JP2002190588A - Heterojunction fet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the performance of a heterojunction FET. SOLUTION: The heterojunction FET comprises a GaAs semiconductor substrate 1, GaAs buffer layer 2, InGaAs channel layer 3, AlGaInP spacer layer 4, donor impurity-doped GaxIn1-xP electron-supplying layer 5, AlGaInP Schottky layer 6, divided GaAs cap layer 7, source electrode 8 and drain electrode 9 which are formed on each divided part of the cap layer 7 and provide ohmic contact, and gate electrode 10 which is formed on the Schottky layer 6 and provides a Schottky contact. The mole fraction x of Ga of the electron-supplying layer is to satisfy 0.55<=x<=0.7, and the composition of GaP is increased, as compared with one, with which GaP is lattice-matched with the semiconductor substrate 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field effect transistor using a heterojunction, and particularly to a field effect transistor having no deep donor impurity level, low noise amplification characteristics, high reliability, and high current driving capability, and easy device fabrication. And a field effect transistor having a high yield.

[0002]

2. Description of the Related Art Hitherto, a field effect transistor using a heterojunction of GaAs and AlGaAs has been considered as a high-speed and high-performance device, and has been applied particularly as a low-noise device and a high-power amplifier device. For example, as shown in FIG. 6, a non-doped GaAs having a large electron affinity (energy required to raise one electron from the conduction band lower limit Ec to a vacuum level).
s, a spacer layer 24 made of non-doped AlGaAs having a small electron affinity, and an electron supply layer 25 made of AlGaAs doped with a donor impurity (Si).
A high electron mobility field effect transistor (HEMT) using a two-dimensional electron gas as an active layer by using a heterojunction with a heterojunction is known.

[0003] Here, the two-dimensional electron gas refers to a discontinuous conductor ΔEc (Conduction-band offsequence) from the electron supply layer 25.
(t: difference in electron affinity at the interface) refers to an electron group that has a degree of freedom only in a two-dimensional direction parallel to the surface (interface) among the electrons supplied to the channel layer 23 which is a potential well formed by the potential well. The electrons supplied from the electron supply layer 25 to the channel layer 23 may move in the Z direction, which is a direction perpendicular to the surface, when the thickness of the potential well is reduced to about 100 Å or less, which is the electron de Broglie wavelength. The vertical motion is restricted in the X and Y directions parallel to the surface by being quantized and taking discrete energy ranks, so that the electron motion has a degree of freedom only in the two-dimensional direction. become.

The above-mentioned HEMT is characterized in that the channel layer 23 in which electrons travel and the electron supply layer 25 for supplying electrons are made of materials having different electron affinities. Because of the confinement, it can be separated from the donor impurities in the electron supply layer 25, and the scattering of electrons can be reduced.

In this HEMT, in order to further reduce scattering, a non-doped spacer layer 24 made of the same material as the electron supply layer 25 is used between the electron supply layer 25 and the channel layer 23. In addition, the improvement of the controllability of the threshold voltage, the trapping effect (Wa in which the state of the charge trapped in the semiconductor surface order changes and the current-voltage characteristic of the gate changes)
In order to reduce the effect such as the lkout phenomenon) and to improve the gate breakdown voltage, the electron supply layer 25 is made as thin as possible by increasing the donor impurity concentration, and non-doped with the same material as the electron supply layer 25 on the electron supply layer 25. Schottky layer 26 is provided.

In order to improve the performance of the HEMT, it is effective to increase the mobility of the channel layer 23 and the concentration of the two-dimensional electron gas in the channel layer 23. Channel layer 2
Pse in which material 3 was changed from GaAs to InGaAs with high mobility
udomorphic HEMT (PHEMT) has been developed. Where I
The mobility of nGaAs increases as the In composition increases,
If it is too large, crystal defects will occur due to lattice mismatch with GaAs as a substrate material.

Therefore, the composition x of In of In _x Ga _1-x As is 0.15
The thickness of the compound semiconductor mixed crystal in the range of ≦ x ≦ 0.25
When the thickness is equal to or less than a certain thickness called the critical thickness, the crystal lattice is distorted and deformed, but high-quality crystal growth without dislocation or other disorder in the lattice can be achieved.

For example, the electron supply layer 25 and the channel layer 23
Is Al_0.3Ga_0.7As, In_0.2Ga _0.8If As
The conduction band discontinuity ΔEc is ΔEc = 0.4 eV,
When the metal layer 23 is GaAs_0.3Ga_0.7As / GaAs
Conduction band discontinuity at the terror interface compared to ΔEc = 0.24 eV
So that quantum effects work effectively
Can trap many electrons and increase the electron concentration.
There is also an advantage that both high mobility and high mobility can be achieved.

[0009]

SUMMARY OF THE INVENTION However, the conventional
The PHEMT uses AlGaAs doped with Si for the electron supply layer 25. This Si-doped AlGaAs has a deep impurity level called a DX center related to a donor in addition to the oxygen which forms a deep impurity level. This hinders an increase in the concentration of the electron supply layer 25 that is effective for the formation. In addition, it is said that the state of the charge trapped in the semiconductor surface state changes and the current-voltage characteristics of the gate change.
There were problems such as the ut phenomenon and frequency dispersion of the drain current-voltage characteristic caused by the semiconductor surface state.

In a conventional AlGaAs / GaAs PHEMT device fabrication process, a gate electrode is formed on a Schottky layer 26 made of AlGaAs, so that a portion corresponding to the gate electrode 10 as shown in FIG. It is necessary to remove the n-type GaAs cap layer 7. However, AlGaAs is
There is a problem that the difference in the etching rate in the wet etching for removing GaN is not sufficiently large, sufficient uniformity in the threshold control of the HEMT cannot be obtained, and the yield is poor.

In order to solve the above-mentioned problems of the deep impurity level called the DX center and the selective etching of GaAs, the electron supply layer 25 and the Schottky layer 26 are made of Ga _0.5 In _0.5 P, and the channel layer 23 is made of the same. Although PHEMT using InGaAs has been developed, Ga _0.5 in _0.5 P / InGaAs
The conduction band discontinuity ΔEc of the heterojunction is smaller than that of the AlGaAs / InGaAs heterojunction, and a high electron concentration cannot be obtained. At this time, metal / Ga _0.5 In _0.5 P
The height of the Schottky barrier is 0.75 eV, which is considerably lower than 1.1 eV in the AlGaAs system.
In the HEMT in the enhanced mode of V or higher, there is a problem that the voltage amplitude of the input signal cannot be sufficiently increased.

It is an object of the present invention to provide an electron supply layer which is free from a deep impurity level called a DX center for a donor and can eliminate unstable operation relating to a surface level, thereby increasing the electron supply layer concentration and increasing the free electron concentration. The source resistance can be reduced, and a Schottky barrier that can provide a sufficiently large voltage amplitude of the input signal can be realized.
An object of the present invention is to provide a heterojunction field-effect transistor that facilitates threshold control of an HEMT and eliminates problems of high-frequency performance and a process control process.

[0013]

According to a first aspect of the present invention, there is provided a semiconductor substrate of GaAs, a first semiconductor layer of InGaAs formed on the semiconductor substrate, and a semiconductor substrate of InGaAs. A second semiconductor layer formed on the first semiconductor layer and made of a material different from the first semiconductor layer and having a higher conduction band potential energy than the first semiconductor layer; and G formed on the layer and having a higher conduction band potential energy than the first semiconductor layer.
a _x In _1-x P, wherein the composition x of Ga satisfies 0.55 ≦ x ≦ 0.7, and a third semiconductor layer to which a donor impurity is added; and a third semiconductor layer formed on the third semiconductor layer. A fourth semiconductor layer made of a material different from the first semiconductor layer and having a higher conduction band potential energy than the first semiconductor layer, wherein the first semiconductor layer and the second and second A two-dimensional electron gas formed by a heterojunction with the third and fourth semiconductor layers is used as an active layer, and is supplied to the two-dimensional electron gas from a source electrode or a drain electrode via the second, third, and fourth semiconductor layers. The carrier is supplied, and the traveling of the two-dimensional electron gas is controlled by an electric field applied to the gate electrode.

[0014] In a second aspect based on the first aspect,
At least one of the second semiconductor layer and the fourth semiconductor layer;
Material is lattice-matched with the semiconductor substrate (Al_yGa_1-y)
_0.5In _0.5P, the composition y of the Al is 0.2 ≦ y ≦ 0.4
It was configured as such.

According to a third aspect, in the first aspect, at least one material of the second semiconductor layer or the fourth semiconductor layer is made of Al _z Ga _{1 -z} As, and the composition z of the Al is 0.2
≦ z ≦ 0.35.

According to a fourth aspect of the present invention, any one of the first to third aspects is provided.
In one aspect of the invention, the semiconductor device further includes a fifth semiconductor layer formed on the fourth semiconductor layer, wherein the fifth semiconductor layer is a material having a higher conduction band potential energy than the first semiconductor layer. Ga _x In _1-x P is formed, and the composition x of the Ga is
0.55 ≦ x ≦ 0.7, and the gate electrode is formed on the fifth semiconductor layer.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [Principle Configuration of the Present Invention] First, the principle of the HEMT device of the present invention will be described. FIG. 1 shows an HE according to the present invention.
FIG. 3 is a diagram illustrating a cross section of the structure of an MT element. In FIG. 1, on a semi-insulating GaAs substrate 1, a GaAs buffer layer 2, an InGaAs channel layer (first semiconductor layer) 3, a spacer layer (second semiconductor layer) 4, and Ga _x In _1-x Electron supply layer of P (0.55 ≦ x ≦ 0.7)
(Third semiconductor layer) 5, Schottky layer (fourth semiconductor layer)
6, a GaAs cap layer 7 is formed.
Reference numerals 8 and 9 denote a source electrode, a drain electrode, and 10 a gate electrode, respectively. The spacer layer 4, the electron supply layer 5, and the Schottky layer 6 are made of a material having a higher conduction band potential energy Ec than the channel layer 3.

Here, the spacer layer 4 and the Schottky layer 6
Is formed of non-doped AlGaInP (or AlGaAs). The semiconductor layers from 1 to 7 are formed by MBE or M
After being sequentially formed by the OCVD method, a source electrode 8 and a drain electrode 9 which are ohmic electrodes are formed on the cap layer 7 of GaAs. Next, the GaAs cap layer 7 at the gate electrode portion is removed, and the gate electrode 10 is replaced with the Schottky layer 6.
The HEMT element is completed by forming it on the upper side.

[First Embodiment] In the above basic structure,
And the spacer layer 4 and the Schottky layer 6
Lattice matching (Al_yGa_1-y)_0.5In_0.5Form with P (0.2 ≦ y ≦ 0.4)
(Al_yGa_1-y)_0.5In _0.5P / InGaAs heterostructure
FIGS. 2 to 4 show the HEMT device according to the first embodiment of the present invention.
This will be described below. Figure 2 shows a cross-sectional view of the structure of the HEMT device.
And In_0.2Ga_0.8As for the channel layer 3, Ga_0.65In_0.35P
This is used for the electron supply layer 5.

First, the manufacturing process of the first embodiment will be described. A buffer layer 2 made of non-doped GaAs having a thickness of 500 nm, a channel layer 3 made of non-doped In _0.2 Ga _0.8 As having a thickness of 12 nm, and a non-doped thickness formed on a semi-insulating GaAs semiconductor substrate 1 by MBE. 30nm (Al
_0.3 Ga _0.7 ) _0.5 In _0.5 P spacer layer 4, 6 × 10 ¹⁸ a
Ga _0.65 In _0.35 P 7 nm thick doped with tom / cm ³ Si
Electron supply layer 5 composed of non-doped (Al
_0.3 Ga _0.7 ) _0.5 In _0.5 P Schottky layer 6, thickness 5
A semiconductor substrate on which a cap layer 7 made of GaAs doped with 0 nm and 6 × 10 ¹⁸ atom / cm ³ of Si is sequentially prepared is prepared.

A photoresist is applied on the cap layer 7 to pattern a region where a source electrode and a drain electrode are to be formed. A multi-layered ohmic metal consisting of Ni / Ge / Au / Ni / Au is deposited, a source electrode 8 and a drain electrode 9 are formed by lift-off, respectively, and heat-treated at 400 ° C. for 10 seconds to form an ohmic metal between the cap layer 7 Contact is obtained. Next, a photoresist is applied on the cap layer 7 to pattern a gate electrode forming region,
The GaAs cap layer 7 in the region where the gate electrode is to be formed is selectively etched away by an etchant composed of H ₃ PO ₄ : H ₂ O ₂ : H ₂ O (3: 1: 50), and the etchant is removed from Ti / Pt / Au. A multi-layered metal thin film is sequentially deposited, and the gate electrode 10 is formed by lift-off. Thus, the HEMT device is completed.

The effect of the above HEMT device will be described.
In FIG. 3, a voltage of 0 V is applied to the gate electrode 10 of the HEMT device.
FIG. In FIG.
 (Al _xGa_1-x)_0.5In_0.5Conduction band of heterojunction of P / GaAs
Continuous ΔEc and valence band discontinuity ΔEv (Valance-band offse
t) Dependence of each Al composition x (M.0.Watanabee
etal., Appl. Phys. Lett., vol. 50, pp. 906-908, 1987). Value
Electronic band discontinuity ΔEv is (Al_xGa_1-x)_0.5In_0.5P / GaAs
The conduction band discontinuity ΔEc is calculated from the difference between the forbidden energy gaps.
This is the value after subtraction.

From FIG. 4, (Al _0.3 Ga _0.7 ) _0.5 In _0.5 P / GaAs
ΔEc of (Al composition x = 0.3) is 0.27 eV (= 0.19 eV +
0.08 eV) and In _0.5 Ga _0.5 P / In _0.2 Ga _0.8 As
ΔEc is 0.35 eV, so (Al _0.3 Ga _0.7 ) _0.5 In _0.5
ΔEc of the P / In _0.2 Ga _0.8 As (spacer layer 4 / channel layer 3) heterojunction is given as 0.43 eV (= 0.35 eV + 0.08 eV).

This value can be calculated from ΔEc = 0.40 eV of a heterojunction of Al _0.3 Ga _0.7 As / In _0.2 Ga _0.8 As, which is a conventional heterojunction of a spacer layer / channel layer, and Ga _0.5 In _0.5 P / In _0.2 G
The value is larger than ΔEc = 0.35 eV of the heterojunction of a _0.8 As, and the quantum effect works more effectively than the conventional one, so that many electrons can be confined and the electron concentration increases.

In this case, Ga _x In _1-x P in which the composition of GaP is larger than that of the channel layer 3 of Ga _0.5 In _0.5 P lattice-matched to the buffer layer 2 of GaAs is used for the electron supply layer 5. As the composition x of Ga increases, Eg (energy gap between conduction band and valence band) and ΔEc increase, and x = 0.65
At this time, the conduction band discontinuity ΔEc with the (Al _0.3 Ga _0.7 ) _0.5 In _0.5 P spacer layer 4 almost disappears. The electron supply layer 5 of Ga _0.65 In _0.35 P does not have a deep impurity level such as the DX center, so that the electron supply layer 5 can be doped at a high concentration, and the device performance can be improved.

The metal and the Schottky layer 6 (Al _0.3 Ga
The height of the _0.7 ) _0.5 In _0.5 P Schottky barrier is 1.0 eV, which is the height of the conventional metal / AlGaAs-based Schottky barrier.
There is no inferiority to 1.1 eV, which is sufficiently larger than the metal / Ga _0.5 In _0.5 P Schottky barrier height of 0.75 eV.
Sufficient performance can be obtained even in enhanced mode HEMT.

In the device fabrication process, the gate electrode 1
0 to (Al_0.3Ga_0.7)_0.5In_0.5Above P's Schottky layer 6
GaAs cap layer 7 is partially etched to form
Will be removed, but H_ThreePO_Four: H_TwoO_Two: H_TwoFrom O (3: 1: 50)
(Al _0.3Ga_0.7)_0.5In_0.5P
Etching of yoke key layer 6 with GaAs cap layer 7
The selection ratio is as large as 6000: 1. In contrast
GaAs wet etching for conventional AlGaAs
The selectivity is at most 100: 1. As a result, the semiconductor film thickness
Processing control is easy, and variations in element characteristics are suppressed.
And reliability is also improved.

[0028] GaInP has an advantage in comparison with GaAs and AlGaAs.
High balunsh breakdown voltage, (Al_xGa _1-x)_0.5In_0.5In P
Is a characteristic that the breakdown voltage further increases as the Al composition x increases.
Have signs. (Al_xGa_1-x)_0.5In_0.5P is the Al composition x
= 0.5 to 0.7 and the transition from direct transition to indirect transition
Is used for the Schottky layer 6 and the spacer layer 4 of the HEMT device.
Therefore, it is desirable that the Al composition x is x ≦ 0.4.
No. Also, the conduction band discontinuity ΔEc and Schottky barrier height
In order to obtain a sufficient value, the Al composition x must be x ≧ 0.2
Is desirable. The Al composition x is 0.2 ≦ x ≦ 0.3 (Al_x
Ga_1-x)_0.5In_0.5P moves from a direct transition to an indirect transition
Because it is far enough from the crossover point,
Deeper impurity levels such as DX center compared to GaAs
Quite a few, the aforementioned Walkout phenomenon,
Unstable operation of the device such as occurrence can be suppressed.

The above direct transition, indirect transition, and crossover point are as follows. A semiconductor having the same k value (wave number vector: k = nπ / L) at the top of the valence band and the bottom of the conduction band is called a direct transition type. In this direct transition type semiconductor, the electrons in the conduction band are valence electrons. Upon recombination with the holes in the band, there is no change in k, ie, momentum (= hk), and the momentum is conserved. On the other hand, a semiconductor in which the k value at the top of the valence band is different from the k value at the bottom of the conduction band is called an indirect transition type. The direct transition semiconductor has a small effective mass and a high mobility, and is therefore used for high-speed electronic devices. A direct transition type is also used for a light emitting device such as a laser. The point at which the bottom of the conduction band shifts from the Γ valley to the X valley is referred to as the above-mentioned crossover point (x = 0.7 in FIG. 4).

[0030] In this first embodiment, the spacer layer 4 and the Schottky layer _{6 (Al 0. 3 Ga 0.7)} 0.5 In 0.5 P
But one or both are Al _0.33 Ga
It may be replaced with an AlGaAs-based material such as _0.67 As. Even in this case, as long as GaInP is used for the electron supply layer 5, it does not hinder the high concentration. When an AlGaAs-based material is used for the Schottky layer 6, the controllability of selective etching is inferior to that when a GaInP-based material is used, but it is not fatal.

[Second Embodiment] FIG. 5 shows a second embodiment of the present invention.
1 shows a structural cross-sectional view of the HEMT device of the embodiment. The difference from the first embodiment shown in FIG. 2 is that the Schottky layer is made of a non-doped Al _0.33 Ga _0.67 As first Schottky layer (fourth embodiment).
Semiconductor layer) 6A and second layer of non-doped Ga _0.65 In _0.35 P
And a Schottky layer (fifth semiconductor layer) 6B.

Schottky layer 16A of Al _0.33 Ga _0.67 As
And the electron supply layer 5 of Ga _0.65 In _0.35 P have almost no conduction band discontinuity, and the first Schottky layer 6A of Al _0.33 Ga _0.67 As which has many deep impurity levels such as a DX center. Ga _0.65 In _0.35 P with no deep impurity level
By providing the second Schottky layer 6B, the unstable operation of the device such as the walkout phenomenon and the occurrence of the current collapse described above can be suppressed, and the Schottky layer of Al _0.33 Ga _0.67 As of oxygen which forms a deep impurity level can be suppressed. Mixing into the layer 6A can be prevented. The first Schottky layer 6A
Is, Al _0.33 Ga _0.6 described in the first embodiment instead of the ₇ As (Al _0.3 Ga _0.7) may be formed so that a _0.5 In _0.5 P.

[0033]

As described above, according to the heterojunction field-effect transistor of the present invention, GaInP having a GaP composition that is higher than the composition lattice-matched to the semiconductor substrate is added to the third semiconductor layer serving as the electron supply layer. Since it is used, there is no deep impurity level unlike the DX center, and the electron supply layer can be doped at a high concentration, and the device performance can be improved.

An AlGaInP / InGaAs (second semiconductor layer / first semiconductor layer) heterojunction is formed of AlGaAs / InGaAs or
Since it has a conduction band discontinuity larger than that of the GaInP / InGaAs heterojunction, many electrons can be confined and the electron concentration can be improved.

Further, since the Schottky layer is composed of the AlGaInP layer as the fourth semiconductor layer or the GaInP layer as the fifth semiconductor layer, the Walkouw due to the deep impurity level is formed.
Unstable operation of the device such as occurrence of t phenomenon and current collapse can be suppressed.

Further, in the device fabrication process, if the gate electrode is formed on the AlGaInP layer as the fourth semiconductor layer or the GaInP layer as the fifth semiconductor layer, a step of etching and removing a part of the GaAs cap layer is performed. In this case, the selectivity of etching with GaAs becomes extremely large, processing control of the semiconductor film thickness becomes easy, variation in element characteristics is suppressed, and reliability is improved.

[Brief description of the drawings]

Fig. 1 InGaAs for the channel layer and GaIn for the electron supply layer
Non-doped AlGaIn for P, spacer layer and Schottky layer
FIG. 3 is a cross-sectional structural view of a heterojunction field effect transistor of the present invention using P (or AlGaAs).

FIG. 2 is a sectional structural view showing a first embodiment of the heterojunction field effect transistor of the present invention.

FIG. 3 is a diagram showing an energy band diagram of a heterojunction field effect transistor according to the first embodiment.

FIG. 4 is a characteristic diagram showing the dependence of the Al composition x on the conduction band discontinuity ΔEc and the valence band discontinuity ΔEv of the (Al _x Ga _1-x ) _0.5 In _0.5 P / GaAs heterojunction.

FIG. 5 is a sectional structural view showing a second embodiment of the heterojunction field effect transistor of the present invention.

FIG. 6 is a sectional structural view of a conventional heterojunction field effect transistor.

[Explanation of symbols]

1: semi-insulating GaAs substrate (semiconductor substrate) 2: buffer layer made of GaAs 3: channel layer made of InGaAs (first semiconductor layer) 4: spacer layer made of AlGaInP (or AlGaAs) (second semiconductor layer) 5: GaP rather than GaInP lattice-matched to GaAs doped with Si
Electron supply layer (third semiconductor layer) made of GaInP with increased composition of 6: Schottky layer made of AlGaInP (or AlGaAs)
(Fourth semiconductor layer) 6A: First Schottky layer made of AlGaAs (fourth semiconductor layer) 6B: Second Schottky layer made of GaInP (fifth semiconductor layer) 7: Cap layer made of GaAs 8: Source electrode 9: Drain electrode 10: Gate electrode

Claims (4)

[Claims] 1. A semiconductor substrate made of GaAs, a first semiconductor layer made of InGaAs formed on the semiconductor substrate, and a material different from the first semiconductor layer formed on the first semiconductor layer. A second semiconductor layer made of a material having a higher conduction band potential energy than the first semiconductor layer; and a conduction band potential energy formed on the second semiconductor layer and higher than the first semiconductor layer. Ga _x In
_A third semiconductor layer comprising _1-x P, wherein the composition x of Ga is 0.55 ≦ x ≦ 0.7, and a donor impurity is added; and the first semiconductor layer formed on the third semiconductor layer. A fourth semiconductor layer made of a material different from the semiconductor layer and having a higher conduction band potential energy than the first semiconductor layer, wherein the first semiconductor layer and the second, third and The two-dimensional electron gas formed by the heterojunction with the semiconductor layer of No. 4 is used as the active layer, and the second, third and fourth electron
A carrier is supplied from a source electrode or a drain electrode via the semiconductor layer of (1), and the traveling of the two-dimensional electron gas is controlled by an electric field applied to a gate electrode. 2. The semiconductor device according to claim 2, wherein at least one of the second semiconductor layer and the fourth semiconductor layer is made of (Al _y Ga _1-y ) _0.5 In _0.5 P lattice-matched to the semiconductor substrate, and has a composition of Al. y is
2. The heterojunction field effect transistor according to claim 1, wherein 0.2 ≦ y ≦ 0.4. 3. The second semiconductor layer or the fourth semiconductor.
At least one material of the layer is Al_zGa _1-zAs
Characterized in that the composition z of Al satisfies 0.2 ≦ z ≦ 0.35
The heterojunction field effect transistor according to claim 1. 4. The heterojunction field-effect transistor according to claim 1, further comprising: a fifth semiconductor layer formed on said fourth semiconductor layer. The layer is made of a material having a higher conduction band potential energy than the first semiconductor layer, Ga _x In _1-x P
Wherein the composition x of Ga is 0.55 ≦ x ≦ 0.7, and the gate electrode is formed on the fifth semiconductor layer.