JP2011254058A - Compound semiconductor epitaxial wafer and high-frequency semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compound semiconductor epitaxial wafer and a high-frequency semiconductor device having a structure that can reduce an HEMT channel resistance.SOLUTION: The compound semiconductor epitaxial wafer sequentially laminates: a semi-insulative semiconductor substrate 10; a buffer layer 11 for improving crystal growth potential; a first electron supply layer 12 having a delta-doped region in a uniformly doped region; a first spacer layer 13 for forming an electronic potential barrier; a channel layer 14 for generating a two-dimensional electron; a second spacer layer 15 for forming an electronic potential barrier; a second electron supply layer 16 having a delta-doped region in the uniformly doped region; and a cap layer 17 for taking an ohmic contact.

Description

  Embodiments described herein relate generally to an epitaxially grown compound semiconductor wafer and a high-frequency semiconductor device using the compound semiconductor wafer.

  Compound semiconductor devices having a heterojunction represented by HEMT (High Electron Mobility Transistor) are more efficient and gainable than GaAs MESFET (Metal Semiconductor FET) and GaAs JFET (Junction FET). Due to its excellent distortion characteristics, it is becoming a mainstream device of MMIC (Microwave Monolithic IC).

FIG. 4 shows a cross-sectional structure example of a conventional p-HEMT double hetero type epitaxial wafer. First, on a semi-insulating GaAs semiconductor substrate 40, grown i-GaAs buffer layer 41 in order to improve the crystal growth _{properties, n-Al x Ga 1-} x As electron supply layer 42, i-Al thereon the _x Ga _1-x As spacer layer _{43, i-In x Ga 1} -x As channel layer _{44, i-Al x Ga 1} -x As spacer layer _{45, n-Al x Ga 1} -x As electron supply layer 46 A HEMT double heterostructure is formed by sequentially laminating. Further, an n ⁺ -GaAs cap layer 47 subjected to high concentration doping for forming an ohmic contact electrode is formed. Here, n- in the name of the epitaxial layer indicates that the epitaxial layer is n-type and i- is undoped.

  In the above structure, ionized donors are formed in the electron supply layers 42 and 46. As a result, electrons fall from the electron supply layers 42 and 46 to the quantum well potential formed in the channel layer 44, and a two-dimensional electron gas is formed in the channel layer 44. These electrons move two-dimensionally and exhibit extremely high mobility because of very little impurity scattering.

  Further, in the double hetero structure having the electron supply layers 42 and 46 on both the substrate side and the growth surface side across the channel layer 44, electrons are supplied from the two electron supply layers 42 and 46 to the channel layer 44. Therefore, the channel resistance of the HEMT device can be reduced as compared with the case where there is one electron supply layer.

  By reducing the channel resistance of the HEMT device, the performance of the HEMT device can be further improved, for example, the insertion loss is reduced in the FET switch and the transfer conductance (gm) is increased in the FET amplifier.

In general, the doping structure of the electron supply layers 42 and 46 is often composed of either a uniform doped structure or a delta doped structure. (See Patent Document 1)
The uniform doped structure is a structure in which doping is performed uniformly throughout the electron supply layers 42 and 46 while continuing crystal growth, but the doping concentration is not necessarily constant within the thickness of the electron supply layers 42 and 46. Instead, the case where the doping concentration changes stepwise may be included.

  This uniform doped structure has a problem that, when the doping level is increased to a certain level or more, it has a characteristic of approaching a conductor such as a metal, so that the characteristic as a semiconductor is impaired. Also, due to crystal growth problems, no matter how much the doping level is raised, the donor level in the semiconductor cannot be raised above a certain level. Therefore, in order to adjust the electron density of the two-dimensional electron gas excited by the channel layer 44, it is necessary to control the film thickness such as making the doping levels constant and increasing the thickness of the electron supply layers 42 and 46. It is.

  The delta doped structure is a structure called pulse doping, planar doping or the like. As the name implies, the delta doped structure is a technique for forming a highly doped layer having a very thin film thickness. For example, in the AlGaAs electron supply layers 42 and 46, during the growth, the supply of Al or Ga is temporarily stopped to stop the growth, and only the Si raw material serving as a dopant is supplied to form a very thin layer of Si. Therefore, it is possible to put a large amount of dopant in the electron supply layers 42 and 46, which is sharper than the uniform doping.

  It is known that this delta-doped structure has excellent linearity, that is, low distortion characteristics, and does not decrease transfer conductance (gm) even in a high drain current region and exhibits excellent flatness. On the other hand, however, there are cases where problems such as the electrical characteristics are likely to be abnormal because a large amount of electrons are confined, and the Si dopant diffuses to the channel layer 42 due to the thin film thickness. In order to prevent this, it is necessary to increase the thickness of the channel layer 34, or to separate the delta doped layer from the channel layer 44 by a certain distance, so that optimization of the device structure becomes complicated.

Japanese Patent Laid-Open No. 2004-221364 FIG. 1 and FIG.

  In order to reduce the channel resistance of the HEMT device, it is necessary to increase the doping level of the electron supply layers 42 and 46. However, as described above, in both the uniform doped structure and the delta doped structure, the electrical characteristics and the epitaxial growth are required. Due to this limitation, there is a limit that the doping level cannot be increased beyond a certain concentration. Moreover, if the doping level is changed, the layer thickness or the like must be changed, which may affect device characteristics other than channel resistance.

  The present invention has been made to solve the above problems, and an object thereof is to provide a compound semiconductor epitaxial wafer and a high-frequency semiconductor device having a structure capable of reducing the channel resistance of the HEMT.

  According to an embodiment of the present invention, a semi-insulating semiconductor substrate, a buffer layer for improving crystal growth, and a first electron supply layer having a delta-doped region in a uniformly doped region A first spacer layer that forms an electron potential barrier, a channel layer that generates two-dimensional electrons, a second spacer layer that forms an electron potential barrier, and delta doping in a uniformly doped region A compound semiconductor epitaxial wafer comprising a second electron supply layer having a region formed and a cap layer for making ohmic contact, and a high frequency semiconductor device using the same I will provide a.

It is sectional drawing of the compound semiconductor epitaxial wafer which concerns on one Embodiment of this invention. It is a figure explaining the electron density of the epitaxial wafer for HEMT which has a double hetero structure. (A) is a uniformly doped structure, (b) is a delta doped structure, and (c) is a case where both a delta doped structure and a uniformly doped structure are used for the electron supply layer. It is a figure showing an example of section composition of a HEMT device using a compound semiconductor epitaxial wafer concerning one embodiment of the present invention. 1 is a cross-sectional view of a compound semiconductor epitaxial wafer according to an embodiment of the present invention.

  An embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a cross-sectional view of a compound semiconductor epitaxial wafer according to an embodiment of the present invention.

  In the present embodiment, as shown in FIG. 1, a buffer layer 11 is grown on a semi-insulating semiconductor substrate 10 in order to improve crystal growth, and a first electron supply layer 12, a first electron supply layer 12, and a first electron supply layer 12 are formed thereon. The spacer layer 13, the channel layer 14, the second spacer layer 15, and the second electron supply layer 16 are sequentially stacked to constitute a HEMT double heterostructure. Further, a cap layer 17 subjected to high concentration doping for forming an ohmic contact electrode is formed.

  When fabricating a GaAs HEMT device, the semi-insulating semiconductor substrate 10 is made of GaAs material, and the buffer layer 11 is made of non-doped GaAs material. The first and second electron supply layers 12 and 16 are made of n-type AlGaAs material, the first and second spacer layers 13 and 15 are made of non-doped AlGaAs material, and the channel layer 14 is made of non-doped GaAs material. The cap layer 17 is an n-type GaAs material doped at a high concentration.

  One or more delta doped structures 18 and 19 are formed in the uniformly doped structure in the electron supply layers 12 and 16. During the growth of the n-type AlGaAs electron supply layers 12 and 16 using the Si raw material as a dopant, the supply of Al or Ga is temporarily stopped to stop the growth, and only the Si raw material serving as the dopant is supplied, thereby thin Si delta doping. Layers 18 and 19 are formed. After the delta doped structure is formed, Al and Ga are supplied again to grow the n-type AlGaAs electron supply layers 12 and 16 so as to have a predetermined layer thickness. Therefore, the two n-type AlGaAs electron supply layers 12 and 16 grown with the channel layer 14 in between have a structure in which the delta doped layers 18 and 19 are sandwiched by the uniformly doped layers, respectively.

  The characteristics of the n-type AlGaAs electron supply layers 12 and 16 thus formed will be discussed.

  It is well known that the resistivity ρ is expressed by the equation (1) using the carrier density n per unit volume, the carrier mobility μ, and the carrier charge q (q = −e in the case of electrons). .

ρ = 1 / (q · μ · n) (1)
Therefore, it can be seen that the resistivity ρ of the channel layer 14 can be lowered by increasing the electron mobility μ or the carrier density (electron density) n.

  In the embodiment of this patent, in order to double the electron density contributing to the channel layer, a double hetero structure having two electron supply layers is employed.

  FIG. 2 is a view for explaining the electron density of an HEMT epitaxial wafer having a double hetero structure. FIG. 2A shows a conventional uniformly doped structure, in which the doping level per unit volume of the electron supply layer 16 on the growth layer side is Nd1, and the thickness of the electron supply layer 16 is T1. Further, the doping level per unit volume of the electron supply layer 12 on the substrate side is Nd2, and the thickness of the electron supply layer 12 is T2.

  In the following discussion, it is assumed that all electrons from the doped donor are activated and fall into the two-dimensional electron layer formed in the channel layer 14.

  Then, the electron density ns (hereinafter referred to as sheet electron density) per unit area of the channel layer 14 is expressed by the following equation (2).

ns = Nd1 · Tn1 + Nd2 · Tn2 (2)
FIG. 2B shows a conventional delta-doped structure, in which the doping level per unit area of the delta-doped electron supply layer 16 on the growth layer side is Sp1, and the doping per unit area of the delta-doped electron supply layer 12 on the substrate side. Assuming that the level is Sp2, the sheet electron density ns of the channel layer 14 is as shown in Equation (3).

ns = Sp1 + Sp2 (3)
FIG. 2C shows a case where both a delta doped structure and a uniform doped structure are used in one embodiment of the present invention. When the thicknesses and doping levels of the electron supply layers 12 and 16 are the same as those in FIGS. 4A and 4B, the sheet electron density of the channel layer 14 is expressed by Equation (4).

ns = Nd1 · Tn1 + Nd2 · Tn2 + Sp1 + Sp2 (4)
In this way, by uniformly doping the electron supply layers 12 and 16 and forming a delta-doped structure therein, the electron density contributing to electron conduction is further increased to about twice that of the conventional structure even with simple calculation. can do. Although the electron mobility μ is actually a function of the carrier density (electron density) n, it is known that the electron density per unit volume is almost independent until the electron density per unit volume is about 10 ⁻¹⁹ cm ^−3. If the doping level is increased to an electron density at which the product of the carrier density n and the electron mobility μ does not decrease in (1), the channel resistivity can be decreased.

  Further, according to the equation (4), even when the same electron density is obtained, the number of variables increases, so that the thicknesses Td1 and Td2 and doping levels Nd1, Nd2, Sp1, and Sp2 of the electron supply layers 12 and 16 are set. Since it can be adjusted independently, the degree of freedom of design such as the ability to increase the electron density without sacrificing electrical characteristics other than the resistivity ρ is widened, and it becomes possible to design optimum performance as a HEMT. .

  That is, in the structure of the present embodiment, the positions of the delta doped layers 18 and 19 in the electron supply layers 12 and 16 can be adjusted and formed, so that abnormal electrical characteristics can be prevented. Furthermore, it is also possible to provide a plurality of delta doped layers 18 and 19 in each of the electron supply layers 12 and 16 by performing a treatment such as lowering the donor level of the delta doped layers 18 and 19.

  As described above, according to the embodiment of the present invention, the dopant is uniformly doped and the delta doping is applied to each of the electron supply layers arranged on the substrate side and the growth surface side with respect to the channel layer. With the structure, the number of conduction electrons contributing to the conduction in the channel layer can be increased. As a result, the channel resistance can be reduced.

  Since the channel resistance can be lowered in this way, when an HEMT device is actually fabricated using this epitaxial wafer, the on-resistance of the FET switch is reduced due to the reduction of the channel resistance, so that the insertion loss is reduced. In addition, an improvement in gm can be obtained in the FET amplifier.

  In the example of the present embodiment, the GaAs HEMT has been described. However, similar results can be obtained with other materials.

  That is, in the InGaAs-based p-HEMT, the semi-insulating semiconductor substrate 10 is made of GaAs material, the buffer layer 11 is made of non-doped GaAs material, and the first and second electron supply layers 12 and 16 are made of n The first and second spacer layers 13 and 15 are non-doped AlGaAs materials, the channel layer 14 is non-doped InGaAs material, and the cap layer 17 is highly doped n. A type GaAs material may be used. The dopant of the delta doped layers 18 and 19 is Si or the like.

  In the GaN-based HEMT, the semi-insulating semiconductor substrate 10 is made of a GaN material, and the buffer layer 11 is made of a non-doped GaN material. The first and second electron supply layers 12 and 16 are made of n-type AlGaN material, the first and second spacer layers are made of 13, 15 and non-doped AlGaN material, and the channel layer 14 is made of non-doped GaN. As a material, the cap layer may be an n-type GaN material doped at a high concentration. The dopant of the delta doped layers 18 and 19 is Si or the like.

  FIG. 3 is a diagram showing a cross-sectional configuration example of a HEMT device using a compound semiconductor epitaxial wafer according to an embodiment of the present invention. Since the layer structure is the same as that in FIG. 1, description thereof is omitted, but a source electrode 31, a drain electrode 32, and a gate electrode 33 for operating as a HEMT device are formed.

  In order to manufacture a HEMT device using the epitaxial wafer of FIG. 1, a source electrode 31 and a drain electrode 32 are formed on the cap layer 17. Since the cap layer 17 is doped with Si or the like at a high concentration, a low-resistance ohmic contact can be obtained.

  The source electrode 31 and the drain electrode 32 are good by performing a sintering process after sequentially depositing materials such as Ge / Au / Ni / Ti / Au using AuGe / Ni / Au or Ge and Au alone. Ohmic characteristics can be obtained.

For the gate electrode 33, the cap layer 17 is removed to form a recess structure, and a Schottky junction is formed on the electron supply layer 16. The gate electrode 33 is formed using a material such as Ti / Pt / Au.

  In the HEMT device formed in this way, each of the two electron supply layers has a structure in which the dopant is uniformly doped and further subjected to delta doping, so that the conduction electrons contributing to the conduction in the channel layer are generated. Can be increased. As a result, the channel resistance can be reduced.

  Therefore, due to the decrease in channel resistance, in the case of the FET switch, the on-resistance is reduced, so that the insertion loss is reduced, and in the case of the FET amplifier, the gm is improved.

  Further, the present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined. As long as the technical idea of the present invention is used, these modifications are also included in the present invention.

DESCRIPTION OF SYMBOLS 10 ... Semi-insulating board | substrate 11 ... Buffer layer 12, 16 ... Electron supply layer 13, 15 ... Spacer layer 14 ... Channel layer 17 ... Cap layer 18, 19 ... Delta dope layer (structure)
31 ... Source electrode 32 ... Drain electrode 33 ... Gate electrode

Claims (5)

A semi-insulating semiconductor substrate;
A buffer layer for improving crystal growth;
A first electron supply layer having a delta doped region within the uniformly doped region;
A first spacer layer forming an electron potential barrier;
A channel layer for generating two-dimensional electrons;
A second spacer layer forming an electron potential barrier;
A second electron supply layer having a delta doped region within the uniformly doped region;
A cap layer for making ohmic contact;
A compound semiconductor epitaxial wafer formed by sequentially laminating layers.   The semi-insulating semiconductor substrate is made of GaAs material, the buffer layer is made of non-doped GaAs material, the first and second electron supply layers are made of n-type AlGaAs material, and the first and second spacers are made. 2. The compound semiconductor epitaxial wafer according to claim 1, wherein the layer is an undoped AlGaAs material, the channel layer is an undoped GaAs material, and the cap layer is a high-concentration n-type GaAs material.   The semi-insulating semiconductor substrate is made of GaAs material, the buffer layer is made of non-doped GaAs material, the first and second electron supply layers are made of n-type AlGaAs material, and the first and second spacers are made. 2. The compound semiconductor epitaxial wafer according to claim 1, wherein the layer is made of a non-doped AlGaAs material, the channel layer is made of a non-doped InGaAs material, and the cap layer is a high-concentration n-type GaAs material.   The semi-insulating semiconductor substrate is made of GaN material, the buffer layer is made of non-doped GaN material, the first and second electron supply layers are made of n-type AlGaN material, and the first and second spacers are made. 2. The compound semiconductor epitaxial wafer according to claim 1, wherein the layer is made of a non-doped AlGaN material, the channel layer is made of a non-doped GaN material, and the cap layer is a high-concentration n-type GaN material.   A high frequency semiconductor device using the compound semiconductor epitaxial wafer according to claim 1.

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