HIGH ELECTRON MOBILITY TRANSISTOR

Abstract: A high electron mobility transistor comprises a semiinsulating substrate (11), an undoped channel layer (12) formed on the semiinsulating substrate and made of undoped gallium arsenide (GaAs), a doped carrier supplying layer (13) formed on the undoped channel layer and made of a II-VI family compound semiconductor having an electron affinity smaller than that of the undoped gallium arsenide and having an energy gap greater than that of the undoped gallium arsenide, and source, drain and gate electrodes (14, 15, 16) formed on the doped carrier supplying layer.

Description: The present invention generally relates to high electron mobility transistors, and more particularly to a high electron mobility transistor which uses as a channel a two dimensional electron gas layer formed in a vicinity of a hetero interface. Conventionally, a high electron mobility transistor (hereinafter simply referred to as a HEMT) is known as a high-speed field effect transistor. The HEMT generally comprises a semi-insulating gallium arsenide (GaAs) substrate, an undoped (or intrinsic) GaAs channel layer formed on the semi-insulating GaAs substrate, and an n-type aluminum gallium arsenide (AlGaAs) carrier supplying layer. A two dimensional electron gas layer formed in a vicinity of a hetero interface on the side of the undoped GaAs channel layer is used as the channel between a source and a drain. The density of two dimensional carrier gas in the two dimensional electron gas layer is controlled by a voltage applied to a gate which is formed on the n-type AlGaAs carrier supplying layer. In addition to the combination of GaAs having a large electron affinity and AlGaAs having a smaller electron affinity, the combination may be GaAs and indium gallium phosphide (InGaP), indium gallium arsenide (InGaAs) and indium phosphide (InP), or indium gallium arsenide (InGaAs) and indium aluminum arsenide (InAlAs). According to the conventional HEMT, the energy difference between the conduction bands of GaAs and Al x Ga 1-x As (x=0.3) is approximately 0.2 eV and is extremely small. Even in the case of the combination of GaAs and InGaP, the energy difference between the conduction bands is approximately 0.4 eV and is not much larger. Generally, when the energy difference between the conduction bands is small, a known phenomenon occurs wherein the carriers accelerated in the two dimensional electron gas layer are transferred to the carrier supplying layer and the carrier mobility becomes low. This phenomenon is referred to as the real space carrier transfer to carrier supplying layer. Therefore, when the energy difference between the conduction bands is large, the carriers in the two dimensional electron gas layer are prevented from moving into the carrier supplying layer even when there is a high voltage applied across the drain and source. For this reason, the HEMT can operate stably even when a high voltage is applied across the drain and source. In addition, the density of the two dimensional carrier gas in the two dimensional electron gas layer of the HEMT is proportional to the energy difference between the conduction bands. Consequently, the larger the energy difference is between the conduction bands, it is possible to obtain a larger current output. The Japanese Laid-Open Patent Application No.58-140167 discloses a HEMT wherein undoped InGaAs, indium gallium arsenide phosphide (InGaAsP) or the like is used as the channel layer and doped InGaAsP, zinc selenide (ZnSe) or the like is used as the carrier supplying layer. According to this previously proposed HEMT, it is possible to obtain a large energy difference between the conduction bands compared to that obtainable in the conventional HEMT described before. However, because the channel layer is a three-element or four-element layer, there are problems in that scattering of alloy occurs at a low operation temperature, and the carrier mobility becomes low. For example, when undoped InGaAs is used as the channel layer and doped InAlAs is used as the carrier supplying layer, it is only possible to obtain a carrier mobility µ in the order of 5×10⁴ cm²/Vsec to 6×10⁴ cm²/Vsec at a temperature of 77K. On the other hand, the carrier mobility µ obtainable in the conventional HEMT using undoped GaAs as the channel layer and doped AlGaAs as the carrier supplying layer is of the order of 10⁵cm²/Vsec to 2x10¹⁵ cm²/Vsec at a temperature of 77K. That is, in the case where the channel layer is made of a three-element compound semiconductor, the carrier mobility is low compared to the case where the channel layer is made of a two-element compound semiconductor. Accordingly, in an HEMT having the InGaAs channel layer and the ZnSe carrier supplying layer, the effect of preventing real space carrier transfer to the carrier supplying layer is small even when the energy difference between the conduction bands is large. According to the present invention there is provided a high electron mobility transistor comprising a semi-insulating substrate, an undoped channel layer formed on said semi-insulating substrate and made of undoped gallium arsenide (GaAs), a doped carrier supplying layer formed on said undoped channel layer, and source, drain and gate electrodes formed on said doped carrier supplying layer, characterised in that said doped carrier supplying layer comprises doped cadmium zinc sulfide (CdZnS), the lattice of which matches that of said undoped gallium arsenide, having an electron affinity smaller than that of said undoped gallium arsenide and an energy gap greater than that of said undoped gallium arsenide. A high electron mobility transistor embodying the present invention can provide a large energy difference between the conduction bands of the channel layer and the carrier supplying layer. For this reason, the carriers in a two dimensional electron gas layer will not move into the carrier supplying layer even when a large voltage is applied across the drain and source, and stable operation of the high electron mobility transistor is ensured. Further, it is possible to obtain a large current output since the density of the two dimensional carrier gas in the two dimensional electron gas layer is high. In the case where the channel layer consists only of two elements, there is no scattering of alloy in a cooling stage of the production process, and the carrier mobility is high compared to previously-proposed high electron mobility transistors. Reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a cross-sectional view showing a high electron mobility transistor embodying the present invention; Fig. 2 shows an energy band diagram for the transistor shown in Fig. 1; and Fig. 3 shows a drain-source voltage versus drain-source current characteristic of the transistor shown in Fig. 1. The HEMT of Fig. 1 comprises a semi-insulating GaAs substrate 11, an undoped (intrinsic) channel layer 12 made of gallium arsenide (GaAs), a doped carrier supplying layer 13 made of n-type cadmium zinc sulfide (CdZnS), a source electrode 14, a drain electrode 15, and a gate electrode 16. The source electrode 14 comprises a metallized region 14A, and the drain electrode 15 has a metallized region 15A. A two dimensional electron gas layer (hereinafter simply referred to as a 2DEG layer) 17 is formed in a vicinity of a hetero interface on the side of the channel layer 12. Compared to cadmium zinc sulfide (CdZnS), gallium arsenide (GaAs) has a large electron affinity and a large carrier mobility, but a small energy band gap. Further, the energy difference between the conduction bands of the channel layer 12 and the carrier supplying layer 13 is large. Thus, even when a large voltage is applied across the drain electrode 15 and the source electrode 14, the carriers in the 2DEG layer 17 will not move into the carrier supplying layer 13, and it is possible to accumulate high density two dimensional carrier gas in the 2DEG layer 17. Thus, the operation of the HEMT is stable. In addition, since the density of the two dimensional carrier gas in the 2DEG layer 17 increases, it is possible to obtain a large current. Preferably, n-type Cd 0.43 Zn 0.57 S doped with silicon (Si) is used for the carrier supplying layer 13. In a preferred embodiment of the present invention, the undoped GaAs channel layer 12 has a film thickness of 5000Å (500nm) and the n-type Cd 0.43 Zn 0.57 S carrier supplying layer 13 has a film thickness of 500Å (50nm). The source electrode 14 and the drain electrode 15 are formed from gold germanium (AuGe) to a film thickness of 300Å (30nm) or from gold (Au) to a film thickness of 3000 Å (300nm). The gate electrode 16 is formed from aluminum (Al) to a thickness of 3000 Å (300nm). In this embodiment, the energy difference between the conduction bands of the undoped GaAs channel layer 12 and the n-type Cd 0.43 Zn 0.57 S carrier supplying layer 13 is extremely large, and a satisfactory lattice matching is obtained between the undoped GaAs channel layer 12 and the n-type Cd 0.43 Zn 0.57 S carrier supplying layer 13, as will now be described with reference to Figs. 2 and 3. In Fig. 2, the parts which are the same as those corresponding parts in FIG.1 are designated by the same reference numerals. In FIG.2, E F denotes the Fermi level, E C denotes the bottom of the conduction band, E V denotes the top of the valence band, B C denotes the energy difference between the conduction bands of the channel layer 12 and the carrier supplying layer 13, G2 denotes the width of the forbidden band of the channel layer 12 and G3 denotes the width of the forbidden band of the carrier supplying layer 13. In the embodiment described, B C = 0.7 eV, B V = 1.02 eV, G2 = 1.42 eV and G3 = 3.14 eV. The value 0.7 eV for the energy difference B C is approximately three to four times that obtainable in the conventional HEMT described before. Since the energy difference B C is large, the carriers in the 2DEG layer 17 will not move into the carrier supplying layer 13 even when a large voltage is applied across the drain electrode 15 and the source electrode 14. Furthermore, because of the large energy difference B C' a density N S of the two dimensional carrier gas in the 2DEG layer 17 is approximately 4x10¹² cm⁻² which is approximately four times the density obtainable in the conventional HEMT described before. In Fig. 3 in which the ordinate denotes the drain-source current I DS' and the abscissa denotes the drain-source voltage V DS' the characteristic of the above-mentioned embodiment of the present invention is indicated by a solid line, and for the purpose of comparison, the characteristic of the conventional HEMT using undoped GaAs as the channel layer and doped Al 0.3 Ga 0.7 As as the carrier supplying layer is indicated by a phantom line. It may be seen from the phantom line in FIG.3 that the drain-source current I DS decreases when the drain-source voltage V DS exceeds 5 V in the case of the conventional HEMT. But according to the present invention, it may be seen from the solid line in FIG.3 that the drain-source current I DS does not decrease even when the drain-source voltage V DS reaches 10 V. The data shown in FIG.3 are obtained when the length of the gate is 0.5 µm, the width of the gate is 200 µm, and the gate voltage is set 0.8 V higher than a threshold voltage which makes the drain-source current I DS zero. The formation of the channel layer 12 and the carrier supplying layer 13 can be carried out with ease. For example, the undoped GaAs channel layer 12 may be formed by a metal organic chemical vapor deposition (hereinafter simply referred to as MOCVD) by using trimethyl gallium (TMG: (CH₃)₃Ga) and arsine (AsH₃) as the source gas, and the doped CdZnS carrier supplying layer 13 can be formed by a MOCVD using dimethyl cadmium (DMC: (CH₃)₂Cd), dimethyl zinc (DMZ: (CH₃)₂Zn) and hydrogen sulfide (H₂S) as the source gas. Because the undoped channel layer 12 is a two-element layer (i.e. GaAs), the carrier mobility therein is quite high. Furthermore, the energy difference between the conduction bands of the carrier supplying layer and the channel layer is so large that such high mobility carriers are effectively prevented from transferring from the channel layer to the carrier supplying layer. Such effect of preventing the real space carrier transfer to carrier supplying layer is much larger than that obtainable in the previously proposed HEMT described before using the three-element or four-element layer as the channel layer. Consequently, from the total point of view, the carrier mobility µ is sufficiently high in the present embodiment.

Claims: A high electron mobility transistor comprising a semi-insulating substrate (11), an undoped channel layer (12) formed on said semi-insulating substrate and made of undoped gallium arsenide (GaAs), a doped carrier supplying layer (13) formed on said undoped channel layer, and source, drain and gate electrodes (14, 15, 16) formed on said doped carrier supplying layer (13), characterised in that said doped carrier supplying layer (13) comprises doped cadmium zinc sulfide (CdZnS), the lattice of which matches that of said undoped gallium arsenide, having an electron affinity smaller than that of said undoped gallium arsenide and an energy gap greater than that of said undoped gallium arsenide. A transistor as claimed in claim 1, wherein said doped cadmium zinc sulfide is Cd 0.43 Zn 0.57 S. A transistor as claimed in claim 1 or 2, in which a density of two dimensional carrier gas in a two dimensional electron gas layer (17) formed in a vicinity of a hetero interface on the side of the undoped channel layer (12) is approximately of the order of 4x10¹²cm-².

Inventors: TAKIKAWA, MASAHIKO
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: H01L29
IPC codes: H01L29, H01L21