JPH0864616A - Compound semiconductor device - Google Patents

Abstract

PURPOSE: To inhibit a decrease in mutual conductance, when a gate voltage is in the vicinity of a threshold voltage, by using a material for a first active layer which has a larger degree of electron migration than that for a second active layer as the first and the second active layers, in which electrons move, are formed in that order. CONSTITUTION: An intermediate layer 12 comprising GaAs of 1.5μm and a buffer layer 13 comprising an AlGaAa film, which is 200nm thick, to inhibit electrons from seeping to the substrate side are formed on a Si substrate 11. And an active layer, in which the electrons move, to be formed on the buffer layer 13 is made of a two-layer structure. For a material of a first active layer 14 in contact with the buffer layer 13, InGaAs which has a large degree of electron migration as compared to a second active layer 15 is used. In the vicinity of a threshold voltage, as a result of an expansion of a depletion layer 19 from a gate electrode 16, an electron i runs concentratedly through the first active layer 14 comprising InGaAs. Consequently, a conventional phenomenon where the degree of migration decreases in this vicinity to cause a substantial reduction of mutual conductance can be inhibited.

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, and more particularly to a compound semiconductor device having a field effect transistor.

[0002]

2. Description of the Related Art A power FET used as a microwave device for satellite communication or mobile communication is required to have a high operating frequency, a long life, high power efficiency, and a small size. ing. A compound semiconductor field effect transistor was manufactured in consideration of such requirements, and one of them is a GaAs MESFET (Metal-Semico
nductor Field Effect Transistor).

Conventionally commonly used GaAs MESF
The ET is the same on the GaAs substrate 1 as shown in FIG.
An intermediate layer of GaAs 2, an AlGaAs layer 3 of a buffer layer, and a Si-GaAs layer 4 of an electron transit layer are sequentially formed.
A gate electrode 5 made of WSi made of WSi is formed on the aAs layer 4, and source / drain electrodes 6, 7 made of AuGe / Au made of AuGe / Au are formed on both sides of the gate electrode 5. Referred to as MESFET).

The AlGaAs layer 3 serving as a buffer layer is a layer provided to prevent electrons from seeping out to the substrate side due to the electric field generated by the voltage applied between the electrodes.
Although the GaAs MESFET having such a structure has already been put into practical use at present, attempts have been made to further reduce the cost due to the recent increase in demand. An example of this is shown in FIG.
It shows in (b). The structure of this GaAs MESFET is almost the same as that shown in FIG. 4A, but the cost is greatly reduced by using an inexpensive Si substrate instead of the GaAs substrate as the material of the substrate. (Hereinafter, this structure is referred to as Si substrate MESFET).

[0005]

However, the Si substrate MESFET shown in FIG. 4 (b) above is a GaAs substrate MES.
Although it is cheaper than the FET, the device characteristics are deteriorated. This point will be described below. Figure 5 is ME
The relationship between the gate-source voltage Vgs (V) and the mutual conductance Gm (mS / mm) in the SFET is shown in the Si substrate M.
It is a graph which shows the experimental result which examined about each ESFET and GaAs substrate MESFET.

According to this graph, Si substrate MESFET
Shows that the transconductance Gm decreases more in the vicinity of the threshold voltage Vth than in the GaAs substrate MESFET. Therefore, since the cutoff frequency depending on the transconductance Gm is also reduced, the Si substrate MESFET is
There is a problem that the operation becomes slower than that of the GaAs substrate MESFET.

The present invention has been made in view of the above problems, and provides a compound semiconductor device capable of reducing the cost of an electronic device and increasing the speed as compared with a conventional Si substrate MESFET. The purpose is to

[0008]

[Means for Solving the Problems]
As illustrated in FIG. 2, a buffer layer formed on a silicon substrate to suppress leaching of electrons into the substrate, a first active layer sequentially formed on the buffer layer, and on which electrons travel, A second active layer; a gate electrode formed on the second active layer and in Schottky contact with the second active layer; and a second active layer formed on both sides of the gate electrode. A compound semiconductor device having a source / drain electrode in ohmic contact, wherein the second active layer is made of a material having a higher electron mobility than the first active layer. .

The compound semiconductor device is characterized in that part or all of the buffer layer has a superlattice structure. The second active layer is Si-doped GaAs, and the first active layer is Si-doped InGaAs;
In addition, the problem is solved by a compound semiconductor device characterized in that the InAs content is 5% or more and 20% or less and the Si concentration is 5 × 10 ¹⁶ cm ⁻³ or more and 5 × 10 ¹⁷ cm ⁻³ or less.

The first active layer is solved by a compound semiconductor device having GaAsSb.

[0011]

[Operation] According to the consideration of the inventor of the present invention, many crystal defects called dislocations are generated in GaAs grown on a Si substrate, but the transconductance Gm is greatly reduced in the vicinity of the threshold voltage Vth which has been conventionally generated. The phenomenon was confirmed to be due to this dislocation. In general, the electron density n is represented by the formula: n = Ncexp {-(Ef-Ec) / kT}. Here, Nc is the density of states, Ef is the Fermi energy, and Ec is the energy of the conduction band.

In the MESFET, the electron concentration decreases near the gate voltage near the threshold voltage Vth, but in such a low electron density state, the Fermi energy, that is, the electron energy becomes low, as is apparent from the above equation. There is. FIG. 2A is a result of a simulation regarding the relationship between electron energy in a crystal having dislocations, that is, electron density and mobility.

From FIG. 2B, it can be seen that the electron mobility decreases as the electron energy decreases due to electron scattering caused by the dislocation of electrons in the crystal. Since the transconductance Gm is an amount that is approximately proportional to the mobility, it is considered that the decrease in the mobility causes the decrease in the transconductance Gm.

In the vicinity of the threshold voltage Vth, as shown in FIG. 1, electrons are concentrated and flow near the AlGaAs layer which is the buffer layer in the Si-GaAs layer which is the traveling layer. Therefore, in the present invention, as described above, two active layers in which electrons travel are provided, and as the material of the first active layer in contact with the buffer layer,
A material having a higher electron mobility than that of the second active layer is used.

Therefore, especially when the gate voltage is the threshold voltage Vth.
In the vicinity of the second, as shown in FIG.
Since electrons travel intensively in the active layer of, the reduction of the mutual conductance Gm of the MESFET can be suppressed as compared with the conventional Si substrate MESFET. As a result, M depending on the mutual conductance Gm
It is possible to prevent the cutoff frequency of the ESFET from decreasing and the operating speed of the ESFET from decreasing.

In the present invention, InGaAs is used as an example of the first active layer. Since this InGaAs has a smaller effective mass than GaAs, the mobility of electrons is large, so
It is suitable as a material for the first active layer. Further, in the present invention, as shown in FIG. 2A, the buffer layer for suppressing the seepage of electrons has a superlattice structure.

As a result, as shown in FIG. 2B, it is possible to prevent dislocations generated on the substrate from advancing upward, so that the active layer is simply made to have a two-layer structure and has a large mobility. Compared with the case where the material is used for the first active layer in contact with the buffer layer, the number of dislocations in the compound semiconductor device is small, so that the characteristics of the device can be further improved.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1A shows an ME according to the first embodiment of the present invention.
It is sectional drawing which shows the structure of SFET. As shown in FIG. 1, the MESFET according to the first embodiment of the present invention includes a Si substrate 11
An intermediate layer 12 made of GaAs having a thickness of 1.5 μm and a thickness of 2
The buffer layer 13 is made of AlGaAs of 00 nm and prevents the electrons from seeping out to the substrate side by the electric field generated by the voltage applied between the electrodes, and the Si-doped film thickness 100
nm, a first active layer 14 made of InGaAs, and a second active layer 15 made of Si-doped GaAs having a thickness of 400 nm are sequentially formed, and a Schottky junction made of WSi is formed on the second active layer 15. The gate electrode 16 is formed, and AuGe is formed on both sides of the gate electrode 16.
Source / drain electrode 1 of ohmic junction made of / Au
It is a Si substrate MESFET in which 7, 18 are formed.

According to the MESFET of this embodiment, as shown in FIG. 1, the active layer formed on the buffer layer in which electrons travel has a two-layer structure, and the first active layer 14 in contact with the buffer layer is formed. As the material of InP, InGaAs, which is a material having a higher electron mobility than that of the second active layer 15, is used. In the vicinity of the threshold voltage Vth where the transconductance Gm has been remarkably reduced in the past, the depletion layer 1 spreading from the gate electrode 1
As shown in FIG. 1, the electrons i intensively flow in the second active layer 15 made of InGaAs due to the spread of 9 so that the mobility is reduced in the vicinity and the mutual conductance Gm is significantly reduced as in the conventional case. It is possible to suppress that as much as possible.

The effect is shown in the graph of FIG. FIG. 2B shows a conventional Si substrate MESFET, GaAs substrate MESFET, and MESF according to the first embodiment of the present invention.
It is a graph which shows the experimental result which shows the relationship between gate-source voltage Vgs and transconductance Gm about three MESFETs of ET. As shown in FIG. 2 (b),
According to the MESFET of the first embodiment, although the degree of decrease in the transconductance Gm near the threshold voltage is larger than that of the GaAs substrate MESFET, the conventional Si substrate MES is used.
It can be seen that it is smaller than the FET.

Therefore, as compared with the conventional Si substrate MESFET, the mobility decreases in this vicinity and the transconductance G is reduced.
The degree of decrease of m is small. As a result, even when the gate voltage is near the threshold voltage Vth, it is possible to prevent the cut-off frequency of the element depending on the transconductance Gm from being remarkably reduced, as compared with the conventional Si substrate MESFET, so that the reduction is remarkable. The MESFET
It is possible to suppress the decrease in the operating speed of the device as much as possible.

If the thickness of the first active layer 14 made of InGaAs is too thin, the depletion layer spreads from the buffer layer 13 side made of AlGaAs so that electrons do not flow into the first active layer 14 and the second active layer 14 does not flow. Since the electrons flow only in the active layer 15 of the above, the above-described effect of improving the mobility cannot be seen. The width of the depletion layer is about 500Å according to the result of the experiment conducted by the inventor of the present invention. Therefore, the film thickness is preferably 500Å or more (1000Å in this embodiment).

When the content of InAs in InGaAs in the first active layer 14 is 10% or more, lattice-like dislocations called crosshatch occur on the surface and the characteristics are deteriorated. The rate is preferably 10% or less. Further, the doping concentration of Si is set to 1.5 × 10 ¹⁸ cm ⁻³ in order to make it the same as that of the conventional structure.

(Second Embodiment) A MESFET according to the second embodiment of the present invention will be described below. Note that items common to the first embodiment will not be described because they overlap. FIG. 3A shows the MESF of the second embodiment of the present invention.
It is sectional drawing which shows the structure of ET. As shown in FIG. 3A, the MESFET according to the second embodiment of the present invention is a Si substrate 2
1, an intermediate layer 22 made of GaAs having a film thickness of 1.5 μm, a buffer layer 23 for preventing electrons from seeping out to the substrate side by an electric field generated by a voltage applied between the electrodes, and a Si-doped film A first active layer 24 made of InGaAs having a thickness of 100 nm and a second active layer 25 made of Si-doped GaAs having a thickness of 400 nm are sequentially formed, and a Schottky junction made of WSi is formed on the second active layer 25. Is a Si substrate MESFET having ohmic junction source / drain electrodes 27 and 28 made of AuGe / Au formed on both sides thereof.

The buffer layer 23 has a film thickness of 5 nm.
Of the first embodiment in that InAlGaAs and AlGaAs having a film thickness of 5 nm are alternately laminated 50 times.
The structure is significantly different from ET. MES according to the present embodiment
According to the FET, since the buffer layer 23 has the superlattice structure as described above, as shown in FIG.
It is possible to prevent dislocations generated on the Si substrate 21 from traveling upward and reaching the first and second active layers 14 and 15 as much as possible. Of the dislocations in the first and second active layers 14 and 15 of the compound semiconductor device, as compared with the MESFET of the first embodiment in which a material having a large size is used for the first active layer in contact with the buffer layer. Therefore, the characteristics of the device can be further improved as compared with the MESFET of the first embodiment.

(Other Embodiments) In the first and second embodiments described above, In is used as the material of the first active layer 14.
Although GaAs is used, the present invention is not limited to this, and if the material has a higher mobility than GaAs, the same effect is obtained, and GaAsSb, for example, also exhibits the same effect. Further, as a method of achieving the above-mentioned object, it is possible to use InGaAs for the entire active layer without forming the active layer into a two-layer structure.
Since the lattice constant is larger than As, if the film thickness becomes thicker, dislocations are newly generated in this operating layer, or three-dimensional growth occurs and the surface is roughened to deteriorate the characteristics. Without doing so, the active layer has a two-layer structure and the first
The active layer 14 is made of a material having high mobility.

[0027]

As described above, according to the present invention, two active layers in which electrons travel are provided as described above, and the second active layer is used as a material of the first active layer in contact with the buffer layer. Since a material having a higher electron mobility is used, the conventional gate voltage is close to the threshold voltage Vth.
It becomes possible to suppress the reduction of the mutual conductance Gm of the MESFET as compared with the Si substrate MESFET.

As a result, it is possible to prevent the cutoff frequency of the MESFET depending on the transconductance Gm from decreasing and the operating speed of the MESFET from decreasing.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a MESFET according to a first embodiment of the present invention.

FIG. 2 is a graph illustrating the function and effect of the MESFET according to the first embodiment of the present invention.

FIG. 3 is a sectional view illustrating a structure of a MESFET according to a second embodiment of the present invention.

FIG. 4 is a sectional view illustrating a conventional MESFET.

FIG. 5 is a graph illustrating a problem of a conventional MESFET.

[Explanation of symbols]

 11 Si Substrate 12 Intermediate Layer 13 Buffer Layer 14 First Active Layer 15 Second Active Layer 16 Gate Electrode 17 Source / Drain Electrode 18 Source / Drain Electrode 19 Depletion Layer 21 Si Substrate 22 Intermediate Layer 23 Buffer Layer 24 First Active layer 25 Second active layer 26 Gate electrode 27 Source / drain electrode 28 Source / drain electrode 29 Depletion layer

Claims (4)

[Claims] 1. A buffer layer formed on a silicon substrate for suppressing leaching of electrons into the substrate; a first active layer and a second active layer sequentially formed on the buffer layer, through which electrons travel. An active layer, a gate electrode formed on the second active layer and in Schottky contact with the second active layer, formed on both sides of the gate electrode and in ohmic contact with the second active layer. And a source / drain electrode, and the second active layer is made of a material having a higher electron mobility than the first active layer. 2. The compound semiconductor device according to claim 1, wherein a part or all of the buffer layer has a superlattice structure. 3. The second active layer is Ga doped with Si.
As, the first active layer is Si-doped InGaAs, and the InAs content is 5% or more and 20% or less,
3. The compound semiconductor device according to claim 1, wherein the concentration of Si is 5 × 10 ¹⁶ cm ⁻³ or more and 5 × 10 ¹⁷ cm ⁻³ or less. 4. The compound semiconductor device according to claim 1, wherein the first active layer contains GaAsSb.