JP2003197646A - Field effect transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a field effect transistor with its InGaN layer serving as a channel layer, with the effect of point defects or uneven composition remarkably reduced and effective utilization of its characteristic high potential for obtaining a high output and high speed. <P>SOLUTION: In an HEMT3, the channel layer 119 has an Si-doped InGaN layer 119a, and the use of Si-doped InGaN inhibits electrons scattering in the layer 119 effectively. The layer 119 may be constituted as a multiple quantum well layer including an InGaN layer 119a serving as a well layer. The state density of electrons near the ground state increases when the channel layer 119 is constituted as the multiple quantum well layer, and this increases an electron density leading to an increased electric current. <P>COPYRIGHT: (C)2003,JPO

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 high speed field effect transistor using a semiconductor heterojunction.

[0002]

2. Description of the Related Art In recent years, HEMT (High Electron Mobility T
ransistor). HEMT is GaAs / AlGa
One using an As heterojunction has been put to practical use, and due to its excellent microwave and millimeter wave characteristics, low noise and high speed field effect transistors (FE
Widely used as T: Field Effect Transistor. Specifically, the main part of the n-type doped A
This is a semiconductor multi-layer structure in which a non-doped GaAs channel layer (i-GaAs layer) is heterojunctioned with an 1 GaAs electron supply layer. Since the Fermi level of the n-type AlGaAs electron supply layer has higher energy than the conduction band bottom of the i-GaAs layer, some electrons flow from the n-type AlGaAs electron supply layer to the i-GaAs channel layer, and the hetero An inverted triangular potential well is formed on the i-GaAs channel layer side of the junction interface. In this potential well, the electrons are confined in a form spatially separated from the donor impurities, forming a two-dimensional electron gas (hereinafter, referred to as “2DEG” in this specification) layer that is not easily affected by impurity scattering. . As a result, the electrons in the i-GaAs channel layer exhibit extremely high electron mobility along the heterojunction interface, and a high-speed field effect transistor can be realized.

On the other hand, in recent years, HEMTs using GaN-based compounds instead of GaAs-based compounds have been attracting attention as next-generation high-speed FETs. Since GaN-based compounds have a wide band gap and have both a relatively high electron mobility and a high saturated electron mobility, there is a possibility that a high-frequency device with a higher output, a higher breakdown voltage, and a higher temperature operation can be realized. Are overlaid.

As a HEMT structure using a GaN-based compound, an n-type doped AlGaN electron supply layer has a Ga
Many heterojunctions of N-channel layers have been tried. However, since electrons in the GaN layer have a large theoretically estimated effective mass, there is a drawback in that the electron mobility cannot be increased as much as expected, and the device has a high speed. The problem remains from the standpoint of becoming a company. Therefore, instead of GaN, it is possible to configure the channel layer with InGaN having a smaller effective mass of electrons,
It is considered to be more advantageous from the viewpoint of improving the electron mobility and, in turn, increasing the speed of the device.

[0005]

However, the MOV
When the InGaN layer is grown by a vapor phase growth method such as PE or MBE, there are drawbacks that a high concentration of point defects is likely to occur in the layer and that compositional nonuniformity is likely to occur. As a result, although the obtained InGaN layer has a high theoretically predicted electron mobility, the effect of electron scattering becomes conspicuous as a result, and there is a problem that sufficient characteristics cannot be obtained.

An object of the present invention is to use an InGaN layer as a channel layer and to significantly reduce the effects of point defects and compositional nonuniformity, and by utilizing the high possibility of its characteristics sufficiently effectively to achieve high output and high speed. An object of the present invention is to provide a field effect transistor that can be realized.

[0007]

In order to solve the above problems, the field-effect transistor of the present invention has a first structure in which an electron supply layer made of a compound semiconductor and an electron supply layer are provided. A field effect transistor having a heterojunction channel layer made of a compound semiconductor, wherein the channel layer has a Si-doped InGaN layer. Si-doped InGa
By adopting N, electron scattering in the channel layer can be effectively suppressed, and the high possibility of electron mobility of InGaN can be effectively brought out, and a field effect transistor capable of coping with higher speed can be realized. it can.

The InGaN layer epitaxially grown by the vapor phase epitaxy method has a vacancy-like point defect (N vacancy) due to the defect of the point defect described above, particularly N (nitrogen) which is a group V atom as shown in FIG. Holes) easily. Such N vacancies strongly scatter the electrons moving in the crystal, which becomes a factor of lowering the electron mobility. Therefore, if the InGaN layer that becomes the channel layer is doped with Si as in the first configuration of the present invention, the N vacancies that cause electron scattering are blocked by Si, and as a result, the crystallinity improves. Therefore, electron scattering can be effectively suppressed.

A second structure of the field effect transistor of the present invention is a field effect transistor having an electron supply layer made of a compound semiconductor and a channel layer made of a compound semiconductor heterojunction to the electron supply layer, The channel layer is configured as a multiple quantum well layer including an InGaN layer as a well layer.

When the channel layer is formed as a multiple quantum well layer, the influence of point defect generation and composition variation, which are problems during InGaN growth, is reduced by thinning, and a well layer in which electron scattering is unlikely to occur can be obtained. As a result, InGa
The effect of high electron mobility peculiar to N is effectively exhibited, and a field effect transistor capable of coping with higher speed can be realized.

In this case, by combining the above-mentioned first configurations, that is, by doping the InGaN layer in the multiple quantum well layer forming the channel layer with Si,
An InGaN well layer having better crystallinity can be obtained, and the performance of the field effect transistor can be further improved.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 schematically shows a laminated structure of a HEMT 1 which is an example of the field effect transistor of the present invention. The HEMT 1 comprises a buffer layer 10 formed on a single crystal substrate 101 made of SiC or sapphire.
2, the element layer 103 is formed by a heteroepitaxial growth method. Specifically, a well-known vapor phase growth method such as MOVPE (Metalorganic Vapor Phase E) is used.
pitaxy: Metalorganic vapor phase epitaxial growth method is used.

The element layer 103 is a Si-doped InGaN channel layer 10 from the side close to the buffer layer 102.
9, an AlGaN electron supply layer 110 n-doped with Si or the like, and an n-type GaN layer 111 functioning as a contact layer with an electrode are laminated in this order. Then, the drain electrode 106 and the source electrode 107 are formed on the n-type GaN layer 111, and the n-type GaN layer 1 is formed.
The gate electrode 108 is formed on the n-type AlGaN layer 110 exposed in the non-formation region 11. Drain electrode 10
6 and the source electrode 107 are made of a metal (for example, Ti / Al) that forms an ohmic junction between the n-type GaN layer 111 and the n-type GaN layer 111, and the gate electrode 108 serves as the n-type AlGaN electron supply layer 1.
10 and a metal (for example, Pd / Au) forming a Schottky junction with each of them.

InGaN has a larger lattice constant than GaN, and the effective mass of electrons moving in the crystal is smaller than that of GaN. As a result, the theoretically estimated electron mobility is G
It becomes larger than aN. The composition of InGaN, the mole fraction of InN as _x, when the In x _{Ga 1 -x} N, in order to achieve a significant improvement in electron mobility, it is desirable to the x 0.05 or more. On the other hand, when x exceeds 0.5, the lattice mismatch with the n-type AlGaN electron supply layer 110 becomes serious and the performance is deteriorated, which is not preferable.

As shown in FIG. 2, between the n-type AlGaN electron supply layer 110 and the InGaN channel layer 109,
Due to the difference in electron affinity between the two, some electrons flow from the n-type AlGaN electron supply layer 110 into the InGaN channel layer 109, and an inverted triangular potential well is formed closer to the InGaN channel layer 109 than the heterojunction interface. . In this potential well, electrons are confined in a form that is spatially separated from donor impurities (Si in the electron supply layer 110), and thus 2DE that is not easily affected by impurity scattering.
The G layer is formed.

However, if point defects such as N vacancies or composition nonuniformity occur in the InGaN channel layer 109, electron scattering becomes remarkable, and even if the 2DEG layer is formed, a high-speed element is not always realized. It is not possible. In the present invention, by doping the InGaN channel layer 109 with Si, N which becomes an electron scattering factor as shown in FIG.
As a result of the pores being filled with Si and repaired, crystallinity is improved and electron scattering can be effectively suppressed.

The Si doping amount is 1 × 10 ¹⁶ / cm ³
It is preferable to adjust in the range of 1 × 10 ²⁰ / cm ³ . S
When the i doping amount is less than 1 × 10 ¹⁶ / cm ³ , the crystallinity improving effect is not remarkable, and conversely, the doping amount is 1 × 1.
When it exceeds 0 ²⁰ / cm ³ , the electron mobility rather decreases due to the influence of impurity scattering.

As shown in FIG. 9, the channel layer 10
When the concentration of Si doped in 9 increases, the electron mobility μ exhibits a maximum value corresponding to a certain doping amount Cp that is necessary and sufficient for repairing holes. On the other hand, since the excessively doped Si that does not contribute to the repair of vacancies behaves as a donor in InGaN, the electron concentration (which can be measured as the sheet carrier concentration Ns) that contributes to the current is higher than a certain Si concentration. It increases monotonically on the side. It is considered that the electron transport characteristics in the channel layer 109 are governed by the product μ · Ns of the electron mobility μ and the sheet carrier concentration Ns, and the larger this value, the larger the current can flow. As is clear from the figure, the maximum value of μ · Ns is at the position of the concentration Ck shifted to a higher concentration side than the concentration Cp at which μ is maximized. By doping Si in excess of the required concentration, a faster field effect transistor can be realized. Considering this point, the Si doping amount is 1 × 1.
It is more desirable to adjust in the range of 0 ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ³ .

As the impurities contributing to the improvement of the crystallinity of the channel layer, it is also possible to use, for example, C, Ge or the like other than Si.

Next, as a concrete means for improving the electron transport characteristics of the InGaN channel layer, the above-mentioned Si is used.
There are several possibilities other than doping. First, InG
aN tends to cause non-uniform concentration, particularly non-uniform concentration of In during vapor phase growth, and such a non-uniform concentration structure is likely to cause electron scattering along with the N vacancies described above. Therefore,
As in the HEMT 4 (reference invention) of FIG. 7, by forming the InGaN channel layer 129 as a thin layer having a layer thickness t of 10 nm or less, it is possible to suppress the occurrence of the nonuniform concentration region as described above, and Scattering can be made less likely to occur. However, if the layer thickness t is less than 5 nm, it means that 2DE
The electron confinement effect due to the G layer formation is impaired, and the electron transport property is rather deteriorated. Although the InGaN channel layer 129 is formed as a non-doped layer in FIG. 7, as in the HEMT 5 shown in FIG.
The i-doped InGaN channel layer 139 may be formed.

Further, in the HEMT 2 shown in FIG. 4,
MQW (Multip) in which the channel layer has a multiple quantum well structure
le Quantum Well) The channel layer 119. In the MQW channel layer 119, a plurality of Ins as shown in FIG. 6 are used instead of the triangular potential structure described above.
The GaN well layer 119a forms a 2DEG layer.
Barrier layers 119b made of a compound semiconductor having a band gap larger than that of the well layer 119a are formed on both sides of the well layer 119a in order to form a potential barrier between the well layer 119a and the well layer 119a. The barrier layer 119b is the well layer 1
In order to improve the electron confinement effect in 19a, it is preferable to use GaN or AlGaN, for example.

In order to realize a sufficient electron confinement effect, the well layer 119a is preferably adjusted to have a thickness t1 of 10 nm or less, preferably 3 nm or less. When the InGaN well layer 119a is thinned to this extent by adopting the multiple quantum well structure, the influence of defect generation and composition fluctuation, which are problems during InGaN growth, is reduced, and electron scattering is less likely to occur. You can Therefore,
The effect of high electron mobility peculiar to InGaN is effectively brought out, and a field effect transistor capable of coping with higher speed can be realized. However, the thickness t1 of the well layer 119a is set to 5 nm.
When the amount is less than the above, it becomes difficult to maintain the crystal structure for obtaining a band structure suitable for realizing the quantum well structure, and therefore it is not preferable.

If the thickness of the barrier layer 119b is less than 5 nm, it is difficult to maintain a crystal structure for obtaining a band structure suitable for realizing a quantum well structure, which is not preferable. On the other hand, when the thickness of the barrier layer 119b exceeds 20 nm, the wave function coupling between the adjacent well layers 119a becomes insufficient, and the multiple quantum well structure cannot be maintained.
The electron confinement effect becomes insufficient.

Although the InGaN well layer 119a is formed as a non-doped layer in FIG. 4, as in the HEMT 5 shown in FIG. 5, the Si-doped well layer 119 is formed.
a may be formed. Thereby, the electron transport characteristics of the MQW channel layer 119 can be further improved.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an embodiment of a first configuration of a field effect transistor of the present invention.

FIG. 2 is a diagram schematically showing the band structure of the heterojunction portion.

FIG. 3 is a diagram for explaining the action of Si doping on a channel layer.

FIG. 4 is a schematic diagram showing an embodiment of a second configuration of the field effect transistor of the present invention.

5 is a schematic diagram showing a modification of the field effect transistor of FIG.

FIG. 6 is a diagram schematically showing a band structure of an MQW channel layer.

FIG. 7 is a schematic diagram showing a field effect transistor of the present invention.

FIG. 8 is a schematic diagram showing a modified example of the first configuration of the field effect transistor of the present invention.

FIG. 9 is a schematic diagram showing the relationship between electron mobility of a channel layer and sheet carrier concentration.

[Explanation of symbols]

1, 2, 3, 4, 5 HEMT (field effect transistor) 110 n-AlGaN electron supply layer 104, 139 Si-doped InGaN channel layer 119 MQW channel layer 119a InGaN well layer

Claims (6)

[Claims] 1. A field effect transistor having an electron supply layer made of a compound semiconductor and a channel layer made of a compound semiconductor heterojunction to the electron supply layer, wherein the channel layer has an Si-doped InGaN layer. A field effect transistor characterized by the above. 2. The channel layer comprises one layer or two or more layers of I.
Each of the InGaN layers has a thickness of 10 including an nGaN layer.
The field effect transistor according to claim 1, wherein the field effect transistor has a thickness of not more than nm. 3. A field effect transistor having an electron supply layer made of a compound semiconductor and a channel layer made of a compound semiconductor heterojunction to the electron supply layer, wherein the channel layer includes an InGaN layer as a well layer. A field-effect transistor characterized by being configured as a quantum well layer. 4. The field effect transistor according to claim 3, wherein the InGaN layer is doped with Si. 5. The multiple quantum well layers are each formed of a plurality of Ins.
The field effect transistor according to claim 3 or 4, wherein the GaN layer and the GaN layer are laminated. 6. The In forming the multiple quantum well layer
The thickness of each GaN layer is 10 nm or less.
The field effect transistor according to claim 5, wherein each layer of the aN layer has a thickness of 20 nm or less.