JP2003059946A - GaN-based semiconductor device - Google Patents

Abstract

[PROBLEMS] To provide a GaN-based semiconductor device having a HEMT structure, which has a low on-resistance during operation and can operate at a large current. SOLUTION: The whole is made of a GaN-based semiconductor material,
A layer structure of a lower layer 3 made of the first undoped material and an upper layer 4 made of the second undoped material having a band gap energy larger than that of the first undoped material is formed on the substrate 1. , On which a gate electrode G, a source electrode S, and a drain electrode D are formed
In the system semiconductor device A, the thickness of the upper layer 4 in the formation region 4A of the source electrode S and the drain electrode D is
A GaN-based semiconductor device thinner than the thickness in B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a GaN-based semiconductor device, and more specifically, H made of a GaN-based semiconductor material.
The present invention relates to a GaN-based semiconductor device that has an EMT structure and has a significantly reduced on-resistance during operation as compared with the related art, and is capable of high-current operation.

[0002]

2. Description of the Related Art GaN, InGaN, AlGaN, Al
GaN-based semiconductor materials such as InGaN are, for example, GaA.
Compared to s-based materials, its bandgap energy is large, its heat resistance is high, and its high-temperature operation is excellent.
Using these materials, especially GaN, field effect transistors (FETs) and high mobility transistors (High Electorn Mobility Transistors).
Research and development of electronic devices such as HEMTs is underway.

An example of a GaN type HEMT structure is shown in FIG. In this HEMT structure, on the semi-insulating substrate 1 such as a sapphire substrate, the buffer layer 2 made of, for example, GaN, the undoped GaN layer 3, and the undoped A that is extremely thin as compared with the undoped GaN layer 3 are used.
A layered structure is formed by sequentially stacking the lGaN layers 4.

On the undoped AlGaN layer 4, a gate electrode G, a source electrode S and a drain electrode D are arranged in a plane. In that case, the gate electrode G
Are directly formed on the undoped AlGaN layer 4. However, the source electrode S and the drain electrode D are
In general, on the surface of the undoped AlGaN layer 4, once S, which is an n-type impurity, is temporarily formed in a region where these electrodes are formed.
A contact layer 5 of n-AlGaN is formed by doping i with a high concentration, and is placed on the contact layer 5. The reason is that these electrodes and undoped AlGa
This is because the resistance between the N layers 4 is reduced to reduce the on-resistance during operation to realize a large current operation.

The source electrode S and the drain electrode D are
It may be formed directly on the undoped AlGaN layer 4, but in this case, these electrodes and the undoped AlG layer 4 are formed.
Since there is a high resistance between the aN layers 4 and it is difficult to realize a large current operation, as described above, the contact layer 5 is formed between them.
The structure in which is interposed is common. In the HEMT structure shown in FIG. 5, the bandgap energy of undoped GaN is smaller than the bandgap energy of undoped AlGaN. And while undoped GaN is a single crystal, undoped AlGaN is a mixed crystal of AlN and GaN. Therefore, at the heterojunction interface between the two layers, a piezo electric field is generated by the piezo-piezoelectric effect due to crystal strain, and the two-dimensional electron gas layer 6 is formed immediately below the interface between the two.

The electron gas concentration in the two-dimensional electron gas layer formed at the heterojunction interface of the GaN-based material is 5 ×.
It is about 10 ¹⁸ to 1 × 10 ²⁰ / cm ³ , and this value is about 5 × 10 ¹⁷ to 1 × 10 ¹⁸ / cm ^{3 when} the electron gas concentration of the two-dimensional electron gas layer formed of a GaAs material is about 5 × 10 ¹⁷ to 1 × 10 ¹⁸ / cm ³ . Compared with some, the concentration is higher by one digit or more. This HE
In the MT structure, when the source electrode S and the drain electrode D are activated, the undoped AlGaN layer 4 functions as an electron supply layer and supplies electrons to the undoped GaN layer 3.
The supplied electrons move at high speed by the action of the two-dimensional electron gas layer 6 and travel to the drain electrode D. At this time, by operating the gate electrode G to generate a depletion layer having a desired thickness immediately below the gate electrode G, various modulation operations can be realized in this HEMT structure.

[0007]

By the way, in the case of the HEMT structure shown in FIG. 5, a HEM in which the source electrode S and the drain electrode D are directly formed on the undoped AlGaN layer 4.
Although the on-resistance during operation is smaller than in the case of the T structure, a high resistance undoped AlGaN layer 4 is interposed between these electrodes and the two-dimensional electron gas layer with a predetermined thickness. Therefore, there is a limit to the reduction of the on-resistance during operation.

Further, the contact layer 5 is usually formed by selective growth, but if it is possible to manufacture a HEMT structure in which the ON resistance during operation is lowered without performing this selective growth, it is industrially possible. The merit will be great. The present invention solves the above-mentioned problems in the conventional HEMT structure shown in FIG. 5 and has a novel HEMT structure having a novel HEMT structure capable of realizing a reduction in ON resistance during operation without forming a contact layer. An object is to provide a semiconductor device.

[0009]

The inventors of the present invention have found that the two-dimensional electron gas layer formed at the heterojunction interface of a GaN-based material has a high electron gas concentration and, for example, undoped AlGa.
At the heterojunction interface between N and undoped GaN, the thinner undoped AlGaN becomes, the more undoped GaN
Attention was paid to the phenomenon that the resistance decreases because the carriers in the undoped layer effectively increase due to the seeping-out effect of the two-dimensional electron gas layer inside.

Then, if the undoped AlGaN layer in the region where the source electrode and the drain electrode are formed is selectively thinned, that region may exhibit the same function as a conventional contact layer. Having an idea, various experiments were repeated and the correctness of the idea was confirmed, and the GaN-based semiconductor device of the present invention was developed. That is, the GaN-based semiconductor device of the present invention is entirely composed of a GaN-based semiconductor material, and is composed of a lower layer made of the first undoped material and a second undoped material having a bandgap energy larger than that of the first undoped material. A layer structure with an upper layer is formed on the substrate, and on the surface of the upper layer,
In a GaN-based semiconductor device in which a gate electrode, a source electrode, and a drain electrode are formed, a thickness of the upper layer in a region where the source electrode and the drain electrode are formed is smaller than a thickness in other regions. It is characterized by

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION 1 of the GaN-based semiconductor device of the present invention
Example A is shown in FIG. This device A comprises a buffer layer 2 made of, for example, GaN on a semi-insulating substrate 1, a first layer described later.
Of the undoped material and the upper layer 4 of the second undoped material are sequentially stacked to form a layered structure. The entire surface is covered with a protective film 7 such as SiO ₂ .

The partial region 4A of the upper layer 4 is thinner than the other region 4B, and the source electrode S and the drain electrode D are formed in the thin region 4A, and the thicker region 4B is formed. A gate electrode G is formed on the. The first undoped material of the lower layer 3 and the second of the upper layer 4
All of the undoped materials are GaN-based semiconductor materials. In that case, as the second undoped material and the first undoped material, the former bandgap energy (E
The material of g) is selected larger than that of the latter, and both are used in combination. As a result, lower layer 3 and upper layer 4
The two-dimensional electron gas layer 6 is formed immediately below the heterojunction interface of, that is, at the uppermost part of the lower layer 3.

As the first undoped material of the lower layer 3,
Usually, GaN (Eg = 3.40 eV) is used. At that time, as the second undoped material of the upper layer 4, for example, AlGaN (Eg = 4.16 eV), AlInGaN (E
g = 3.8 eV), AlGaNAs (Eg = 4.5 eV), A
lGaNP (Eg = 4.2 eV), AlGaInNAsP
(Eg = 4.0 eV), AlGaNAsP, etc. can be used.

The first undoped material is not limited to GaN and may be various GaN-based mixed crystals. In that case, the second undoped material has more Eg than the mixed crystal. It is necessary to use large GaN-based semiconductor materials. Here, the thickness of the lower layer 3 is not particularly limited, but is usually set to about 1000 to 4000 nm.

Further, the electrode forming region 4A in the upper layer 4 is formed.
The thickness of the two-dimensional electron gas layer 6 depends on the relationship between the constituent material of the upper layer and the constituent material of the lower layer 3 combined with the upper layer.
It is appropriately determined in relation to the level of electron gas concentration in (3) and crystal defects. If the thickness of the formation region 4A is too thin, a film defect occurs and the continuous two-dimensional electron gas layer 6 does not proceed. On the contrary, if the thickness is too thick, the undoped layer may have a high resistance. The thickness of the formation region 4A is preferably set to about 1 to 15 nm.

When the thickness of the formation region 4A is within the above range, the condition of the two-dimensional electron gas layer 6 immediately below the formation region 4A is good, and the electron gas concentration thereof is also high, so that it is sufficient between the electrode and the lower layer 3. Functions as a contact layer. This device A can be manufactured as follows. First, on the semi-insulating substrate 1, a gas source molecular beam epitaxial growth method (GSMBE) or a metal organic chemical vapor deposition method (M
A layer structure A ₀ shown in FIG. 2 is manufactured by sequentially forming a buffer layer 2 made of, for example, GaN, a lower layer 3 made of, for example, undoped GaN 3, and an upper layer 4 made of, for example, undoped AlGaN by a crystal growth method such as OCVD. To do.

Here, a sapphire substrate is usually used as the substrate 1, but SiC, GaAs, Si, Ga are used.
It may be a substrate such as N. Then, for example, SiO ₂ is deposited on the entire surface of the layer structure A ₀ , patterned with a resist there, and further subjected to a reactive ion beam etching method (RIBE) in a region other than the region where the gate electrode is to be formed. Dry etching is performed to the depth to be used.

As a result, as shown in FIG. 3, the layer structure A ₁ having the upper layer 4 in which the region 4A having a desired thickness and the region 4B which is not etched are formed is manufactured. Then, after removing the resist of the layer structure A ₁ and the SiO ₂ film,
Again, for example, a SiO ₂ film is formed on the entire surface, the SiO ₂ film is patterned with a resist, openings are formed at the locations where the source electrode and the drain electrode are to be formed, and an electrode material is deposited there by, for example, a sputtering method.

As a result, as shown in FIG. 4, the layer structure A ₂ in which the source electrode S and the drain electrode D are formed in the region 4A of the upper layer 4 is manufactured. Note that Ta-Si, Al / Ti, Ti / Au, or the like can be used as the electrode material. All of them make ohmic contact with the upper layer 4. Further, it is preferable to heat-treat the layered structure A _{2 in,} for example, an N ₂ gas atmosphere furnace after forming the source electrode and the drain electrode. This is because the ohmic contact with the upper layer of these electrodes becomes better. At that time, the heat treatment temperature is preferably set to 400 to 800 ° C. and the treatment time is preferably set to 5 to 30 minutes.

Finally, again, for example, SiO _{2 is formed on the} entire surface.
After forming a film, the device A shown in FIG. 1 is manufactured by forming a gate electrode on the upper region 4B in the same manner as described above. In the above series of steps,
InGaN or GaN is further formed on the surface of the region 4A of the layer structure A _1.
When a thin layer such as is formed, the layer structure A ₂ shown in FIG.
Source electrode S and drain electrode D and region 4A in
The ohmic contact between and becomes even better.

[0021]

EXAMPLE The device A shown in FIG. 1 was manufactured as follows. On the sapphire substrate 1, radicalized nitrogen (3 × 1
0 ^-6 Torr), Ga metal (5 × 10 ^-7 Torr), Si metal
(5 × 10 ⁻⁹ Torr) is used to form a buffer layer 2 of n-GaN having a thickness of 50 nm at a growth temperature of 640 ° C. by the GSMBE method, and further, a metal Ga (1 × 10 ⁻⁶ Tor) is formed thereon.
rr) and ammonia (5 × 10 ⁻⁵ Torr) were used to form an undoped GaN layer 3 having a thickness of 1000 nm at a growth temperature of 850 ° C. And on top of that, metal Al (1 × 10 ⁻⁷
Torr), metallic Ga (1 × 10 ⁻⁷ Torr), ammonia (5
X10 ^-6 Torr) at a growth temperature of 850 ° C and a thickness of 30 nm
Of the undoped AlGaN layer 4 of FIG.
Formed ₀ .

The entire surface of the layer structure A ₀ is covered with a SiO ₂ film having a thickness of 100 nm by the P-CVD method and then patterned with a resist, and buffered hydrofluoric acid is applied to regions other than the region 4B where the gate electrode is to be formed. The wet etching used was performed to remove the SiO ₂ film. Then, the exposed undoped AlGaN layer 4 was dry-etched by RIBE to obtain the layer structure A ₁ shown in FIG.

At this time, by changing the etching depth as shown in Table 1, the layer structure A in which the thickness of the region 4A is different.
Manufactured ₁ . Then, the resist on the region 4B and SiO ₂
After removing the film, a SiO ₂ film is formed again on the entire surface, and patterning is performed with a resist to open a portion where a source electrode and a drain electrode are to be formed, Ta-Si is sputtered there, and lift-off is performed to remove the source. The electrode S and the drain electrode D were formed (FIG. 4). After that, the whole temperature is 1050
The heat treatment was performed for 60 minutes in a N ₂ gas atmosphere furnace at ℃.

Finally, the SiO ₂ film on the region 4B was opened, and Pt / Au was vapor-deposited thereon to form the gate electrode G, thereby completing the device A shown in FIG. About the obtained device,
The electrical characteristics, that is, the on-resistance of the FET and the current between the source and the drain were measured. The results are shown in Table 1. For comparison, on top of the layer structure of FIG.
-AlGaN (Si doping concentration 1 × 10 ¹⁹ / cm ³ )
A HEMT having the same layer structure as that of the example was manufactured except that a source electrode S and a drain electrode D were formed on the contact layer having a thickness of 50 nm. The characteristics are shown in Table 1 as a conventional example.

[0025]

[Table 1]

The following is clear from Table 1. (1) The on-resistance of the device of the embodiment is lower than that of the conventional example in which the contact layers are laminated, and the current between the source and the drain is significantly large. (2) In the device of the embodiment, when the thickness of the region 4A becomes thicker than 15 nm, the resistance of the semiconductor layer in the region 4A becomes high,
On the other hand, when the thickness is less than 1 nm, the strain becomes small and the effect of the two-dimensional electron gas layer becomes weak, and as a result, the resistance of the semiconductor layer becomes high. Therefore, the thickness of the region 4A is preferably set to 1 to 15 nm.

[0027]

As is apparent from the above description, in the GaN-based semiconductor device of the present invention, the on-resistance during operation is reduced by reducing the thickness of the region where the source electrode and the drain electrode are formed in the upper layer. Current operation is possible. Further, this device does not need to form a contact layer by selective growth as in the conventional case, and can be manufactured with high productivity.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing one example A of the device of the present invention.

2 is a cross-sectional view showing a layer structure A ₀ used for manufacturing the device A. FIG.

FIG. 3 is a cross-sectional view showing a layer structure A _{1 in} which regions 4A and 4B are formed in an upper layer of the layer structure A ₀ .

FIG. 4 is a cross-sectional view showing a layer structure A ₂ in which a source electrode and a drain electrode are formed.

FIG. 5 is a cross-sectional view showing an example of a conventional GaN-based HEMT structure.

[Explanation of symbols]

1 Semi-Insulating Substrate 2 Buffer Layer 3 Lower Layer 4 Made of First Undoped Material Lower Layer 4A Made of Second Undoped Material Region 4B for Source and Drain Electrodes Formed in Upper Layer 4 Region 5 in Upper Layer 4 Other than Region 4A Contact Layer 6 Two-dimensional electron gas layer 7 Protective film

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 5F045 AA04 AA05 AB14 AB17 AB18                       AB19 AB32 AC12 AD10 AD12                       AF02 AF03 AF04 AF09 BB16                       CA07 CB10 DA53 DA63 HA16                 5F102 FA02 FA03 GB01 GC01 GJ02                       GJ03 GJ04 GJ05 GJ10 GK04                       GK08 GL04 GM04 GS01 GT01                       GT03 GV07 HC01 HC19 HC21

Claims (3)

[Claims] 1. The whole is composed of a GaN-based semiconductor material,
A lower layer made of a first undoped material and a second layer having a bandgap energy larger than that of the first undoped material.
A GaN-based semiconductor device in which a layered structure with an upper layer made of an undoped material is formed on a substrate, and a gate electrode, a source electrode, and a drain electrode are formed on the surface of the upper layer. And a thickness of the upper layer in a region where the drain electrode is formed is smaller than a thickness in other regions. 2. The GaN-based semiconductor device according to claim 1, wherein the first undoped material is GaN and the second undoped material is AlGaN. 3. The upper layer in the formation region of the source electrode and the drain electrode has a thickness of 1 to 15 nm.
GaN-based semiconductor device.