JPH06120259A - Field-effect transistor - Google Patents

Abstract

PURPOSE:To provide a field-effect transistor, in which electron scattering at the interface between a buffer layer and a channel layer is suppressed, and two-dimensional electron density and electron velocity are increased. CONSTITUTION:A field-effect transistor includes a GaAs substrate 21 on which are provided a buffer layer 22 (undoped GaAs), a gradient-composition layer 23 (InGaAs) and a uniform composition layer 24 (InGaAs). In the gradient- composition layer, the proportion of In is zero at the interface with the buffer layer and increases continuously to 0.2. This prevents an abrupt heterojunction between the buffer layer 22 and the channel layer 25, thus preventing electron scattering. The uniform composition layer 24 contributes to the increase in electron density and electron velocity.

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, and more particularly to a two-dimensional electron gas (2DEG).
High Electron Mobility Transistor (H), which is a compound semiconductor device using layers
EMT) involved.

[0002]

2. Description of the Related Art Conventionally, the problems to be solved by the invention
As a field effect transistor using a two-dimensional electron gas layer of this kind, a two-dimensional electron gas layer formed in a channel layer in the vicinity of a hetero interface between a channel layer and an electron supply layer is used as a channel, An n-type AlGaAs / GaAs HEMT using an n-type AlGaAs layer and a non-doped GaAs layer as a channel layer is known. For such n-type AlGaAs / GaAs HEMT, GaA is provided between the n-type AlGaAs layer and the GaAs layer.
n-type AlGaAs / GaAs by interposing an InGaAs layer of a constant composition having a larger electron affinity than s
It has been proposed to increase the two-dimensional electron gas density by increasing the step ΔEc of the conduction band edge (Ec) at the hetero interface. In this case, since the lattice constant of InGaAs is larger than the lattice constant of GaAs, InGaA
There is a lattice mismatch between the s layer and the GaAs layer. As a result, lattice strain occurs in this InGaAs layer. For this reason, this InGaAs layer is called a strained layer (Pseudomorphic layer). FIG. 10 shows this I
It is explanatory drawing which shows the epistructure of the cross section of HEMT which used the nGaAs strained layer for the channel layer. In the figure, 1 is semi-insulating Ga.
A buffer layer 2 which is an As substrate and which is a high resistance semiconductor (P ⁻ -GaAs) layer is formed on the GaAs substrate 1. Then, on the buffer layer 2, InGaAs (I
A channel layer 3 having an n composition of 0.20) is formed, and non-doped AlGaA is sequentially formed on the channel layer 3.
A spacer layer 4 made of s, an electron supply layer 5 made of Si-doped AlGaAs, and a cap layer 6 made of Si-doped GaAs are formed. The structure of the energy band corresponding to the epi structure of the buffer layer 2, the channel layer 3, the spacer layer 4, and the electron supply layer 5 is shown in the band diagram of FIG. As shown in this band diagram, since the electron potential of the channel layer 3 using InGaAs decreases,
The two-dimensional electron gas density is increased, the electron velocity is increased, and the electron confinement effect is improved, and in terms of DC, the mutual conductance (gm) and the output resistance are improved. In addition, e in FIG. 11 indicates a two-dimensional electron gas. However, the buffer layer (GaAs) 2 and the channel layer (In
At the interface with GaAs: In = 0.2) 3, electron scattering occurs due to the mismatch of lattice constants, and the performance is not necessarily improved in terms of RF due to the scattering. That is, there is a problem in that the high frequency characteristics mainly including the noise figure NF are not necessarily improved due to electron scattering.

Therefore, as a countermeasure against this problem, FIG.
As shown in FIG. 5, the In composition (y) of the entire channel layer 3 from the buffer layer 2 toward the hetero interface is changed from y = 0 to y.
= 0.2, and as shown in the band diagram of FIG.
The buffer layer 2 and the channel layer 3 are continuously changed so that the conduction band edge Ec becomes lower toward the electron supply layer side. However, in such a HEMT, although electron scattering at the interface between the buffer layer 2 and the channel layer 3 can be suppressed, a new problem arises in that the density and speed of the two-dimensional electron gas decrease.

When AlGaAs is used as the buffer layer, depending on the non-doped level of the buffer layer,
A conductive channel may be generated at the interface with the channel layer. FIG. 14 shows this band diagram,
Is a buffer layer, 8 is a channel layer, 9 is a spacer layer, 10
Is an electron supply layer, e is a two-dimensional electron gas, and c is a conductive channel.

Further, as a field effect transistor of this type, as shown in FIG. 15, a semi-insulating InP substrate 1 is used.
1, a buffer layer 12 made of AlInAs, GaIn
A channel layer 13 made of As, a spacer layer 14 made of AlInAs, an electron supply layer 15 made of AlInAs, and AlI.
Schottky contact layer 16 made of nAs, GaIn
It is known that the cap layer 17 made of As is sequentially formed. FIG. 16 is a band diagram showing the buffer layer 12 made of AlInAs and GaInAs.
A steep heterojunction is formed between the channel layers 13. Therefore, especially metal organic chemical vapor deposition (MOCVD)
Since the non-doped level of AlInAs is poor in the case of manufacturing by the method, as shown in FIG. 16, a conductive channel c due to two-dimensional electrons is generated at the interface of the buffer layer / channel layer, and HE
There is a problem of degrading the high frequency characteristics of MT. In addition, electron scattering occurs in the above heterojunction where the composition changes rapidly,
There is a problem that becomes a noise source. Furthermore, a layer having an abnormal composition may be present at such an interface, which causes a conductive path and electron scattering, which causes a problem.

The present invention was made in view of such conventional problems. The two-dimensional electron density and electron velocity are increased, the electron confinement effect is high, and the buffer layer / channel layer is also provided. It is intended to obtain a field effect transistor having excellent high frequency characteristics by suppressing electron scattering at the interface.

[0007]

The invention according to claim 1 is
A semi-insulating substrate has a buffer layer made of a high-resistance first semiconductor, and a channel layer made of a high-purity second semiconductor having an electron affinity higher than that of the first semiconductor is provided on the buffer layer. In the field effect transistor having a spacer layer made of a third semiconductor and an electron supply layer made of a fourth semiconductor on the channel layer, the channel layer is formed from the junction surface with the buffer layer. The solution is to form a transition composition layer that continuously changes from the composition of 1) and a uniform composition layer that is located on the spacer layer side and has a uniform composition.

According to a second aspect of the present invention, the buffer layer is made of GaAs and the channel layer is made of InGaAs.

The third aspect of the present invention is characterized in that the buffer layer is made of AlGaAs and the channel layer is made of GaAs.

The invention according to claim 4 is characterized in that the film thickness of the uniform composition layer is 10 nm or more.

The invention according to claim 5 is characterized in that the composition of the transition composition layer is linearly changed.

The invention according to claim 6 is the semi-insulating InP.
A buffer layer made of high-purity AlInAs, a channel layer made of high-purity GaInAs, a spacer layer made of undoped AlInAs, and an n-type AlInAs are sequentially formed on the substrate.
In a field effect transistor having an electron supply layer consisting of, an intervening AlGaInAs layer whose composition continuously changes between the channel layer and the buffer layer is a solution.

The invention according to claim 7 is the above AlGaAs.
It is characterized in that the composition of the layer changes linearly.

[0014]

According to the present invention, since the transition composition layer is provided, the heterojunction between the channel layer and the buffer layer is eliminated, electron scattering at the interface is suppressed, and noise is prevented. Further, since it has a uniform composition layer, it has the effect of increasing the two-dimensional electron density and electron velocity. Particularly, in the invention according to claim 3, as the buffer layer, AlG is used.
By using a transition composition layer, it is possible to avoid the occurrence of a conductive channel (conductive path) at the interface between the buffer layer and the channel layer when aAs and GaAs are used as the channel layer. The inventions according to claim 6 and claim 7 are AlGaI
By interposing the nAs layer between the channel layer and the buffer layer and continuously changing the composition between the channel layer and the buffer layer, a conductive channel is prevented from occurring at the interface between the channel layer and the buffer layer. There is an action. Therefore, it is possible to prevent the noise figure (NF) and the gain (Ga) from deteriorating.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the field effect transistor according to the present invention will be described below with reference to the embodiments shown in the drawings.

(Embodiment 1) As shown in FIG. 1, the field effect transistor according to the present embodiment is first of all a semi-insulating Ga.
On the As substrate 21, a buffer layer 22 made of undoped GaAs as a high-resistance first semiconductor is formed to a thickness of 500 nm. On the buffer layer 22, a transition composition layer 23 made of InGaAs in which the composition (y) of indium (In) changes from 0 to 0.2 and an In composition of y =
Uniform composition layer 2 made of InGaAs having a uniform composition of 0.2
4 are formed. The transition composition layer 23 and the uniform composition layer 24 are composed of InGaAs as the second semiconductor, and this InGaAs has a larger electron affinity than the non-doped GaAs forming the buffer layer 22 and both layers 23, 24.
Constitutes the channel layer 25.

The transition composition layer 23 is the buffer layer 22.
From the bonding surface of the buffer layer 22 to the composition ratio (in this case, y =
0) linearly from the composition ratio of the uniform composition layer 24 (y = 0.
It is designed to change continuously in 2). InG
The y of aAs is the In composition ratio (InyGa _1-y As).

On the channel layer 25, a spacer layer 26 made of AlGaAs as a third semiconductor, an electron supply layer 27 made of AlGaAs doped with silicon (Si) as a fourth semiconductor, and a cap made of GaAs doped with Si. Layers 28 are sequentially formed.

When the transition composition layer 23 is formed by, for example, a metal organic chemical vapor deposition (MOCVD) method, it can be realized by continuously changing the flow rate of trimethylindium (CH ₃ ) ₃ In. Further, in this embodiment,
The thickness of the uniform composition layer 24 is 10 nm. this is,
As shown in FIGS. 4 and 5, since the peak of the two-dimensional electron gas density distribution is near 8 nm from the interface with the spacer layer 26, most of the two-dimensional electrons are in the uniform composition layer 24. Is. Therefore, it is necessary to set the thickness of the uniform composition layer 24 to 10 nm or more. However, due to strain, when the In composition ratio is 0.2, the transition composition layer 23
And the thickness of the channel layer 25 including the uniform composition layer 24 is 25 n
It is preferably m or less.

FIG. 2 is a band diagram of this embodiment. As can be seen from the figure, the interface between the buffer layer 22 and the channel layer 25 has the transition composition layer 23, and therefore the interface having a sharp composition is formed. Therefore, the generation of electron scattering is suppressed,
It is possible to prevent the generation of noise. Further, since the uniform composition layer 24 having a thickness of 10 nm is provided, the two-dimensional electron density is large, the electron velocity is increased, and
It also has the effect of confining electrons.

As described above, in the present embodiment, not only the electron scattering at the interface of the buffer layer is prevented as in the conventional case, but also the two-dimensional electron density and the electron velocity are increased and the electron confinement effect is achieved. It becomes possible. FIG. 3 is a cross-sectional explanatory view of a field effect transistor in which the Schottky gate 29 and the source / drain 30a, 30b are provided in the above-mentioned epitaxial structure. E in FIG. 3 is
A two-dimensional electron gas is shown.

(Embodiment 2) This embodiment is an example in which the constituent semiconductors of the buffer layer 22 and the channel layer 25 of Embodiment 1 are changed, as shown in FIG.

That is, the buffer layer 2 on the GaAs substrate 21
2 is non-doped AlGaAs and its thickness is 80n
m. The transition composition layer 23 is made of non-doped Al.
The composition ratio of Al (x) of GaAs is continuously changed from 0.3 to 0 from the interface with the buffer layer 22, and the uniform composition layer 24 has a uniform composition in which the composition ratio (x) of Al of AlGaAs is 0. It is composed of GaAs. In addition, AlGa
The x of As is the Al composition ratio (AlxGa _1-x As). The other configuration of this embodiment is the same as that of the first embodiment. The band diagram of this example is as shown in FIG.

Also in this embodiment, as in the first embodiment,
Electron scattering is prevented, a two-dimensional electron density and an electron velocity are increased, and an electron confinement effect is achieved.

(Embodiment 3) FIG. 8 is a sectional view showing the epitaxial structure of the field effect transistor of this embodiment.

In this embodiment, first, Fe-doped semi-insulating material I is used.
On the nP substrate 31, non-doped Al ₀ .48 In _{0 .52} As
A buffer layer 32 composed of is formed. The transition composition layer 33 formed on the buffer layer 32 is made of Ga _y Al _x I
n _1-xy As, the Ga composition ratio y is 0 to 0.47, the Al composition ratio x is 0.48 to 0, and the In composition ratio 1-xy is 0.52 from the interface of the buffer layer 32. From 0.53
Is continuously changed linearly.

Next, a uniform composition layer 34 made of non-doped Ga ₀ .47 In _{0 .53} As is formed on the transition composition layer 33 to form a channel layer together with the transition composition layer 33. Further, on the uniform composition layer 34, the electron supply layer 3 made of undoped Al _{0 · 48} In _{0 · 52} spacer layers 35 made of As, Si-doped ^{_{n + -Al 0 · 48 In 0}} · 52 As
6, the Schottky contact layer 37 made of non-doped _{Al 0 · 48 In 0 · 52} As, Si -doped n ⁺ -Ga _{0 · 47} I
A cap layer of n _0.53 As is sequentially formed.

The composition ratio of each layer described above is the most general value, and is not limited to this value. The thickness of the transition composition layer 33 made of GaAlInAs is
Strain (lattice mismatch) occurs depending on the composition, but it may be within a range not affected by the device.

FIG. 9 is a band diagram of this embodiment. As shown in FIG. 9, since there is no heterojunction between the buffer layer 32 and the uniform composition layer 34, the buffer layer (AlInA
Even if the non-doped level of s) 32 deteriorates, it is possible to prevent the generation of a conductive path (conductive channel) at the interface.

Further, the thickness of the AlInAs buffer layer can be increased correspondingly (there is a path flowing through the AlInAs bulk, but the interface component is lost, so there is a margin in the bulk component), and the introduction of dislocations and impurities from the substrate is suppressed. be able to. Then, since there is no steep junction surface between the buffer layer and the channel layer in this way, electron scattering at the interface can be prevented and noise generation can be suppressed.

In the field effect transistor of this embodiment, the Schottky gate is made of Al or Ti / Au and is AlI.
The Schottky contact layer 37 made of nAs is formed by recess etching, and the ohmic metal is AuGe /
It may be Au.

Although the respective embodiments have been described above, the present invention is not limited to these, and various design changes associated with the gist of the configuration can be made.

[0033]

As is apparent from the above description, according to the inventions of claims 1 to 7, the two-dimensional electron density and electron velocity can be increased, and the electron confinement effect can be achieved. Further, the electron scattering at the interface between the buffer layer and the channel layer is suppressed, and the noise figure (NF) and the gain (G
It has the effect of improving the high-frequency characteristics mainly in (a).

[Brief description of drawings]

FIG. 1 is a sectional explanatory view showing an epitaxial structure of a field effect transistor of Example 1 of the present invention.

FIG. 2 is a band diagram showing an energy band of Example 1.

FIG. 3 is an explanatory cross-sectional view of the field effect transistor of Example 1.

FIG. 4 is a graph showing the relationship between the distance from the interface between the spacer layer and the channel layer and the potential in Example 1.

FIG. 5 is a graph showing the relationship between the distance from the interface between the spacer layer and the channel layer and the electron density in Example 1.

FIG. 6 is a sectional explanatory view showing an epitaxial structure of Example 2 of the present invention.

7 is a band diagram showing an energy band of Example 2. FIG.

FIG. 8 is an explanatory sectional view showing an epitaxial structure of Example 3 of the present invention.

9 is a band diagram showing an energy band of Example 3. FIG.

FIG. 10 is a sectional explanatory view of a conventional example.

FIG. 11 is a band diagram showing an energy band of a conventional example.

FIG. 12 is a cross-sectional explanatory view of a conventional example.

FIG. 13 is a band diagram showing an energy band of a conventional example.

FIG. 14 is a band diagram showing an energy band of a conventional example.

FIG. 15 is a sectional explanatory view of a conventional example.

FIG. 16 is a band diagram showing an energy band of a conventional example.

[Explanation of symbols]

 21 ... GaAs substrate 22 ... Buffer layer 23 ... Transition composition layer 24 ... Uniform composition layer 25 ... Channel layer 26 ... Spacer layer 27 ... Electron supply layer 28 ... Cap layer

Claims (7)

[Claims] 1. A high-purity second semiconductor having a buffer layer made of a high-resistance first semiconductor on a semi-insulating substrate, and having a higher electron affinity than the first semiconductor on the buffer layer. A channel layer formed on the channel layer, and a third layer is sequentially formed on the channel layer.
A field effect transistor having a spacer layer made of a semiconductor and an electron supply layer made of a fourth semiconductor, wherein the channel layer is a transition composition layer that continuously changes from the composition of the buffer layer from the junction surface with the buffer layer. And a uniform composition layer formed on the spacer layer side and having a uniform composition. 2. The field effect transistor according to claim 1, wherein the first semiconductor forming the buffer layer is GaAs, and the second semiconductor forming the channel layer is InGaAs. 3. A first semiconductor forming the buffer layer is AlGaAs, and a second semiconductor forming the channel layer.
The field effect transistor according to claim 1, wherein said semiconductor is GaAs. 4. The field effect transistor according to claim 1, wherein the film thickness of the uniform composition layer is 10 nm or more. 5. The field effect transistor according to claim 1, wherein the composition of the transition composition layer changes linearly. 6. A buffer layer made of high-purity AlInAs and a high-purity GaIn layer on a semi-insulating InP substrate, in that order.
In a field effect transistor having a channel layer made of As, a spacer layer made of undoped AlInAs, and an electron supply layer made of n-type AlInAs, the composition between the channel layer and the buffer layer is continuous between both layers. A field-effect transistor characterized by interposing an AlGaInAs layer which changes into 7. The field effect transistor according to claim 6, wherein the composition of the AlGaInAs layer changes linearly.