JPH09283745A - High-electron mobility transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To raise the 2DEG(two-dimensional electron gas) density and mobility by forming a second channel layer between a carrier feed layer and first channel layer and specifying the relation of the conduction band ends of these layers. SOLUTION: Between an n-type AlGaAs carrier layer 26 and i-position InGaAs channel layer 22 an i-type GaAs channel layer 24 is formed, having a conduction band located in a band-discontinuous area between the conduction bands of both. The conduction band end Ec3 , of the channel layer 24 is esp. set to meet Ec1 >Ec3 >Ec2 where the conduction band end of the carrier layer 26 is Ec1 and the conduction band end of the channel layer 22 is Ec2 This raises the total 2DEG density since the 2DEG is also formed in the second channel layer, the i-type GaAs layer 24.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compound semiconductor device, particularly a high electron mobility transistor (HEMT).

[0002]

2. Description of the Related Art High electron mobility transistors (hereinafter referred to as HE)
It is called MT. ) Is a two-dimensional electron gas
electron gas; hereinafter may be referred to as 2DEG. )
Is a compound semiconductor device having a heterostructure. The structure of the conventional HEMT is described in, for example, the document “Properties of Lattice-Matched and Strained Indiu”.
mGallium Arsenide, p290, INSPEC (1993) ”. The configuration example of this document is a semi-insulating GaA.
This is a strained lattice (Pseudomorphic) HEMT structure using a strained InGaAs quantum well layer as a channel on an s substrate. InGa
On the As channel layer, non-doped AlGaAs (i-
AlGaAs) spacer layer and Si-doped AlGaA
The s (n-AlGaAs) carrier supply layers are sequentially provided, and the HEMT having such a structure is usually called a forward structure HEMT. In addition to this, a reverse structure type HEMT having a structure in which a carrier supply layer is provided under a channel layer
Or a double hetero type HEMT (double doped HEMT) having a structure in which carrier supply layers are provided above and below the channel layer.
Also called. ) Etc. are known. Inverse structure type H
The EMT has excellent pinch-off characteristics, and the double hetero type HEMT has a high carrier density and is suitable for high output.

[0003]

In many cases, a spacer layer is inserted between the carrier supply layer and the channel layer constituting the HEMT to enhance the mobility of two-dimensional electrons. For example, the carrier supply layer may be n-AlG.
In the case of the configuration example of the literature in which the aAs layer is used and the channel layer is the i-InGaAs layer, in order to make the doping selectivity of Si impurities between the carrier supply layer and the channel layer more effective, the spacer layer is formed of i. -AlGaAs layer is provided. By providing the spacer layer in this way,
Since the influence of Coulomb scattering due to impurities in the carrier supply layer can be further reduced, the mobility of two-dimensional electrons moving in the channel layer is increased. It is generally known that the mobility of two-dimensional electrons increases with the thickness of the spacer layer.

However, if the spacer layer is thickened for the purpose of increasing the mobility of two-dimensional electrons, there is a problem that the 2DEG density is reduced. This has been proved both theoretically and experimentally. This is the conduction band (and accompanying donor level) of the carrier supply layer that becomes the barrier.
Occurs because of approaching (or crossing) the Fermi level in the spacer layer. Therefore, even if an attempt is made to utilize high mobility, if the 2DEG density is low, it is difficult to form an ohmic contact and it is difficult to apply it to a device.

Therefore, the 2DEG density is higher than in the conventional case,
Moreover, the emergence of HEMTs exhibiting high mobility has been desired.

[0006]

According to the high electron mobility transistor of the present invention, in a high electron mobility transistor having a carrier supply layer and a first channel layer, the carrier supply layer and the first channel layer are provided. When the conduction band edge of the carrier supply layer is E _C1 and the conduction band edge of the first channel layer is E _C2 , a conduction band edge of the second channel layer is provided between The end E _C3 is set so that E _C1 > E _C3 > E _C2 .

Here, the conduction band edge means the energy level at the lower end (bottom) of the conduction band in each layer. Thus, the conduction band edges E _C1 and E _C2 of the carrier supply layer and the first channel layer, respectively.
A second channel layer having a conduction band edge E _C3 between the levels is provided between the carrier supply layer and the first channel layer. By doping the carrier supply layer with an appropriate amount of impurities in advance, the conduction band edge E _C3 (all or part) can be formed at a level lower than the Fermi energy. The electrons generated from the donor impurities in the carrier supply layer move to the first channel layer due to the electron affinity, and the conduction band edge E _C2
Accumulated in the potential well formed above, 2DEG
To form Further, since the potential well is also formed at the conduction band edge E _C3 of the second channel layer, the electrons generated from the carrier supply layer also move to the second channel layer to form 2DEG. Therefore, the total number of two-dimensional electrons has increased, and therefore 2DE
The density of G increases.

According to a preferred configuration example of the HEMT of the present invention, the carrier supply layer is an n-AlGaAs layer, and the first channel layer is an i-InG having a strained quantum well structure.
a as layer, and the second channel layer is i-GaA
It is characterized in that it is an s layer.

As described above, n-A is used as the carrier supply layer.
1GaAs layer is provided, and i-InG is used as the first channel layer.
When the aAs channel layer is provided, the i-GaAs layer is provided as the second channel layer between the aAs channel layer, so that the second layer is provided between the conduction band edges E _C1 and E _C2 of the carrier supply layer and the first channel layer, respectively. An energy state is obtained such that there is a conduction band edge E _{C3 of the} channel layer. Further, in this case, it is preferable that the film thickness of the second channel layer is 10 to 100Å.

The 2DEG formed in the first channel layer is
Since it is spatially separated from the ionized donor impurities in the carrier supply layer by the second channel layer, it is less affected by the ionized impurity scattering and exhibits a high mobility as in the past. However, since 2DEG formed in the second channel layer is generated in a region close to the carrier supply layer, it is insufficiently spatially separated from impurities and exhibits a relatively low mobility. Also, in terms of physical properties, electron mobility of GaAs is lower than that of InGaAs, so that 2DEG formed in the second channel layer has lower mobility than 2DEG formed in the first channel layer. 2DE of the whole
The mobility of G is expressed by the average of these, and as the film thickness of the second channel layer increases, the 2DEG of the first channel layer is less and less affected by impurities, so the total 2D
Although the mobility of EG increases, 2DEG of the second channel layer
Since the influence of impurities is strongly felt, the mobility of the entire 2DEG starts to decrease when the thickness of the second channel layer exceeds a certain level. Therefore, the second channel layer (i-
The desired mobility and density can be obtained by setting the film thickness of the GaAs layer) to an appropriate value (10 to 100Å).

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. Note that the drawings are merely schematic representations such that the shape, size, and arrangement relationship of the configuration of the present invention can be understood, and the numerical conditions and the like described below are merely examples. The invention is not limited to this embodiment.

FIG. 1 is a sectional view showing the structure of this embodiment. The configuration example of this embodiment is a HEMT having a double hetero structure, which includes two carrier supply layers. As the base 10, a semi-insulating GaAs substrate 12, i-
It has a structure in which a GaAs buffer layer 14 and an i-AlGaAs buffer layer 16 are sequentially stacked, and a HEMT structure is formed above the underlayer 10. First,
n-AlGa is formed on the i-AlGaAs buffer layer 16.
The As carrier supply layer 18 is laminated, and is referred to as i-
AlGaAs spacer layer 20, i-InGaAs channel layer 22, i-GaAs channel layer 24 and n-Al
The GaAs carrier supply layer 26 is sequentially stacked.
Thus, in the configuration example of this embodiment, the i-InGaAs channel layer 22 is used as the first channel layer, the i-GaAs channel layer 24 is used as the second channel layer, and the n-AlGaAs carrier supply layer is used as the carrier supply layer. Two
This is a configuration including 6. The first and second channel layers 22 and 24 have a strained quantum well structure.

Each of the above layers is a semi-insulating GaAs substrate 12
Is formed on the upper side by a molecular beam epitaxy (MBE) method. The film thickness of each layer is set as follows.

I-GaAs buffer layer 14 1000Å i-AlGaAs buffer layer 16 1000Å n-AlGaAs carrier supply layer 18 100Å i-AlGaAs spacer layer 20 40Å i-InGaAs channel layer 22 100Å i-GaAs channel layer 24 50Å n-AlGaAs carrier Supply layer 26 500 Å Incidentally, n-AlGaAs carrier supply layer 18 and n-A
The impurity doping amount of the 1 GaAs carrier supply layer 26 is 3 ×
It is set to 10 ¹⁸ cm ^-3 . In addition, each AlGaAs described above
When the Al composition of the layers 16, 18, 20 and 26 is expressed as Al _x Ga _1-x As due to the limitation of the DX center, x =
0.28, and the In composition of the InGaAs layer 22 is expressed as In _x Ga _1-x As from the lattice matching relationship, x =
It is set as 0.2.

Here, the DX center is Al _x Ga _1-x.
When As is doped with impurities such as Si to form an n-type semiconductor, in the range of x larger than x = 0.3, the donor level becomes gradually deeper as the energy level at the bottom of the conduction band increases as x increases. This is the phenomenon of going.

When the composition is set as described above, in order to grow the InGaAs layer 22 in a good crystal state without causing a misfit transition, the thickness of the InGaAs layer 22 is 100Å as described above ( The limit is 150Å.). Further, in order to prevent 2DEG from being generated in the i-GaAs buffer layer 14, i-AlGa is used.
The film thickness of the As buffer layer 16 is 1000 Å, which is sufficiently large, and the distance between the i-GaAs layer 14 and the n-AlGaAs layer 18 is sufficiently large.

Further, in order to form the source electrode 28 and the drain electrode 30 so as to form ohmic contact, an n ⁺ -GaAs cap layer 32 of 1000 Å is provided between these electrodes 28 and 30 and the n-AlGaAs carrier supply layer 26. I have inserted it. The n ⁺ -GaAs cap layer 32 is doped with impurities of 4 × 10 ¹⁸ cm ⁻³ . The electrodes 28 and 30 are made of AuGe / Ni / Au.
A layer (a layer in which AuGe, Ni, and Au are laminated in this order) is used. In addition, H that operates at a desired threshold value
In order to form the EMT, an n ⁺ -GaAs cap layer 32 is formed in the region between the source electrode 28 and the drain electrode 30.
To the middle of the n-AlGaAs carrier supply layer 26, a recess structure is formed, and a Ti / Al layer (a layer in which Ti and Al are laminated in this order) is provided as the gate electrode 34 there.

FIG. 2 is a diagram showing an energy state of a configuration example of this embodiment. In the drawing, the height of energy is taken in the vertical direction in the drawing, and the region of each layer 12 to 26 constituting the HEMT is taken in the horizontal direction in the drawing. The solid line a represents the conduction band edge E _C of each of the layers 12 to 26, and the broken line b represents the Fermi level E _F. Especially,
The conduction band edge of the n-AlGaAs carrier supply layer 26 is represented by E _C1 , and the i-InGaAs channel layer 2
The edge of the conduction band of 2 is designated E _C2 , and i-
The conduction band edge of the GaAs channel layer 24 is set to E
It is represented by _C3 . The broken line c represents the density of 2DEG generated in the first channel layer 22 by the height in the vertical direction in the figure, and the broken line d represents 2 generated in the second channel layer 24.
The density of DEG is represented by the height in the vertical direction in the figure.

As described above, the conduction band edge E _C3 of the second channel layer 24 is set so that E _C1 > E _C3 > E _C2 , so that the second channel layer i-
2DEG is also formed in the GaAs channel layer 24. To make it easier for 2DEG to form, E _C1
The energy difference between E _C3 and E _C3 (and E _C2 ) is desired to be set large, but for this purpose, the In composition ratio x of the i-InGaAs layer may be set high. However, because of the problem of misfit transition as described above, x = 0.2-
It is better to set it in the range of 0.3.

FIG. 3 is a graph showing the dependence of the mobility and density of the 2DEG generated in this embodiment on the film thickness of the i-GaAs channel layer 24. The horizontal axis of the figure is i
-The thickness of the GaAs channel layer 24 is 0 to 100 in units of Å.
, The mobility is 10 on the vertical axis on the left side of the figure.
The range is 0 to 10 in ³ cm ² · V ⁻¹ · s ⁻¹ unit, and the vertical axis on the right side of the figure indicates the density in 0 to 10 ¹² cm ⁻² unit.
It is taken in the range of 4. In each data of the configuration example of the embodiment, the mobility is plotted on the graph with the symbol ◯, and the density is plotted on the graph with the symbol □. In the figure, for comparison, a conventional configuration example (i.e., in the configuration example of the embodiment, i-
The film thickness dependence of the mobility and density of 2DEG in a configuration example in which an i-AlGaAs spacer layer is provided instead of the GaAs channel layer 24 is shown. Each data of the conventional configuration example is plotted on the graph by the symbol Δ for mobility and the symbol ▽ for density. These measurements were performed at room temperature (25 ° C). In addition, the measured value is set to the i-GaAs channel layer 24 or i-
The relationship between the film thickness of the AlGaAs spacer layer and the mobility and density is shown in Table 1. In addition, the measurement is i-GaA.
The film thickness of the s channel layer 24 was set to 50Å and 100Å, and the film thickness of the i-AlGaAs spacer layer was set to 10Å, 20Å and 40Å. Also,
The mobility and the density are measured even when the i-GaAs channel layer 24 or the i-AlGaAs spacer layer is not provided, and the results are shown in Table 1 and FIG. 3 as the measurement results when the film thickness is 0Å. .

[0021]

[Table 1]

As is apparent from FIG. 3 and Table 1, i-
When the thickness of the GaAs channel layer 24 is 0 to 50Å, the mobility of 2DEG is 5.66 × 10 ³ cm ² · V ⁻¹ · s.
⁻¹ to 6.93 × 10 ³ cm ² · V ⁻¹ · s ⁻¹ , which shows the same tendency as the film thickness dependence of the i-AlGaAs spacer layer in the conventional structure. To be understood. On the other hand, when the embodiment (symbol □) is compared with the conventional (symbol ▽) for the density of 2DEG, for example, when the film thickness of the i-AlGaAs spacer layer is 40 Å, the density is 3.01 × 10 ¹² cm 2. ^-2 , while i
It is 3.31 × 10 ¹² cm ⁻² when the film thickness of the −GaAs channel layer 24 is 50 Å, and it can be seen that the density decrease is smaller in the embodiment.

The measurement results of FIG. 3 (and Table 1) are qualitatively explained as follows. In this embodiment, n−
Instead of the conventional i-AlGaAs spacer layer, an i-GaAs channel layer 24 having a conduction band between the band discontinuity of the conduction bands of the AlGaAs carrier supply layer 26 and the i-InGaAs channel layer 22 is provided. Provided. The i-GaAs channel layer 24 functions like a conventional spacer layer for the i-InGaAs channel layer 22. Therefore, the mobility of 2DEG increases as the thickness of the i-GaAs channel layer 24 increases. However,
Since 2DEG is also formed in the i-GaAs channel layer 24, if the film thickness of this layer is made too large (50 Å or more in this embodiment), it is relatively low formed in the i-GaAs channel layer 24. The 2DEG of mobility is reflected in the overall mobility, and the mobility of all 2DEG becomes low.

On the other hand, 2DEG is not formed in the conventional i-AlGaAs spacer layer, but since 2DEG is formed in the i-GaAs channel layer 24 of the configuration example of this embodiment, compared to the conventional configuration example. 2DEG density is high. In this way, 2DE as a carrier
Since the density of G is increased, the high frequency characteristics of the HEMT element are increased, and the HEMT element can be used for high output. In consideration of the above-mentioned mobility, it is preferable to set the film thickness of the i-GaAs channel layer 24 in the range of 10 to 100Å in the configuration example of this embodiment, for example, when the film thickness is 50Å. Mobility is 6.93 × 10
^{It is 3} cm ² · V ⁻¹ · s ⁻¹ and the density is 3.31 × 10 ¹²
cm ^-2 .

In this embodiment, the double hetero structure is used as an example of the structure of the HEMT, but the present invention is not limited to this, and the same effect can be obtained even if the structure is a forward structure type or an inverse structure type.

FIG. 4 is a sectional view showing the structure of a forward structure HEMT. In this configuration example, the carrier supply layer is nA.
The I-GaAs channel layer 2 is provided below the carrier supply layer 26.
4 and the i-InGaAs channel layer 22 is provided thereunder. In addition, i-InGa
The As channel layer 22 is formed on the i-GaAs buffer layer 14. Further, a source electrode 28 and a drain electrode 30 are provided on the n-AlGaAs carrier supply layer 26 via an n ⁺ -GaAs cap layer 32, and a recess structure between these electrodes 28 and 30 is formed. Where the gate electrode 34 is n-AlGa
It is provided in contact with a part of the As carrier supply layer 26. The structure described above is formed on the upper side of the semi-insulating GaAs substrate 12.

The double heterostructure type HEM described above
Compared with the configuration example of T, the number of carrier supply layers is smaller, and therefore the density of 2DEG is expected to be reduced accordingly. However, the conventional forward structure HEMT (i-
A HEMT having a configuration in which an i-AlGaAs spacer layer is provided instead of the GaAs channel layer 24. ), The i-GaAs channel layer 24 is
2DEG is also formed, so the entire 2D
The density of EG is high.

FIG. 5 is a sectional view showing the structure of an inverted structure HEMT. In this configuration example, n-AlG is formed on a base 10 (semi-insulating GaAs substrate 12, i-GaAs buffer layer 14 and i-AlGaAs buffer layer 16).
The aAs carrier supply layer 26 is formed, and i is formed thereon.
The -GaAs channel layer 24, the i-InGaAs channel layer 22 and the n-GaAs Schottky contact layer 21 are provided in this order. And n-G
n ⁺ -G is formed on the aAs Schottky contact layer 21.
A source electrode 28 and a drain electrode 30 are provided via an aAs cap layer 32, and a gate electrode 34 is connected to an n-GaAs Schottky contact in a region where a recess structure is formed between these electrodes 28 and 30. It is provided in contact with part of the layer 21.

As described above, it is generally known that the pinch-off characteristic is excellent when the reverse structure type is adopted.
Even in the case of the inverted structure type, by providing the i-GaAs channel layer 24 at the position where the i-AlGaAs spacer layer should be provided in the related art, 2DEG can be generated in the i-GaAs channel layer 24. It is possible to increase the density of 2DEG.

[0030]

According to the HEMT of the present invention, by providing the second channel layer instead of the conventional spacer layer, 2DEG is also formed in this layer, and as a result, the entire 2D
It is possible to increase the density of EG as compared with the conventional one. Further, by appropriately setting the range of the film thickness of the second channel layer, 2DEG having high mobility and high density can be obtained.
Therefore, it is possible to obtain a remarkable effect that the element characteristics (for example, high frequency characteristics) of the HEMT are improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment.

FIG. 2 is a diagram showing an energy state of the embodiment.

FIG. 3 is a diagram showing the dependence of the mobility and density of 2DEG on the film thickness of the i-GaAs channel layer 24.

FIG. 4 is a diagram showing a configuration of an embodiment.

FIG. 5 is a diagram showing a configuration of an embodiment.

[Explanation of symbols]

10: Underlayer 12: Semi-insulating GaAs substrate 14: i-GaAs buffer layer 16: i-AlGaAs buffer layer 18: n-AlGaAs carrier supply layer 20: i-AlGaAs spacer layer 21: n-GaAs Schottky contact layer 22: i-InGaAs channel layer 24: i-GaAs channel layer 26: n-AlGaAs carrier supply layer 28: source electrode 30: drain electrode 32: n ⁺ -GaAs cap layer 34: gate electrode

Claims (3)

[Claims] 1. A high electron mobility transistor comprising a carrier supply layer and a first channel layer, wherein a second electron mobility transistor is provided between the carrier supply layer and the first channel layer.
A channel layer is provided, the conduction band edge of the carrier supply layer is E _C1, and the conduction band edge of the first channel layer is E _{C1.
When C2} , the conduction band edge E _C3 of the second channel layer is
A high electron mobility transistor characterized in that E _C1 > E _C3 > E _C2 is set. 2. The high electron mobility transistor according to claim 1, wherein the carrier supply layer is an n-AlGaAs layer, the first channel layer is an i-InGaAs layer having a strained quantum well structure, and the second A high electron mobility transistor characterized in that a channel layer is an i-GaAs layer. 3. The high electron mobility transistor according to claim 2, wherein the film thickness of the second channel layer is 10 to 100 Å.