JP2004221364A - Epitaxial wafer for high-speed electron mobility transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance electron mobility of a HEMT by confining the two-dimensional electron gas of the HEMT further in two-dimensions. <P>SOLUTION: The epitaxial wafer for HEMT comprises such a stratum as AlGaAs spacer layers 4 and 6 of large band gap, and electron supply layers 3 and 7 are provided on both sides of an InGaAs channel layer 5 of narrow band gap on a semi-insulating substrate. The InGaAs channel layer 5 is provided with the composition gradient where In composition increases gradually from the central part toward a spacer interface. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an epitaxial wafer for a high-speed electron mobility transistor having a heterojunction.
[0002]
[Prior art]
As a compound semiconductor field-effect transistor, there is a HEMT that uses a two-dimensional electron gas (2DEG) induced in a semiconductor having a small energy gap at a heterojunction.
[0003]
A heterojunction HEMT (High Electron Mobility Transistor) having an InGaAs layer as a channel layer and having an electron supply layer on both sides or one side of the channel is capable of high-speed operation only by taking advantage of high-speed electron transfer. In addition, since high-output and high-efficiency operation is possible in an ultra-high frequency band such as a microwave band, it is used for applications such as a transistor mounted in a mobile phone or a satellite communication device.
[0004]
FIG. 4 shows an example of an epitaxial wafer having a double hetero-type HEMT (herein referred to as D-HEMT) structure according to the prior art.
[0005]
Structure as shown in FIG. 4, it comprises an un-GaAs buffer layer 12 on a semi-insulating GaAs substrate _{11, n-Al x Ga 1} -x As electron supply layer 13 in this upper, _un-Al x _{Ga 1- x} As spacer layer _{14, un-in x Ga 1} -x As channel layer _{15, un-Al x Ga 1} -x As spacer layer _{16, n-Al x Ga 1} -x As double by stacking an electron supply layer 17 A hetero-structure HEMT is formed, and an n-GaAs gap layer 18 is formed thereon. That is, the n-AlGaAs electron supply layers 13 and 17 are disposed above and below the un-InGaAs channel layer 15, and undoped between the un-InGaAs channel layer 15 and the upper and lower n-AlGaAs electron supply layers 13 and 17. It has a structure in which AlGaAs spacer layers 14 and 16 are inserted. Note that n- and un- in the names of the epitaxial layers indicate that the epitaxial layers are n-type and undoped, respectively.
[0006]
In the above, a two-dimensional electron gas (2DEG) is formed in the channel layer, and an ionized donor is formed in the electron supply layer. As a result, electrons fall from the donor into the quantum well potential to form 2DEG (see FIG. 3B). Since 2DEG electrons move two-dimensionally, they exhibit extremely high mobility.
[0007]
Here, the un-AlGaAs spacer layers 14 and 16 reliably and spatially separate the un-InGaAs channel layer 15 in which carriers (electrons) travel and the n-AlGaAs electron supply layers 13 and 17 doped with n-type impurities. Has been inserted for. That is, the spacer layers 14 and 16 prevent electrons from being scattered by ionized donors. The function of the un-AlGaAs spacer layers 14 and 16 makes it difficult for carriers (electrons) traveling in the un-InGaAs channel layer 15 to be scattered from the ionized impurities in the n-AlGaAs electron supply layers 13 and 17. As a result, the mobility of carriers (electrons) is improved.
[0008]
With such an epitaxial layer structure, the concentration of the two-dimensional electron gas can be increased, the mobility of electrons can be increased, and the performance of the device can be improved.
[0009]
[Problems to be solved by the invention]
However, in the conventional HEMT structure, an InGaAs layer having a narrow band gap is used for a channel layer, and an un-GaAs layer or an AlGaAs layer is used for a spacer layer in order to perform a carrier confinement effect by a potential barrier generated at a hetero interface. As a result, a two-dimensional electron gas is formed as shown in FIG. 3B, spatially separated from ionized donors serving as scattering sources, reducing Coulomb scattering and exhibiting high-speed mobility.
[0010]
Therefore, the electron concentration becomes high, and as shown in FIG. 3B, the level of the electron concentration exceeds the conduction band energy Ec of the channel layer, or Ec of the channel layer is increased near the interface with the spacer. When the range in which electrons can move is wide because of broadness, there is a problem that electron mobility is reduced.
[0011]
An object of the present invention is to solve the above-mentioned problem and speed up electron transfer in a HEMT by confining a two-dimensional electron gas more two-dimensionally as shown in FIG. It is another object of the present invention to provide an epitaxial wafer for a high-speed electron mobility transistor in which the mobility of a semiconductor crystal laminated thin film is reduced in sheet carrier concentration and the mobility is greatly improved.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is configured as follows.
[0013]
The epitaxial wafer for a high-speed electron mobility transistor according to the first aspect of the present invention has a layer structure in which a spacer layer having a large band gap and an electron supply layer are provided on both sides of a channel layer having a narrow band gap on a semi-insulating substrate. In the epitaxial wafer for a high-speed electron mobility transistor, the composition of the channel layer is gradually changed from a central portion toward a spacer interface.
[0014]
According to a second aspect of the present invention, in the epitaxial wafer for a high-speed electron mobility transistor according to the first aspect, the channel layer is formed of InGaAs, the spacer layer is formed of AlGaAs, and the In concentration of the channel layer is changed from the center to the spacer interface. It is characterized in that it is gradually raised to GaAs at the center of the channel layer and to InGaAs at the interface with the spacer. Here, the center of the channel layer includes a form in which the position of GaAs does not exactly coincide with the center of the channel layer. That is, the graded layer may be formed asymmetrically.
[0015]
According to a third aspect of the present invention, in the epitaxial wafer for a high-speed electron mobility transistor according to the first or second aspect, the electron supply layer is formed of an n-AlGaAs layer or a delta-doped layer of Si.
[0016]
<The gist of the invention>
The gist of the present invention is that the channel layer has a composition gradient that gradually changes from the center to the spacer interface. Specifically, for example, a channel layer is formed of InGaAs and a spacer layer is formed of AlGaAs, and the In concentration of the channel layer is gradually increased from the center toward the spacer interface as shown in FIG. The portion is made of GaAs, and the spacer interface side is made of InGaAs.
[0017]
As shown in FIG. 3A, the In concentration of the InGaAs channel layer is gradually increased from the center of the channel layer toward the spacer interface so that GaAs is formed at the center of the channel layer and InGaAs is formed at the interface side of the spacer. In addition, the two-dimensional electron gas can be confined more two-dimensionally. That is, as shown in FIG. 3B, electrons can move because the level of the electron concentration exceeds the conduction band energy Ec of the channel layer or Ec of the channel layer is broad near the interface with the spacer. The disadvantage that the range is widened is eliminated. Therefore, the speed of electron transfer in the HEMT can be increased.
[0018]
The configuration of the electron supply layer is arbitrary, and can be an n-AlGaAs layer or a delta-doped layer of Si.
[0019]
In order to increase the amplification factor of the HEMT and reduce noise, the electron supply layer is being highly thinned (doped) and the electron supply layer is required to have a delta-doped structure in which a delta-doped layer is formed in an undoped AlGaAs layer. it can. In this method, high-concentration doping is performed by using a technique called delta doping, in which a doping layer is converted into one atomic layer (or several atomic layers that cannot be avoided in growth), and the other portions are undoped or lightly doped. That is, the dopant atoms are distributed at an extremely high concentration only in a part of the delta-doped layer of the electron supply layer. In this delta-doping structure, by optimizing the position of the delta-doped layer containing the dopant atoms in the electron supply layer and optimizing the method of forming the delta-doped layer, the concentration of the electrons remaining in the carrier travel can be greatly reduced. Can be increased somewhat without any loss.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described based on the illustrated embodiments.
[0021]
FIG. 1 shows a structure of an epitaxial wafer for D-HEMT according to an embodiment of the present invention.
[0022]
On a semi-insulating GaAs substrate 1, un-GaAs buffer layer 2, the electron supply layer by δ-doped _{Si 3, un-Al x Ga} 1-x As spacer layer 4, graded _{(graded) un-In x Ga} 1 having a _-x As channel layer _{5, un-Al x Ga 1} -x As spacer layer 6, an electron supply layer 7 by δ doping of Si, un-AlGaAs cap layer 8, n-GaAs cap layer 9 were sequentially stacked .
[0023]
The channel layer 5 has a composition gradient that gradually changes from the center toward the spacer interface. I.e. _{un-In x Ga 1-x} As In concentration in the channel layer 5 (an In composition ratio x), as shown in FIG. 2, gradually increase toward the interface with the spacer layer 4, 6 from the channel layer center GaAs at the center of the channel layer and InGaAs at the interface with the spacer.
[0024]
Therefore, as shown in FIG. 3A, the two-dimensional electron gas can be more two-dimensionally confined than in the conventional FIG. 3B. Therefore, the speed of electron transfer in the HEMT can be increased.
[0025]
In order to confirm the effect of the present invention, a prototype of the following example of an epitaxial wafer for D-HEMT was manufactured.
[0026]
That is, an un-GaAs buffer layer 2 is formed to a thickness of 500 nm on a semi-insulating GaAs substrate 1 by epitaxy using a MOVPE apparatus, and an electron supply layer 3 by δ-doping of Si is formed thereon to a thickness of 0 nm. (1 atomic layer or grown on unavoidable several atomic layers) are formed, the _{un-Al x Ga 1-x} As spacer layer 4 thereon was 5~10nm formed.
[0027]
Then, a graded _{(graded) un-In x Ga} 1-x As channel layer 5 was total 10~15nm formed thereon. This channel layer 5 is deposited with several nm by gradually changing the In composition ratio x from x = 0.20 → 0 to several nm and then gradually changed from x = 0 → 0.2 to deposit several nm. Was 10 to 15 nm in thickness.
[0028]
Then, electrons due on the graded _{un-In x Ga 1-x} As channel layer 5, the _{un-Al x Ga 1-x} As spacer layer 6 and 5~10nm formation, [delta] doped layers of Si thereon The supply layer 7 is formed to have a thickness of 0 nm (one atomic layer or several atomic layers inevitable for growth), and an un-AlGaAs layer 8 and an n-GaAs cap layer 9 (doping concentration 3 × 10 ¹⁸ cm ⁻³ ) are formed thereon. Epitaxy was grown in order.
[0029]
The epitaxial wafer for HEMT (product of the present invention) according to the present embodiment (FIG. 1) has substantially the same mobility as that of the conventional structure (FIG. 4) (although the sheet carrier concentration is substantially the same). Improved. That is, in the conventional structure shown in FIG. 1, the mobility was 5000 to 6500 (cm ² / V · s) when Ns (sheet carrier concentration) was 2.2 to 2.8 × 10 ¹² cm ⁻² . . On the other hand, in the product of the present invention, at Ns: 2.2 to 2.8 × 10 ¹² cm ⁻² , the mobility is 7000 (cm), which is significantly higher than the mobility 6500 (cm ² / V · s) of the conventional product. ² / V · s) or higher mobility was realized.
[0030]
【The invention's effect】
As described above, according to the present invention, in an epitaxial wafer for a high-speed electron mobility transistor in which a spacer layer having a large band gap and an electron supply layer are provided on both sides of a channel layer having a narrow band gap, When the channel layer is made of InGaAs and the spacer layer is made of AlGaAs, the In concentration of the channel layer is gradually increased from the center toward the spacer interface. In this case, GaAs is formed at the center of the channel layer and InGaAs is formed at the interface of the spacer, so that the two-dimensional electron gas can be confined more two-dimensionally, and the electron transfer in the HEMT can be accelerated. it can. Therefore, it is possible to provide an epitaxial wafer for a high-speed electron mobility transistor in which the dependency of the mobility of the semiconductor crystal laminated thin film on the sheet carrier concentration is reduced and the mobility is greatly improved.
[0031]
Therefore, according to the present invention, it is possible to obtain a high-speed electron mobility transistor whose performance is remarkably improved by the above-described high carrier mobility, and a great effect can be obtained when applied to low-noise amplification or the like.
[Brief description of the drawings]
FIG. 1 is a structural sectional view of an epitaxial wafer for HEMT according to one embodiment of the present invention.
FIG. 2 is a diagram showing a relationship between a composition gradient of an In composition of a channel layer and an epitaxial layer structure in the present invention.
FIG. 3 is an energy band diagram of an interface between a channel layer and a spacer layer in the HEMT structure of the present invention and a conventional HEMT structure.
FIG. 4 is a structural sectional view of a conventional epitaxial wafer for HEMT.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 GaAs substrate 2 GaAs buffer layer 3 Si electron supply layer by δ doping 4 AlGaAs spacer layer 5 InGaAs channel layer 6 AlGaAs spacer layer 7 Electron supply layer by Si δ doping 8 AlGaAs cap layer 9 GaAs cap layer

Claims (3)

On a semi-insulating substrate, a high-speed electron mobility transistor epitaxial wafer having a layer structure in which a large band gap spacer layer and an electron supply layer are provided on both sides of a narrow band gap channel layer,
An epitaxial wafer for a high-speed electron mobility transistor, wherein the composition of the channel layer gradually changes from a central portion toward a spacer interface. The epitaxial wafer for a high-speed electron mobility transistor according to claim 1,
The channel layer is formed of InGaAs and the spacer layer is formed of AlGaAs, and the In concentration of the channel layer is gradually increased from the center toward the spacer interface so that GaAs is at the center of the channel layer and InGaAs is at the interface of the spacer. An epitaxial wafer for high-speed electron mobility transistors, characterized in that: The epitaxial wafer for a high-speed electron mobility transistor according to claim 1 or 2,
An epitaxial wafer for a high-speed electron mobility transistor, wherein the electron supply layer comprises an n-AlGaAs layer or a delta-doped layer of Si.

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