaAs layer, which is lattice matched to the InP substrate, the electrons which are leached out of the InAs layer are confined into the above In x Ga 1-x As (wherein 0.65 < x < 0.85) layer and the In y Ga 1-y As (wherein 0.65 < y < 0.85) layer. Further, in the present invention, the electrons which are leached out of the InAs layer are confined into the above In x Ga 1-x As (wherein 0.65 < x < 0.85) layer and the In y Ga 1-y As (wherein 0.65 < y < 0.85) layer, which InGaAs layers are superior in electron transport performance to the InGaAs layer having the InAs composition ratio of 0.53 which is lattice matched to the InP substrate. In consequence, by using the structure of the present invention, a field effect transistor excellent in high frequency characteristic which exhibits a high cut-off frequency and an improved transconductance can be realized. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a cross-sectional view illustrating a conventional field effect transistor; Fig. 2 is a cross-sectional view illustrating an embodiment of the field effect transistor according to the present invention; Fig. 3 is a diagram showing an energy band structure of the embodiment of Fig. 2 for illustrating the change of the conduction band in the active layer and the neighborhood thereof; Fig. 4 is a cross-sectional view illustrating another embodiment of the field effect transistor according to the present invention; and Fig. 5 is a diagram showing an energy band structure of the embodiment of Fig. 4 for illustrating the change of the conduction band in the active layer and the neighborhood thereof. DESCRIPTION OF THE PREFERRED EMBODIMEMTS Next, specific embodiments of the present invention will be described with reference to the accompanying drawings. Fig. 2 is a cross-sectional view illustrating an embodiment of the field effect transistor according to the present invention, and Fig. 3 is a diagram showing an energy band structure of the embodiment of Fig. 2 for illustrating the change of the conduction band in the neighborhood of the active layer. The field effect transistor illustrated in Fig. 2 are manufactured by the following steps: growing an InAlAs buffer layer 102 (for example, InAs composition ratio of 0.52, thickness of 200 nm) on an InP substrate 101; growing an InGaAs layer 103 (for example, InAs composition ratio of 0.53, thickness of 9 nm), an InGaAs layer 104 (for example, InAs composition ratio of 0.8, thickness of 4 nm), an InAs layer 105 (for example, thickness of 4 nm), an InGaAs layer 106 (for example, InAs composition ratio of 0.8, thickness of 2 nm) and an InGaAs layer 107 (for example, InAs composition ratio of 0.53, thickness of 1 nm) sequentially; growing an InAlAs spacer layer 108 (for example, InAs composition ratio of 0.52, thickness of 4 nm), an InAlAs carrier supply layer 109 (for example, InAs composition ratio of 0.52, thickness of 20 nm) and an InAlAs schottky layer 110 (for example, InAs composition ratio of 0.52, thickness of 20 nm) sequentially; and, forming a source electrode 111, a gate electrode 112 and a drain electrode 113 thereon. In the InAlAs carrier supply layer 109, Si is doped with the concentration of, for example, 3 x 10 18 cm -3 . The active layer or channel layer comprises the above InGaAs layer 103, InGaAs layer 104, InAs layer 105, InGaAs layer 106 and the InGaAs layer 107. In Fig. 3 showing the energy band structure, especially the change of the conduction band in the neighborhood of the InGaAs layers, the abscissa is the position along the direction of the thickness of each layer of the structure shown in Fig. 2, and the ordinate is the energy of the lower edge of the conduction band. This Fig. 3 reveals that the conduction band of the In 0. 8 Ga 0. 2 As layer 104, InAs layer 105 and In 0. 8 Ga 0. 2 As layer 106 is lower than Fermi level 114 and that they can confine electrons with a far higher probability than the layer higher in conduction band than Fermi level, for example, In 0. 53 Ga 0. 47 As layer 103, which reveals that most of the electrons within the active layer can be confined into the high InAs composition ratio layers 104, 105, 106 of which a high electron transport performance can be expected. By adopting such a structure, since about 90 % of the electrons are confined into the high InAs composition ratio layers, i.e. InGaAs layer 104 (InAs composition ratio of 0.8, thickness of 4 nm) / InAs layer 105 (thickness of 4 nm) / InGaAs layer 106 (InAs composition ratio of 0.8, thickness of 2 nm), of the active layer, a field effect transistor excellent in high frequency characteristic in which a high electron transport characteristic inherent to the high InAs ratio layers is reflected and which exhibits a high cut-off frequency and high transconductance can be obtained. It was found that the above embodiment realized the cut-off frequency of 260 GHz by using a 1.5 µm long gate electrode and the electron mobility higher by 14 % than that of the structure of Fig. 1. Incidentally, although, in the foregoing embodiment, as the high InAs ratio layers of the active layer was chosen the InGaAs layer 104 (InAs composition ratio of 0.8, thickness of 4 nm) / InAs layer 105 (thickness of 4 nm) / InGaAs layer 106 (InAs composition ratio of 0.8, thickness of 2 nm), the InAs composition ratio for the InGaAs layers 104 and 106 can be each changed within the range of 0.55 to 1. That is, in the field effect transistor, if the active layer comprises two InGaAs layers and an InAs layer inserted between them, and the InAs composition ratio of the InGaAs layers is chosen to be In x Ga 1-x As (wherein 0.65 < x < 0.85) and In y Ga 1-y As (wherein 0.65 < y < 0.85), the electrons which have been leached out of the InAs layer are confined into the high InAs ratio layers excellent in electron transport performance so as to exhibit a high cut-off frequency and an improved transconductance. However, if the InAs composition ratio is near to 0.55 or 1, the difference between the strain energies in the heterointerface tends to become large and the heterointerface tends to be roughened to exert an undesired influence on the electron transport performance. Therefore, the InAs composition ratio of the InGaAs layer contacting the InAs layer is within the range of 0.65 to 0.85. Although, in the foregoing embodiment, the thickness of the InAs layer 105 was set to 4 nm, the thickness of the InAs layer 105 may be set to any arbitrary value as long as the dislocation is not generated and the desired high frequency characteristic can be obtained. It is, for example, within the range of 4 nm to 10 nm. Although, in the foregoing embodiment, the thickness of the InGaAs layer 104 was set to 4 nm, the thickness of the InGaAs layer 104 may be set to any arbitrary value as long as the dislocation is not generated. However, as the thickness of the InGaAs layer 104 increases, the strain energy of the high InAs ratio layers as a whole increases and the degradation of crystallinity caused by dislocation occurs. Therefore, it is preferably within the range of 2 nm to 6 nm. Similarly, although, in the foregoing embodiment, the thickness of the InGaAs layer 106 was set to 2 nm, the thickness of the InGaAs layer 106 may be changed within the range of 1 nm to 6 nm. However, as the thickness of the InGaAs layer 106 increases, the distribution of the electrons becomes more distant from the gate electrode and the improvement in the high frequency characteristic is suppressed. Therefore, it is preferably within the range of 1 nm to 4 nm. In the foregoing embodiment, the active layer comprises the InGaAs layer 103 (InAs composition ratio of 0.53, thickness of 9 nm) and the InGaAs layer 107 (InAs composition ratio of 0.53, thickness of 1 nm) in addition to the InAs layer-105 and InGaAs layers 104, 106. However, the InGaAs layers 103, 107 may be omitted. Fig. 4 is a cross-sectional view illustrating such an embodiment of the field effect transistor according to the present invention. Fig. 5 is a diagram showing an energy band structure of the embodiment of Fig. 4. It was found that this embodiment achieved the similar effect as the embodiment of Fig. 2. Further, the InAs composition ratio of each of InAlAs buffer layer 102, InAlAs spacer layer 108, InAlAs carrier supply layer 109 and InAlAs schottky layer 110 may be each changed within the range of 0 to 1. Still further, these layers may be made of GaAs, AlGaAs, AlAs, InP or AlSb. Further, the concentration of the dopant in the InAlAs carrier supply layer 109 may be set to any desired value. With reference to the dopant, in the foregoing embodiment, since electron was chosen as the carrier, the n-type dopant Si was used. However, the other n-type dopant such as S or Se may be used. If hole is chosen as the carrier, a p-type dopant such as Be, C or the like may be used. As described in the above, according to the present invention, the field effect transistor features that the active layer includes the In x Ga 1-x As (wherein 0.65 < x < 0.85) / InAs / In y Ga 1-y As (wherein 0.65 < y < 0.85) structure, and most of electrons can be accumulated within the InAs layer and the InGaAs layers with the InAs composition ratio of above 0.65 and below 0.85 having a high electron transport performance, so that an excellent high frequency characteristic can be obtained to achieve a high cut-off frequency and high transconductance.

Claims: A field effect transistor disposed on an InP substrate (101), comprising: a carrier supply layer (109); and an active layer (103 - 107) positioned between said substrate (101) and said carrier supply layer (109), said active layer (103 - 107) comprising a first InGaAs layer (104), a second InGaAs layer (106) and a strained InAs layer (105) positioned therebetween, said InAs layer (105) having a thickness within the range of 4 nm to 10 nm, said first InGaAs layer (104) being strained and positioned at a substrate side of said InAs layer (105) and having a thickness more than 2 nm and less than 6 nm and a composition represented by a formula In x Ga 1-x As (wherein 0.65 < x < 0.85), said second InGaAs layer (106) being strained and having a thickness more than 1 nm and less than 4 nm and a composition represented by a formula In Y Ga 1-Y As (wherein 0.65 < y < 0.85). The field effect transistor as set forth in claim 1, wherein said active layer (103 - 107) further comprises a third InGaAs layer (103) positioned at the substrate side of said first InGaAs layer (104) and a fourth InGaAs layer (107) positioned at an anti-substrate side of said second InGaAs layer (106), said third InGaAS layer (103) having a composition represented by a formula In a Ga 1-a As (wherein 0.50 < a < 0.55), said fourth InGaAs layer (107) having a composition represented by a formula In b Ga 1-b As (wherein 0.50 < b < 0.55).

Inventors: MIYAMOTO HIRONOBU, NAKAYAMA TATSUO, MIYAMOTO, HIRONOBU, NAKAYAMA, TATSUO, MIYAMOTO, HIRONOBU, C/O NEC CORPORATION, NAKAYAMA, TATSUO, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: H01L29
IPC codes: H01L29, H01L21