FIELD EFFECT TRANSISTOR

Abstract: A field effect transistor utilizing semiconductor hetero junction with high mutual conductance as well as low noise with source resistance reduced comprises a gallium indium arsenide mixed crystal semiconductor layer (23) providing a current path, indium phosphide layers (22, 34) capable of making easily low resistance formed on or under the gallium indium arsenide mixed crystal semiconductor layer (23) by ion-implantation, a source electrode (6), a gate electrode (5) and a drain electrode (7) which are formed on the surface of an aluminum indium arsenide mixed crystal semiconductor layer (24) formed on an uppermost layer, and an ion-­ implanted layer formed at least in a region to form a source resistance between the source electrode (6) and two-­ dimensional electron layer (8) within the gallium indium arsenide mixed crystal semiconductor layer (23).

Description: The present invention relates to a field effect transistor with a semiconductor hetero junction according to claim 1 applied to a low noise amplifier circuit, a high frequency circuit, a fast logical circuit, their integrated circuits, and an integrated optics or the like. Description of the Prior Art Fig. 1 is a cross sectional view showing a structure of a conventional field effect transistor utilizing semiconductor hetero junction, which is disclosed in, for example, an article by C.Y. Chen et all., IEEE Electron Device Letters, Vol. EDL-3, No. 6, 1982, p.152. In Fig. 1, a conventional field effect transistor comprises a semi-insulating indium phosphide (In P) substrate 1, an aluminum indium arsenide (Al In As) mixed crystal semiconductor layer 2 formed on the substrate 1, a gallium indium arsenide (Ga In As) mixed crystal semiconductor layer 3 formed on the mixed crystal semiconductor layer 2, an aluminum indium arsenide (Al In As) mixed crystal semiconductor layer 4 formed on the mixed crystal semiconductor layer 3, to which n-type impurities are added, a gate electrode 5 formed on the mixed crystal semiconductor layer 4, and a source electrode 6 and a drain electrode 7 provided opposed to each other on both sides of the gate electrode 5. In the field effect transistor with such a structure, a two-dimensional electron layer 8 is formed in the Ga In As mixed crystal semiconductor layer 3 and serves as a channel, so that a current path is formed between the source electrode 6 and the drain electrode 7. The operation of the field effect transistor is achieved by controlling voltage applied to the gate electrode 5 to modulate electron density of the two-dimensional electron layer 8 immediately under the gate electrode 5 so that the drain current is modulated. In the above described structure, the Al In As mixed crystal semiconductor layer 2 is provided as a buffer layer having functions for improving the crystal property of an epitaxially grown layer (the Ga In As mixed crystal semiconductor layer 3) formed thereon and also preventing such as diffusion of impurities from the substrate 1. In this structure, since electron mobility in the two-dimensional electron layer 8 within the Ga In As mixed crystal semiconductor layer 3 has a value of more than 10000 cm²/V.sec at room temperature, a field effect transistor comprising high mutual conductance, low noise figure and high cut-off frequency is expected to be achieved. However, in the field effect transistor with the above described structure, the resistance between the two-dimensional electron layer 8 under the gate electrode 5 and the source electrode 6, that is, the source resistance is not so low and hence, various problems occur in the transistor characteristics. For example, the non-negligible source resistance lowers mutual conductance, cut-off frequency and the like as well as noise figure is degraded. In order to reduce the source resistance, a method is developed in which impurities such as silicon and selenium are ion-implanted into the region between the source electrode 6 and the gate electrode 5 so that the resistance in the region between the source electrode 6 and the two-dimensional electron layer 8 is lowered. The resistance in this region is a parallel resistance formed of the Al In As mixed crystal semiconductor layers 2 and 4 and the Ga In As mixed crystal semiconductor layer 3. However, in the structure shown in Fig. 1, since the uppermost Al In As mixed crystal semiconductor layer 4 is generally very thin such as less than 10⁴ nm (1000 Å) in thickness, and electron mobility in the Al In As mixed crystal semiconductor is approximately 100 cm²/V.sec which is smaller, by more than one order, than that in the Ga In As fixed crystal semiconductor, the source resistance becomes substantially equal to the resistance of the Ga In As mixed crystal semiconductor layer 3. However, the mixed crystal semiconductor layer 3 is generally as thin as 10⁴ nm (1000 Å), and if impurities more than 10¹⁷/cm³ in concentration are implanted to make the layer 3 low resistance, electron mobility is rapidly reduced due to electron scattering by ionized impurities. Thus, it is difficult to effectively reduce the source resistance. Fig. 2 is a cross sectional view showing another structure of the conventional field effect transistor, which is also disclosed in an article by C.Y. Chen et al., IEEE Electron Device Letters, Vol. EDL-3, No.6, 1982, p. 152. The field effect transistor shown in Fig. 2 comprises a semi-insulating indium phosphide (In P) substrate 11, a gallium indium arsenide (Ga In As) mixed crystal semiconductor layer 12 formed on the substrate 11, a non-doped Al In As mixed crystal semiconductor layer 13 formed on the mixed crystal semiconductor layer 12, an aluminum indium arsenide (Al In As) mixed crystal semiconductor layer 14 doped with n-type impurities formed on the mixed crystal semiconductor layer 13, a gate electrode 5 formed on the mixed crystal semiconductor layer 14, and a source electrode 6 and a drain electrode 7 formed on the layer 14 and opposed to each other on respective sides of the gate electrode 5. In this structure, the Al In As mixed crystal semiconductor layers 13 and 14 on the Ga In As mixed crystal semiconductor layer 12 comprises a wide forbidden band gap layer 19. In the field effect transistor shown in Fig. 2, similarly to the field effect transistor shown in Fig. 1, the two-dimensional electron layer 8 is formed along the wide forbidden band gap layer 19 in the Ga In As mixed crystal semiconductor layer 12. Electron density in the two-dimensional electron layer 8 under the gate electrode 5 is modulated by voltage applied to the gate electrode 5 so that the drain current is modulated. As a result, the transistor operation is achieved. In the field effect transistor shown in Fig. 2, the Ga In As mixed crystal semiconductor has high electron mobility of more than 10000 cm²/V.sec at room temperature, so that high values of approximately 440 mS/mm at room temperature and approximately 700 mS/mm at 77°K have been obtained in the device having a gate length of 1 µm as mutual conductance indicative of the performance of the device (refer to proceedings of 12th International Symposium on Gallium Arsenide and Related compounds, by K. Hirose et al., entitled "700 ms/mm 2 DEGFETs fabricated from high mobility MBE-grown n-Al In As/Ga In As hetero structures", pp. 529-534. However, in the field effect transistor with this structure, the source resistance can not be neglected, so that transistor characteristics such as mutual conductance and noise figure is limited by the source resistance. In order to reduce the source resistance, it would be considered that n-type impurities are ion-implanted at least into a region to form a source resistance as described above and then, flash annealing or the like is performed to activate the implanted n-type impurities, so that the region to form a source resistance is made a low resistance. However, in this case as well, similarly to the explanation concerning the field effect transistor shown in Fig. 1, the following has been found from an experiment performed by our inventors. More specifically, in the field effect transistor having a wide forbidden band gap layer comprising only an Al In As mixed crystal semiconductor layer, the rate of activated n-type impurities ion-implanted into the Al In As semiconductor layer is low, and electron mobility in the Al In As mixed crystal semiconductor layer is low such as less than 300 cm²/V.sec, so that it is difficult to reduce Sufficiently the resistivity of the ion-implanted Al In As mixed crystal semiconductor layer. AS a result, even if the ion-implantation technique is employed in the field effect transistor with a conventional structure, the source resistance can not be sufficiently reduced, so that it is difficult to improve transistor characteristics. A FET with another feature is described in a patent, summarized in the Pat. Abs. of Japan, vol. 8, No.217 (E-270)(1654), 4th October 1984. It discloses a structure containing various layers of doped Ga In As , InP and Al In As with the aim to improve the characteristic of leak currents in the FET. In this reference there is no any hint on the elimination of the above characterized disadvantages relating the source resistance in FET s. The problem of contact resistance between the alloyed region and the 2-dimensional electron gas relates the European Patent application EP-A-O 131 379 describing impurity doped regions (GaAs and Al Ga As) directly under the source and drain regions. A further document (EP-A-O 155 215) relates to a high electron mobility transistor with a special layered structure with the aim to prevent ion diffusion in the transistor. There is not shown a reduction of the source resistance. As described above, in the structure of the conventional field effect transistor, there were disadvantages that the source resistance can not be sufficiently reduced and the characteristics of device such as mutual conductance, cut-off frequency, noise figure and the like can not be sufficiently improved. SUMMARY OF THE INVENTION An object of the present invention is to remove the disadvantages of the conventional field effect transistor and to provide a field effect transistor having a sufficiently low source resistance and excellent characteristics of device. The field effect transistor according to the present invention is adapted such that in order to reduce the source resistance, an In P semiconductor layer is provided between a Ga In As mixed crystal semiconductor layer in which a two-dimensional electron layer is formed and a semi-insulating substrate, and n-type impurities are ion-implanted into the region to form a source resistance. The ion-implantation is performed to the depth reaching at least the In P layer. In accordance with the above described structure, the rate of activation of implanted ion in the In P semiconductor layer is very high and, therefore, electron mobility of the ion-implanted In P semiconductor is higher, by more than one order, than that of the Al In As mixed crystal semiconductor. Thus, when a lower resistance of the In P semiconductor layer is provided in parallel with the resistance of the Ga In As mixed crystal semiconductor layer, the source resistance can be effectively reduced, and further a gate electrode is formed on the Al In As mixed crystal semiconductor layer, so that Schottky junction at the gate electrode can be easily formed. These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. Fig. 1 is a cross sectional view showing a structure of a conventional field effect transistor; Fig. 2 is a cross sectional view showing another structure of a conventional field effect transistor; and Fig. 3 is a cross sectional view showing a structure of a field effect transistor according to an embodiment of the present invention. Prior to the specific embodiment, the experimental fact for supporting the present invention is described. According to a report of the result of an experiment in which n-type impurities are implanted into a In P semiconductor and annealing is performed (refer to Applied Physics Letters, Vol. 43, No. 15, 1983, p. 381), the rate of activation of implanted ion of more than 60 % as well as electron mobility in an ion-implanted layer of more than 2000 cm²/V.sec are obtained in an In P semiconductor. Therefore, if n-type impurities of 10¹⁴/cm² are implanted, the sheet resistance of the ion-implanted layer can be less than 50 Ω/□ in the In P semiconductor. On the other hand, according to an experiment by our inventors, if n-type impurities of 10¹⁴/cm² were ion-implanted into a Al In As mixed crystal semiconductor, the sheet resistance became approximately 1 kΩ/□. This is because both electron mobility and the rate of activation of implanted ion in the In P semiconductor are larger than those of Al In As mixed crystal semiconductor. From the foregoing, when the Al In As mixed crystal semiconductor layer in a region to form a source resistance is replaced with the In P mixed crystal having an ion-implanted region, the source resistance can be effectively reduced. Specific embodiments of the present invention are described in detail hereinafter. Fig. 3 is a cross sectional view showing a structure of a field effect transistor according to an embodiment of the present invention. Referring now to Fig. 3, a manufacturing process and a structure of a field effect transistor according to an embodiment of the present invention are described. An In P layer 22, a Ga In As mixed crystal semiconductor layer 23, and an Al In As mixed crystal semiconductor layer 24 are formed, in this order, on an In P semi-insulating substrate 21 by using gas source molecular beam epitaxial growth or metal organic chemical vapor deposition (MOCVD). It is desirable that the In P layer 22 is an n-type doped with impurities of less than 10¹⁵/cm³ or a p-type doped with impurities of less than 10¹⁶ /cm³ in concentration and is, for example, approximately 1 µm in thickness. If ion-implantation is performed into the In P layer 22, electron mobility is 2000 cm²/V.sec, which is larger, by one order than that of the Al In As mixed crystal semiconductor. Therefore, even though the thickness In P layer is more than 1 µm, there occurs no problem in characteristics of device. The Ga In As mixed crystal semiconductor layer 23 is made to be of an n-type or a p-type having an impurity concentration of less than 10¹⁶/cm³ and the thickness thereof is made to be approximately 0.1 µm. In determining an impurity concentration and a thickness of these layers, it is important that the In P semiconductor layer 22 is made to be in a depletion state when a transistor operates. The Al In As mixed crystal semiconductor layer 24 is made to be of an n-type having an impurity concentration of approximately 10¹⁸/cm³ and the thickness thereof is made to be, for example, 400 Å. After forming these epitaxial layers, n-type impurities such as silicon, having, for example, a dose of more than 10¹⁴/cm² at an acceleration voltage of more than 100 KeV are implanted by using ion implantation, whereby an ion-implanted region reaching the In P semiconductor layer 22 is formed. Thereafter, annealing is performed, for example, for approximately ten seconds at 800°C, so that a low resistive region 25 is formed. The ion-implanted region is not formed in the portion underlying the region to form the gate electrode. This is achieved by, for example, forming a dummy gate on the region to form the gate electrode and performing ion-implantation utilizing the dummy gate as a mask. An ohmic electrode of gold-germanium alloy and the like is formed at a predetermined region on the surface of the Al In As mixed crystal semiconductor layer 24 by using evaporation and is alloyed with the mixed crystal semiconductor layer 24, so that both of a source electrode 6 and a drain electrode 7 are formed. Finally, a gate electrode 5 comprising platinum, gold, aluminum or the like is formed by using evaporation on the region of Al In As mixed crystal semiconductor layer 24 in which the low resistive layer 25 is not formed, so that a field effect transistor is completed. In the field effect transistor shown in Fig. 3 as described above, since lower resistance of a conductive layer comprising the In P semiconductor layer 22 is provided in parallel with the resistance of the Ga In As mixed crystal semiconductor layer 23, the resistance between the source electrode 6 and a two-dimensional electron layer 8 under the gate electrode 5, that is, the source resistance, is effectively reduced. In the above described embodiment, since impurity ions are implanted at a high acceleration voltage, impurities can be distributed mainly within the In P layer 22, and the source resistance can be reduced while maintaining a low impurity concentration in the neighborhood of the gate electrode 5. It is important to maintain a low impurity concentration in the neighbourhood of the gate electrode 5 to raise the breakdown voltage of the gate in the transistor. Thus, transistor characteristics can be improved without decreasing the breakdown voltage of the gate in a manner of the above described embodiment.

Claims: A field effect transistor utilizing a semiconductor hetero junction, comprising: a semi-insulating substrate (21); a gallium indium arsenide (Ga In As) mixed crystal semiconductor layer (23) formed on said semi-insulating substrate (21), having a current path formed therein; a first layer formed on said Ga In As mixed crystal semiconductor layer (23), including at least an aluminum indium arsenide (Al In As) mixed crystal semiconductor layer (24); indium phosphide (In P) layer (22) disposed under said Ga In As mixed crystal semiconductor layer (23), wherein said In P layer (22) is formed between said Ga In As mixed crystal semiconductor layer (23) and said semi-insulating substrate (21), contacting said Ga In As mixed crystal semiconductor layer (23), and said first layer of said Al In As mixed crystal semiconductor layer (24); a gate electrode (5) formed in a predetermined region on the surface of said Al In As mixed crystal semiconductor layer (24), a source electrode (6) and a drain electrode (7) provided opposed to each other on respective sides of said gate electrode (5); and ion-implanted regions (25) formed by implanting impurity ions into a region between said gate electrode (5) and said source electrode (6) and between said gate electrode (5) and said drain electrode (7), wherein said ion-implanted regions (25) are formed to the depth reaching at least said In P layer (22). A field effect transistor in accordance with claim 1, wherein said In P layer (22) is approximately 1 µm in thickness. A field effect transistor in accordance with claim 1, wherein said In P layer (22) is formed as an n-type layer having doped impurities of less than 10¹⁵/cm³ in concentration. A field effect transistor in accordance with claim 1, wherein said In P layer (22) is formed as a p-type layer having doped impurities of 10¹⁶/cm³ in concentration. A field effect transistor in accordance with claim 1, wherein said Al In As mixed crystal semiconductor layer (24) is formed as an n-type layer having doped impurities of the order of 10¹⁸/cm³ in concentration. A field effect transistor in accordance with claim 1, wherein said ion-implanted layer (25) is an n-type layer in which n-type impurities are implanted under the condition of acceleration voltage of 100 keV and a dose of more than 10¹⁴/cm².

Inventors: HAYASHI, HIDEKI OSAKA WORKS OF SUMITOMO, SASAKI, GORO OSAKA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1986
Main classification: H01L29
IPC codes: H01L29