Patent Application
20150263155
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-052733, filed Mar. 14, 2014, the entire contents of which are incorporated herein by reference.
Embodiments described herein relate generally to a semiconductor device.
Circuits such as switching power supplies and inverters use semiconductor devices such as switching elements and diodes. These semiconductor devices are required to have high breakdown voltage and low on-resistance. For this reason, between breakdown voltage and on-resistance, there are trade-offs which are determined by the materials of devices.
With advancement of technological development, there have been implemented semiconductor devices having low on-resistance close to the limit of silicon, which is a main material of these devices. In order to further reduce on-resistance, it is necessary to change the materials of devices. If a nitride semiconductor such as GaN or AlGaN, or a wide band gap semiconductor such as silicon carbide (SiC) is used as a switching element material, it is possible to improve the trade-offs discussed above, and to reduce on-resistance.
Devices using nitride semiconductors such as GaN and AlGaN are, in general, devices having low on-resistance, and an example of such the devices includes a high electron mobility transistor (HEMT) using a hetero-junction structure of AlGaN and GaN. The HEMT realizes low on-resistance by high mobility of a channel formed at the hetero-junction, and high electron concentration which is generated by polarization.
However, since the HEMT generates electrons by polarization, electrons exist in high concentration under the gate electrode. For this reason, in general, the HEMT becomes a normally-on type device whose gate threshold voltage is negative. For a safe operation, it is preferred to incorporate a normally-off type device having a gate threshold voltage that is positive. For example, in order to implement a normally-off type device, a p-type semiconductor layer can be provided below the gate electrode. But, in this method, an increase in gate leakage current is a problem.
An embodiment provides a semiconductor device that has a low gate leakage current.
In general, according to one embodiment, a semiconductor device includes: a first nitride semiconductor layer; a second nitride semiconductor layer on the first nitride semiconductor layer and having a band gap wider than a band gap of the first nitride semiconductor layer. A source and a drain electrode are spaced apart on the second nitride semiconductor layer. A third nitride semiconductor layer is on the second nitride semiconductor layer between the source electrode and the drain electrode and has an impurity concentration of 1×1017 atoms/cm3 or less and a band gap narrower than the band gap of the second nitride semiconductor layer. A p-type fourth nitride semiconductor layer is on the third nitride semiconductor layer. A gate electrode is provided on the fourth nitride semiconductor layer.
In this specification, elements identical or similar to each other are denoted by the same reference symbol, and repeated description of an element may be omitted.
In this specification, the term “nitride semiconductor” means, for example, a GaN-based semiconductor. The term “GaN-based semiconductor” is the general term for semiconductors having a composition such as gallium nitride (GaN), aluminum nitride (AlN) and indium nitride (InN), or an intermediate composition of these materials.
In this specification, the term “undoped” means having no impurities intentionally introduced into the material.
A semiconductor device according to the present embodiment includes a first nitride semiconductor layer, a second nitride semiconductor layer that is provided on the first nitride semiconductor layer and has a band gap wider than that of the first nitride semiconductor layer, a source electrode that is provided on the second nitride semiconductor layer, a drain electrode that is provided on the second nitride semiconductor layer, a third nitride semiconductor layer that is provided on the second nitride semiconductor layer between the source electrode and the drain electrode, has an impurity concentration of 1×1017 atoms/cm3 or less, and has a band gap narrower than that of the second nitride semiconductor layer, a p-type fourth nitride semiconductor layer that is provided on the third nitride semiconductor layer, and a gate electrode that is provided on the fourth nitride semiconductor layer.
As shown in
The substrate 10 is formed of, for example, silicon (Si). Besides silicon, for example, sapphire (Al2O3) or silicon carbide (SiC) can also be used as the substrate 10.
On the substrate 10, the buffer layer 12 is provided. The buffer layer 12 has a function of relieving lattice mismatch between the substrate 10 and the channel layer 14. The buffer layer 12 is formed of, for example, a multi-layer structure of aluminum gallium nitride (AlWGa1-WN wherein 0<W<1).
On the buffer layer 12, the channel layer 14 is provided. The channel layer 14 is, for example, an undoped AlXGa1-XN layer (wherein 0≦X<1). More specifically, the channel layer 14 is, for example, an undoped GaN layer. The channel layer 14 has, for example, a film thickness of equal to or larger than 0.5 μm and equal to or less than 3 μm.
On the channel layer 14, the barrier layer 16 is provided. The band gap of the barrier layer 16 is wider than the band gap of the channel layer 14. The barrier layer 16 is, for example, an undoped AlYGa1-YN layer (wherein 0<Y≦1 and X<Y). More specifically, the barrier layer 16 is, for example, an undoped Al0.2Ga0.8N layer. The barrier layer 16 has, for example, a film thickness of equal to or larger than 20 nm and equal to or less than 50 nm.
An interface between the channel layer 14 and the barrier layer 16 becomes a hetero-junction. During an ON operation of the HEMT 100, a two-dimensional electron gas is formed at the hetero-junction interface, and becomes conductive.
On the barrier layer 16, the source electrode 18 and the drain electrode 20 are formed. The source electrode 18 and the drain electrode 20 are, for example, metal electrodes, and the metal electrodes are electrodes containing, for example, aluminum (Al) as a main component. It is desirable for the source electrode 18 and the drain electrode 20 to be in ohmic contact with the barrier layer 16. The distance between the source electrode 18 and the drain electrode 20 is, for example, about 18 μm.
Between the source electrode 18 and the drain electrode 20 on the barrier layer 16, the first cap layer 22 is provided. The first cap layer 22 is a high-resistance layer and has a function of suppressing the gate leakage current.
The impurity concentration of the first cap layer 22 is equal to or less than 1×1017 atoms/cm3. In the first cap layer 22, there is at least a region having the impurity concentration of equal to or less than 1×1017 atoms/cm3. In order to make the first cap layer 22 have high resistance, it is desirable that the impurity concentration of the first cap layer 22 is equal to or less than 1×1016 atoms/cm3, and it is more desirable that the impurity concentration of the first cap layer 22 is equal to or less than 1×1015 atoms/cm3.
The impurity concentration can be analyzed, for example, by secondary ion mass spectrometry (SIMS).
The band gap of the first cap layer 22 is lower than the band gap of the channel layer 14. The first cap layer 22 is, for example, an undoped AlZGa1-ZN layer (wherein 0≦Z<1 and Y>Z). More specifically, the first cap layer 22 is, for example, an undoped GaN layer. The first cap layer 22 has, for example, a film thickness of equal to or larger than 1 nm and equal to or less than 10 nm. The first cap layer 22 is a single crystal.
On the first cap layer 22, the p-type second cap layer 24 is provided. The p-type second cap layer 24 has a function of raising the potential of the channel layer 14, thereby increasing the threshold voltage of the HEMT 100.
The second cap layer 24 is, for example, a p-type AlUGa1-UN layer (wherein 0≦U<1). More specifically, the second cap layer 24 is, for example, a p-type GaN layer. The second cap layer 24 has, for example, a film thickness of equal to or larger than 5 nm and equal to or less than 500 nm.
The p-type impurity contained in the second cap layer 24 is, for example, magnesium (Mg). In order to raise the potential of the channel layer 14, it is desirable that a concentration of the p-type impurity in the second cap layer 24 is equal to or larger than the 1×1018 atoms/cm3, and it is more desirable that the concentration of the p-type impurity in the second cap layer 24 is equal to or larger than 1×1019 atoms/cm3. The second cap layer 24 is a single crystal.
On the second cap layer 24, the gate electrode 26 is provided. The gate electrode 26 is, for example, a metal electrode. The metal electrode is, for example, an electrode mainly having a laminate structure of platinum (Pt) and gold (Au). It is desirable that the gate electrode 26 is in ohmic contact with the second cap layer 24.
An example of a method for manufacturing the semiconductor device according to the first embodiment will be described.
First, the substrate 10, for example, a Si substrate is prepared. Next, the buffer layer 12 is grown on the Si substrate, for example, by epitaxial growth.
Next, on the buffer layer 12, undoped GaN to be the channel layer 14, and undoped Al0.2Ga0.8N to be the barrier layer 16 are formed by the epitaxial growth.
Next, undoped GaN to be the first cap layer 22, and p-type GaN to be the second cap layer 24 are sequentially formed by the epitaxial growth. For example, GaN precursor gases are supplied to an epitaxial growth apparatus in which the substrate 10 is held, whereby undoped GaN is formed on the substrate.
Thereafter, Mg and GaN precursor gases are supplied to the epitaxial growth apparatus, whereby undoped GaN and p-type GaN are sequentially formed. For example, an insulating film which is to be subjected to patterning may be formed on the surface of the barrier layer 16, then patterned, whereby the first cap layer 22 and the second cap layer 24 may be selectively grown on the surface of the barrier layer 16.
Next, on the surface of the barrier layer 16, the source electrode 18 and the drain electrode 20 are formed by forming and patterning a metal layer. Also, on the second cap layer 24, the gate electrode 26 is formed by forming and patterning a metal layer.
The semiconductor device 100 shown in
Subsequently, the operations and effects of the semiconductor device 100 will be described.
In the HEMT 100 according to the first embodiment, since the p-type second cap layer 24 exists immediately below the gate electrode 26, the potential of the channel layer 14 is raised. Therefore, generation of a two-dimensional electron gas is suppressed, and the threshold value of the HEMT 100 increases as compared to a case where the second cap layer 24 does not exist. When the energy of the lower end of the conduction band of the hetero-junction interface becomes higher than the Fermi level, even when the gate voltage is 0 V, the channel layer 14 is depleted, whereby the HEMT 100 has a normally-off operation.
However, if a positive voltage is applied to the gate electrode in order to operate the HEMT 100, a forward voltage is applied to the junction of the barrier layer 16 and the p-type second cap layer 24 between the gate electrode 26 and the source electrode 18 grounded. For this reason, there is a risk that the gate leakage current could increase.
In the first embodiment, the first cap layer 22 having a film thickness less than that of the p-type second cap layer 24 and a low p-type impurity concentration is interposed between the barrier layer 16 and the second cap layer 24. Therefore, since the first cap layer 22 has high resistance, the gate leakage current is suppressed.
Also, the film thickness of the first cap layer 22 is equal to or larger than 1 nm and equal to or less than 10 nm. It is desirable, in some embodiments, that the film thickness of the first cap layer 22 is equal to or larger than 2 nm and equal to and less than 6 nm.
If the film thickness of the first cap layer 22 falls below the above-described range, the resistance of the first cap layer 22 decreases, and thus there is a risk that the effect of suppressing the gate leakage current could not be sufficiently achieved. Also, if the film thickness of the first cap layer 22 exceeds the above-described range, there is a risk that the potential of the channel layer 14 could not be sufficiently raised by the p-type second cap layer 24. That is, since the p-type second cap layer 24 raises the potential of the first cap layer 22, there is a risk that the potential of the channel layer 14 could not be finally and sufficiently raised for conductive operations if the first cap layer is too thick.
As described above, according to the first embodiment, the HEMT 100 which realizes the normally-off operation and suppressing of the gate leakage current is provided.
A semiconductor device according to the second embodiment that the third nitride semiconductor layer is provided on bottom and sides of a recess whose the bottom and sides are positioned in the second nitride semiconductor layer.
As shown in
In the HEMT 200, the first cap layer 22 and the second cap layer 24 are provided in the recess (trench) 30 formed in the barrier layer 16. The bottom 30a and sides 30b of the recess 30 are positioned in the barrier layer.
The HEMT 200 has a so-called recess structure. The first cap layer 22 is provided on the bottom 30a and sides 30b of the recess 30.
The HEMT 200 can be manufactured by a method which is the same as that in the first embodiment except that before the first cap layer 22 and the second cap layer 24 are formed, the recess 30 is formed in the surface of the barrier layer 16 by etching, for example.
Since the HEMT 200 has the p-type second cap layer 24, it is possible to increase the threshold value of the transistor. Also, since the HEMT 200 has the first cap layer 22 having high resistance, the gate leakage current is suppressed.
Also, since the HEMT 200 has the recess structure, a portion of the barrier layer 16 positioned below the gate electrode 26 becomes thin. Therefore, the quantity of piezoelectric polarization decreases, and the concentration of the two-dimensional electron gas below the gate electrode 26 decreases. Therefore, it becomes somewhat easier to realize a normally-off operation.
It is desirable, though not necessarily required, that the film thickness of the first cap layer 22 on the sides 30b is larger than the film thickness of the first cap layer 22 on the bottom 30a as shown in
Also, it is preferred that the interface between the first cap layer 22 and the second cap layer 24 is positioned closer to the gate electrode 26 than to the opposite surface of the barrier layer 16 to the channel layer 14 as shown in
Also, it is preferred that the edge portions of the first cap layer 22 be positioned on the surface of the barrier layer 16 other than within the recess 30. According to this configuration, it is possible to improve the margin (tolerance) of alignment between the recess 30 and the first cap layer 22 during manufacturing, and to implement the HEMT 200 having stable characteristics. In addition, since the first cap layer 22 becomes thinner at the upper corners of the recess 30, it is possible to suppress increase of the gate leakage current.
Since the sides 30b are inclined, it becomes easy to embedded (coat) the recess 30 with the first cap layer 22 and the second cap layer 24. Therefore, it also becomes easier to make the film thickness of the first cap layer 22 on the sides 30b larger than the film thickness of the first cap layer 22 on the bottom 30a.
As described above, it is possible to provide the HEMT 200 and the HEMT 300 which have a normally-off operation and low gate leakage current. Further, since the HEMTs have recess structures, it becomes relatively easy to provide the normally-off operation.
In the described example embodiments, GaN and AlGaN have been used as examples of the material of the nitride semiconductor layers. However, for example, InGaN, InAlN, and InAlGaN containing indium (In) can also be applied to embodiments of the present disclosure. Also, AlN can be the material of the nitride semiconductor layers in embodiments of the present disclosure.
Also, in the example embodiments, a barrier layer, comprising an undoped AlGaN layer has been used. However, an n-type AlGaN layer can also be used as a barrier layer material.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-052733 | Mar 2014 | JP | national |
