This application is based on and claims priority to Japanese Patent Application No. 2020-041232 filed on Mar. 10, 2020, the entire contents of which are incorporated herein by reference.
The embodiments discussed herein relate to a semiconductor device.
With respect to semiconductor devices using compound semiconductors, there are many reports on field effect transistors, particularly high electron mobility transistors (HEMTs). As a HEMT using nitride semiconductors, a HEMT using a GaN layer for a channel layer and an AlGaN layer for a barrier layer is known. In such a GaN-based HEMT, distortion caused by the difference between the lattice constant of AlGaN and the lattice constant of GaN occurs in the AlGaN layer. Due to this distortion, piezo polarization occurs, and a high-concentration two-dimensional electron gas is generated near the upper surface of the GaN layer under the AlGaN layer. Thereby, a high output is obtained.
A quantum confinement structure transistor using a large band offset between AlN and GaN is also proposed in order to enhance mobility.
However, with a conventional quantum confinement structure transistor, it is difficult to reduce the on resistance.
The present disclosure has an object to provide a semiconductor device that can reduce the on resistance.
According to one aspect of the present disclosure, a semiconductor device includes: a base of a first nitride semiconductor; a buffer layer of a second nitride semiconductor provided on or above the base; a channel layer of a third nitride semiconductor provided on or above the buffer layer and having an opening portion; a barrier layer of a fourth nitride semiconductor provided on or above the channel layer; and a contact layer of a fifth nitride semiconductor provided in the opening portion and in contact with the buffer layer and the channel layer, the contact layer being electrically conductive. A ratio of Al in a composition of the second nitride semiconductor is higher than or equal to a ratio of Al in a composition of the third nitride semiconductor. A ratio of Al in a composition of the first nitride semiconductor and a ratio of Al in a composition of the fourth nitride semiconductor are higher than the ratio of Al in the composition of the second nitride semiconductor.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention.
(Outline of Quantum Confinement Structure Transistor)
First, an outline of a quantum confinement structure transistor will be described.
In the semiconductor device 900 according to the reference example, as illustrated in
In the semiconductor device 900, the higher the ratio of Al in the composition in the InAlGaN barrier layer 904, the higher the barrier in the quantum confinement structure. Conversely, as the ratio of Al in the composition increases, the contact resistance between the InAlGaN barrier layer 904 and the source and drain electrodes 906 and 907 increases. Accordingly, in the semiconductor device 900, if increasing the ratio of Al in the composition in the InAlGaN barrier layer 904 in order to obtain a favorable quantum confinement effect, the on resistance becomes high.
In order to reduce the contact resistance, it is considered that portions of the InAlGaN barrier layer 904 and GaN channel layer 903 overlapping the source electrode 906 and the drain electrode 907 in plan view are etched to expose the AlN barrier layer 902 and an n-type GaN contact layer is regrown on the exposed portions. However, when an n-type GaN contact layer is regrown at a temperature of approximately 1000° C., a large thermal stress is applied between the GaN channel layer 903 and the barrier layers 902 and 904, causing damage such as a crack. By re-growing the n-type GaN contact layer at a relatively low temperature, the damage can be suppressed, but in this case, the n-type GaN contact layer regrows with a shape of islands on the AlN barrier layer 902.
Accordingly, the inventor of the present invention diligently investigated a structure such that a contact layer having a low surface roughness is obtained. As a result, it has been found that a contact layer having a low surface roughness is obtained in a structure in which a buffer layer with a predetermined composition is provided between a base and the contact layer.
Based on these findings, the inventor of the present invention has arrived at the embodiments described below. Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the present specification and the drawings, duplicated descriptions of elements having substantially the same functional configuration may be omitted by providing the same reference numerals.
A first embodiment will be described. The first embodiment relates to a semiconductor device including a high electron mobility transistor (HEMT) having a quantum confinement structure.
In the semiconductor device 100 according to the first embodiment, as illustrated in
The substrate 101 is, for example, an AlN-free standing substrate, and the upper surface of the substrate 101 is an Al polar surface. That is, the Miller index of the upper surface of the substrate 101 is (0001). The buffer layer 102 is, for example, an Alx1Ga1-x1N layer (0.00≤x1≤0.20) with a thickness of 30 nm to 100 nm. The channel layer 103 is, for example, a GaN layer having a thickness of 50 nm or less. The barrier layer 104 is, for example, an Iny2Alx2Ga1-x2-y2 layer (0.00≤x2≤1.00, 0.00≤y2≤0.20) with a thickness of 4 nm to 20 nm. The thickness of the channel layer 103 is preferably 50 nm or less, and is more preferably 20 nm or less. This is for obtaining an excellent quantum confinement effect. The thickness of the buffer layer 102 is preferably 20 nm or less and is more preferably 5 nm or less. This is for reducing the thermal resistance of the buffer layer 102. The Miller index of the upper surface of each of the buffer layer 102, the channel layer 103, and the barrier layer 104 is also (0001). The substrate 101 is an example of a base. AlN of the substrate 101 is an example of a first nitride semiconductor. Alx1Ga1-x1N (0.00≤x1≤0.20) of the buffer layer 102 is an example of a second nitride semiconductor. GaN of the channel layer 103 is an example of a third nitride semiconductor. Iny2Alx2Ga1-x2-y2 (0.00≤x2≤1.00, 0.00≤y2≤0.20) of the barrier layer 104 is an example of a fourth nitride semiconductor. The ratio of Al in the composition of GaN is 0.00.
An element isolation area defining an element area is formed in the nitride semiconductor layered structure 130, and an opening portion 111s for source and an opening portion 111d for drain are formed in the barrier layer 104 and the channel layer 103 within the element area. A conductive contact layer 112s for source is formed in the opening portion 111s and a conductive contact layer 112d for drain is formed in the opening portion 111d. For example, the contact layers 112s and 112d are an n-type GaN layer having a thickness of 40 nm to 70 nm. For example, the contact layers 112s and 112d are doped with Si at a concentration of approximately 1×1019 cm−3 as an n-type impurity. The contact layers 112s and 112d may be doped with Ge, 0, or the like as an n-type impurity. Here, n-type GaN of the contact layers 112s and 112d is an example of a fifth nitride semiconductor, which may be contained at a concentration of 1×1017 cm−3 to 5×1020 cm−3.
A source electrode 106 is formed on the contact layer 112s and a drain electrode 107 is formed on the contact layer 112d. The source electrode 106 forms an ohmic contact with the contact layer 112s, and the drain electrode 107 forms an ohmic contact with the contact layer 112d. A passivation film 108 is formed over the barrier layer 104 to cover the source electrode 106 and the drain electrode 107. An opening portion 108g is formed in the passivation film 108 at a position between the source electrode 106 and the drain electrode 107 in plan view, and a gate electrode 109 is formed on the passivation film 108 through the opening portion 108g in contact with the barrier layer 104.
The source electrode 106 and the drain electrode 107 include, for example, a Ta film having a thickness of 10 nm to 50 nm and an Al film having a thickness of 100 nm to 500 nm on the Ta film. The source electrode 106 forms an ohmic contact with the contact layer 112s, and the drain electrode 107 forms an ohmic contact with the contact layer 112d. The gate electrode 109 includes, for example, a Ni film having a thickness of 10 nm to 50 nm and an Au film having a thickness of 300 nm to 500 nm on the Ni film, and forms a Schottky contact with the nitride semiconductor layered structure 130. For example, the passivation film 108 is a film of oxide, nitride, or oxynitride of Si, Al, Hf, Zr, Ti, Ta or W, and is preferably a film of Si nitride (SiN). For example, the thickness of the passivation film 108 is 2 nm to 500 nm, and is preferably approximately 100 nm.
Next, a band structure of the semiconductor device 100 will be described.
Next, a method of manufacturing the semiconductor device 100 according to the first embodiment will be described.
First, as illustrated in
A surface protective film 110 is then formed on the barrier layer 104, as illustrated in
Thereafter, on the surface protective film 110, the barrier layer 104, and the channel layer 103, an opening portion 111s for source and an opening portion 111d for drain are formed, as illustrated in
Subsequently, as illustrated in
Then, in the nitride semiconductor layered structure 130, an element isolation area that defines an element area is formed. In the formation of the element isolation area, for example, a photoresist pattern is formed on the nitride semiconductor layered structure 130 to expose an area where the element isolation area is to be formed, and ion implantation with Ar or the like is performed with the pattern as a mask. Using a chlorine-based gas with the pattern as an etching mask, dry etching may be performed.
Then, as illustrated in
Then, as illustrated in
Then, as illustrated in
Then, as illustrated in
In this manner, the semiconductor device 100 according to the first embodiment can be manufactured.
It is not necessary to stop the dry etching at the upper surface of the buffer layer 102 when forming the opening portions 111s and 111d. Portions of the surficial layer portion of the buffer layer 102 may be over-etched so that the channel layer 103 does not remain at the bottom portions of the opening portions 111s and 111d. The portions of the buffer layer 102 below the contact layers 112s and 112d may be thinner than the portion below the channel layer 103.
The ratio (x1) of Al in the composition of the buffer layer 102 is preferably 0.20 or less, and is more preferably 0.10 or less. This is because if the ratio of Al in the composition is higher than 0.20, the difference between the lattice constant of the buffer layer 102 and the lattice constant of the contact layers 112s and 112d is large, and the surface roughness of the contact layers 112s and 112d may be large.
The test performed by the inventor of the present application will now be described. In this test, an n-type GaN layer having a thickness of 50 nm was grown on an Alx1Ga1-x1N layer, and the surface roughness Ra of the n-type GaN layer was measured. The n-type GaN layer was grown at a temperature of 700° C. to 750° C. under a pressure of 20 kPa.
A nitride semiconductor layer may be included between the substrate 101 and the buffer layer 102.
In the semiconductor device 190 according to the modified example, an intermediate layer 191 is provided between the substrate 101 and the buffer layer 102 as illustrated in
A second embodiment will be described. The second embodiment relates to a semiconductor device including a HEMT having a quantum confinement structure.
In the semiconductor device 200 according to the second embodiment, a nitride semiconductor layered structure 230 is formed over a substrate 101 as illustrated in
The cap layer 205 is, for example, a GaN layer having a thickness of 1 nm to 5 nm. The opening portions 111s and 111d are formed in the cap layer 205, the barrier layer 104, and the channel layer 103. A passivation film 108 is formed over the cap layer 205. The gate electrode 109 is in contact with the cap layer 205. The Miller index of the upper surface of the cap layer 205 is also (0001).
Other configurations of the second embodiment are similar to those of the first embodiment.
Effects similar to those of the first embodiment can also be obtained by the second embodiment. Also, because the cap layer 205 is formed, an electric field spreading from the end portion of the gate electrode 109 on the drain electrode 107 side toward the 2DEG 120 can be alleviated. Further, because the cap layer 205 is formed, separation of In or the like from the barrier layer 104 can be suppressed.
Next, a method of manufacturing the semiconductor device 200 according to the second embodiment will be described.
First, as illustrated in
A surface protective film 110 is then formed on the cap layer 205, as illustrated in
Subsequently, as illustrated in
Then, in the nitride semiconductor layered structure 230, an element isolation area that defines an element area is formed. Then, as illustrated in
Then, as illustrated in
In this manner, the semiconductor device 200 according to the second embodiment can be manufactured.
A third embodiment will be described. The third embodiment relates to a semiconductor device including a HEMT having a quantum confinement structure.
In the semiconductor device 300 according to the third embodiment, a nitride semiconductor layered structure 330 is formed over the substrate 101, as illustrated in
The buffer layer 302 includes a first layer 302A formed on the substrate 101 and a second layer 302B formed on the first layer 302A. The first layer 302A is, for example, an Alx3Ga1-x3N layer (0.00<x3<1.00) with a thickness of 25 nm to 80 nm. The second layer 302B is, for example, an Alx4Ga1-x4N layer (0.00≤x4≤0.20, x4<x3) with a thickness of 5 nm to 20 nm. That is, the ratio of Al in the composition (x4) of the second layer 302B is lower than the ratio of Al in the composition (x3) of the first layer 302A. The thickness of the buffer layer 302 is preferably 100 nm or less and is more preferably 50 nm or less. This is for reducing the thermal resistance of the buffer layer 302. The Miller index of the upper surface of each of the first layer 302A and the second layer 302B is also (0001).
Other configurations of the third embodiment are similar to those of the first embodiment.
Effects similar to those of the first embodiment can also be obtained by the third embodiment. In addition, because the buffer layer 302 includes the first layer 302A and the second layer 302B having compositions different from each other, while suppressing the difference between the lattice constant of the substrate 101 and the lattice constant of the first layer 302A, the difference between the lattice constant of the second layer 302B and the lattice constant of the contact layers 112s and 112d can be suppressed.
It should be noted that the number of nitride semiconductor layers included in the buffer layer 302 may be three or more. The ratio of Al in the composition of the buffer layer 302 may be continuously lowered from the bottom surface to the top surface. In any structure, it is preferable that the ratio of Al in the composition on the surface of the buffer layer 302 in contact with the contact layers 112s and 112d is 0.20 or less. This is because if the ratio of Al in the composition is higher than 0.20, the difference between the lattice constant of the buffer layer 302 and the lattice constant of the contact layers 112s and 112d is large, and the surface roughness of the contact layers 112s and 112d may be large.
Next, a method of manufacturing the semiconductor device 300 according to the third embodiment will be described.
First, as illustrated in
A surface protective film 110 is then formed on the barrier layer 104, as illustrated in
Subsequently, as illustrated in
Then, in the nitride semiconductor layered structure 330, an element isolation area that defines an element area is formed. Then, as illustrated in
In this manner, the semiconductor device 300 according to the third embodiment can be manufactured.
Next, a fourth embodiment will be described. The fourth embodiment relates to a discrete package of a HEMT.
In the fourth embodiment, as illustrated in
Such a discrete package may be fabricated, for example, as follows. First, the semiconductor device 1210 is secured to the land 1233 of a lead frame using the die attach material 1234, such as solder. The gate pad 1226g is then connected to the gate lead 1232g of the lead frame by bonding using wires 1235g, 1235d and 1235s, the drain pad 1226d is connected to the drain lead 1232d of the lead frame, and the source pad 1226s is connected to the source lead 1232s of the lead frame. Thereafter, a transfer mold process is performed using a mold resin 1231 for sealing. The lead frame is then disconnected.
Next, a fifth embodiment will be described. A fifth embodiment relates to a PFC (Power Factor Correction) circuit with a HEMT.
The PFC circuit 1250 is provided with a switch element (transistor) 1251, a diode 1252, a choke coil 1253, capacitors 1254 and 1255, a diode bridge 1256, and an AC power supply (AC) 1257. A drain electrode of the switch element 1251 is connected to an anode terminal of the diode 1252 and one terminal of the choke coil 1253. A source electrode of the switch element 1251 is connected to one terminal of the capacitor 1254 and one terminal of the capacitor 1255. The other terminal of the capacitor 1254 is connected to the other terminal of the choke coil 1253. The other terminal of the capacitor 1255 is connected to a cathode terminal of the diode 1252. A gate driver is also connected to the gate electrode of the switch element 1251. The AC 1257 is connected between the terminals of the capacitor 1254 via the diode bridge 1256. A DC power supply (DC) is connected between the terminals of the capacitor 1255. In the present embodiment, for the switch element 1251, a semiconductor device having the same structure as any of the first to third embodiments is used.
When the PFC circuit 1250 is manufactured, for example, solder or the like is used to connect the switch element 1251 to the diode 1252, the choke coil 1253, and the like.
Next, a sixth embodiment will be described. The six embodiment relates to a power supply apparatus having a HEMT suitable for server power supply.
The power supply apparatus is provided with a high voltage primary circuit 1261, a low voltage secondary circuit 1262, and a transformer 1263 disposed between the primary circuit 1261 and the secondary circuit 1262.
The primary circuit 1261 is provided with a PFC circuit 1250 according to the fifth embodiment and an inverter circuit, such as a full bridge inverter circuit 1260, which is connected between the terminals of the capacitor 1255 of the PFC circuit 1250. The full bridge inverter circuit 1260 is provided with a plurality (four, in this example) of switch elements 1264a, 1264b, 1264c and 1264d.
The secondary circuit 1262 is provided with a plurality (three, in this example) of switch elements 1265a, 1265b and 1265c.
In the present embodiment, a semiconductor device having the same structure as in any of the first to third embodiments is used for the switch element 1251 of the PFC circuit 1250, and the switch elements 1264a, 1264b, 1264c, and 1264d of the full bridge inverter circuit 1260, which form the primary circuit 1261. Conversely, a related art MIS-type FET (field effect transistor) using silicon is used for the switch elements 1265a, 1265b and 1265c of the secondary circuit 1262.
Next, a seventh embodiment will be described. The seventh embodiment relates to an amplifier having a HEMT.
The amplifier is provided with a digital pre-distortion circuit 1271, mixers 1272a and 1272b, and a power amplifier 1273.
The digital pre-distortion circuit 1271 compensates for a nonlinear strain of an input signal. The mixer 1272a mixes the non-linear strain compensated input signal and an AC signal. The power amplifier 1273 includes a semiconductor device having the same structure as any of the first to third embodiments to amplify an input signal mixed with an AC signal. In the present embodiment, for example, by switching the switching elements, an output signal can be mixed with an AC signal by the mixer 1272b, and the mixed signal can be transmitted to the digital pre-distortion circuit 1271. The amplifier can be used as a high-frequency amplifier, or a high-power amplifier. The high-frequency amplifier can be used, for example, in a transceiver for a mobile phone base station, a radar device, and a microwave generator.
If an intermediate layer 191 is provided to serve as a base as in the modified example of the first embodiment, the used substrate may be a silicon carbide (SiC) substrate, a sapphire substrate, a silicon substrate, an AlN substrate, a GaN substrate, or a diamond substrate. The substrate may be electrically conductive, semi-insulating, or insulating.
The structures of the gate electrode and the drain electrode are not limited to those of the embodiments described above. For example, the electrodes may be composed of a single layer. Also, the method of forming these electrodes are not limited to a lift-off process. Further, if ohmic characteristics can be obtained, a heat treatment after the forming of the source electrode and the drain electrode may be omitted. A heat treatment may be performed after the formation of the gate electrode.
As the structure of a gate electrode, although a Schottky type gate structure is used in the embodiments described above, a MIS (metal-insulator-semiconductor) type gate structure may be used.
Preferred embodiments have been described in detail above. However, various alterations and substitutions may be added to the embodiments described above, without being limited to the embodiments described above and without departing from the scope described in claims.
All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventors to further the art, and are not to be construed as limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
Number | Date | Country | Kind |
---|---|---|---|
2020-041232 | Mar 2020 | JP | national |
Number | Name | Date | Kind |
---|---|---|---|
7544963 | Saxler | Jun 2009 | B2 |
20130141156 | Teo et al. | Jun 2013 | A1 |
20140367743 | Inoue | Dec 2014 | A1 |
20150236121 | Chiu | Aug 2015 | A1 |
20160155828 | Sugawara | Jun 2016 | A1 |
20160172476 | Kikkawa | Jun 2016 | A1 |
20160181364 | Stewart | Jun 2016 | A1 |
20160254378 | Inoue | Sep 2016 | A1 |
20170047437 | Nakayama | Feb 2017 | A1 |
20170092751 | Frijlink | Mar 2017 | A1 |
20170125516 | Ishiguro | May 2017 | A1 |
20170125545 | Yamada | May 2017 | A1 |
20170125567 | Yamada | May 2017 | A1 |
20170250274 | Nakayama | Aug 2017 | A1 |
20180076355 | Hayashi | Mar 2018 | A1 |
20180145148 | Yamada | May 2018 | A1 |
20190096879 | Chen | Mar 2019 | A1 |
20190214494 | Yamada et al. | Jul 2019 | A1 |
20190296137 | Yamada et al. | Sep 2019 | A1 |
Number | Date | Country |
---|---|---|
H09330916 | Dec 1997 | JP |
2013-118383 | Jun 2013 | JP |
2017-085060 | May 2017 | JP |
2017-514316 | Jun 2017 | JP |
2019-125600 | Jul 2019 | JP |
2019-165172 | Sep 2019 | JP |
Entry |
---|
Northrup et al. (Physical review B, vol. 55, No. 20, 15, May 1997). |
Northrup et al. (Physical review B, vol. 55, No. 20, May 15, 1997). |
Office Action issued by the Japanese Patent Office in Japanese Patent Application No. 2020-041232, dated Sep. 26, 2023. |
Number | Date | Country | |
---|---|---|---|
20210288169 A1 | Sep 2021 | US |