Patent Application
20150084103
1. Field of the Invention
The present invention relates to a field effect transistor formed in an epitaxially grown crystal layer formed of GaN or AlGaN on a Si substrate, and a MMIC including the field effect transistor.
2. Background Art
A semiconductor device is known in which an epitaxially grown crystal layer formed of GaN or AlGaN is formed on a Si substrate, and in which a field effect transistor is formed in the epitaxially grown crystal layer (see, for example, Japanese patent Laid-Open Nos. 2009-206142 and 2010-67662).
In a conventional semiconductor device, a Si substrate having a thickness of about 100 μm exists below a transistor portion, a pad portion, a wiring portion and a passive circuit portion, with a several micrometer-thick epitaxial layer interposed therebetween, the epitaxial layer comprising GaN or AlGaN. Even the resistivity of a high resistance type of Si substrate is about 104 Ωcm, which is four to five orders of magnitude lower than those of semi-insulating GaAs substrates and SiC substrates used in high-frequency-band semiconductor devices. The conductor loss in a circuit portion is increased thereby.
In ordinary cases, the substrate back surface of a semiconductor device serves as electrical ground. However, a GaN-based epitaxial layer ordinarily has a thickness of about several micrometers. Therefore, in a case where a Si substrate in a semiconductor device has a low resistivity, the stray capacitance between a drain electrode and the substrate back surface (Cds) is larger than that in a semiconductor device using a semi-insulating GaAs substrate or a SiC substrate, and a high-frequency characteristic degrades.
The heat conductivity of Si is about 1.5 W/cm·K, lower than that of SiC used as a substrate in a GaN or AlGaN-based semiconductor device. Therefore, heat generated in a transistor in such a semiconductor device cannot easily be released, and the thermal resistance of the semiconductor device is increased. In the case of use of the semiconductor device as a high-output amplifier in particular, the junction temperature is increased during operation, resulting in a reduction in reliability.
In devices described in Japanese patent Laid-Open Nos. 2009-206142 and 2010-67662, therefore, part of the Si substrate is replaced with a filler having a high insulating property compared with Si and having high heat conductivity to reduce the conductor loss in a circuit portion and limit the increase in stray capacitance in a portion below the transistor (including Cds) so that high-frequency characteristics are improved. Because complete removal of the corresponding portion of the Si substrate is more effective in enabling high-withstand-voltage high-frequency operation, a through hole is formed so as to reach from the back surface of the Si substrate to the epitaxially grown crystal layer and is filled with the filler in the devices described in Japanese patent Laid-Open Nos. 2009-206142 and 2010-67662. However, the lattice constants of the epitaxially grown crystal layer and the filler differ from each other and a defect can occur. It is, therefore, difficult to deposit the filler.
In view of the above-described problems, an object of the present invention is to provide a semiconductor device and a method for manufacturing the same which can reduce the conductor loss in the circuit portion, improve high-frequency characteristics, and easily deposit the filler.
According to the present invention, a semiconductor device includes: a Si substrate having first and second major surfaces facing in opposite directions; a buffer layer formed of AlxGa1-xN (0≦x≦1) on the first major surface of the Si substrate; an epitaxially grown crystal layer formed of AlyGa1-yN (0≦y≦1, x≠y) on the buffer layer; a transistor formed on the epitaxially grown crystal layer; and a filler formed of AlxGa1-x N having the same composition ratio x as a material of the buffer layer, wherein a through hole is formed in the Si substrate so as to reach from the second major surface to the buffer layer, and the through hole is filled with the filler.
In the present invention, a through hole formed in the Si substrate is filled with a filler formed of AlxGa1-xN having the same composition ratio x as a material of the buffer layer. Therefore, the conductor loss in the circuit portion can be reduced, high-frequency characteristics can be improved, and the filler can be easily deposited.
Other and further objects, features and advantages of the invention will appear more fully from the following description.
A semiconductor device and method for manufacturing the same according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted.
An active layer region 6 is formed in a portion of the epitaxially grown crystal layer 5, and an insulation region 7 is formed around the active layer region 6 by insulation injection or the like. A transistor 8 is formed in the epitaxially grown crystal layer 5. More specifically, a pair of field-effect transistors having gate electrodes 9, source electrodes 10 and drain electrodes 11 are disposed parallel to each other on the active layer region 6, each gate electrode 9 being interposed between one of the source electrodes 10 and one of the drain electrodes 11. The active layer region 6 and a region where the three kinds of electrodes are arranged parallel to each other form an operating region. At least one gate pad 12 and at least one drain pad 13 are disposed in such positions on opposite sides of the operating region as to be opposed to each other.
All the gate electrodes 9 in the operating region are combined into one outside the operating region to be connected to the gate pad 12. Similarly, all the drain electrodes 11 in the operating region are combined into one outside the operating region to be connected to the drain pad 13. The source electrodes 10 are connected to source pads 14 ordinarily provided by the side of the gate pad 12. Wiring electrodes that connect the source electrodes 10 and the source pads 14 are disposed on the wiring electrode into which the gate electrodes 9 are merged, with an insulating film and air interposed therebetween.
A through hole 15 is formed in the Si substrate 1 so as to reach from the second major surface to the buffer layer 2. The through hole 15 is filled with a filler 16 formed of AlN. The materials of the above-described layers are only an example. The buffer layer 2 is formed of AlxGa1-xN (0≦x≦1), the epitaxially grown crystal layer 5 is formed of AlyGa1-yN (0≦y≦1, x≠y), and the filler 16 is formed of AlxGa1-xN having the same composition ratio x as the material of the buffer layer 2.
A method of manufacturing the above-described semiconductor device will now be described. The AlN buffer layer 2 is first formed on the first major surface of the Si substrate 1. The epitaxially grown crystal layer 5 is formed on the buffer layer 2. The transistor 8 is formed in the epitaxially grown crystal layer 5. The through hole 15 is thereafter formed by an etching process in which the region in the second major surface of the Si substrate 1 other than the region below the active layer region 6 is masked with a resist and the portion of the Si substrate 1 below the active layer region 6 is dry-etched from the second major surface with a fluorine-based gas such as SF6 by using the buffer layer 2 as a stopper layer. The through hole 15 is filled with the AlN filler 16 by using sputtering or CVD.
In the present embodiment, part of the Si substrate is removed and the hole thereby formed is filled with the filler 16 formed of AlxGa1-xN having a high insulating property compared with Si and having high heat conductivity, thereby reducing the conductor loss in the circuit portion and limiting the increase in stray capacitance below the transistor (including Cds). High-frequency characteristics can thus be improved. Also, heat generated in the transistor 8 can be efficiently released to the metal base of the package through the filler 16. Prevention of reduction in reliability can thus be enabled.
The filler 16 is formed of AlxGa1-xN having the same composition ratio x as the material of the buffer layer 2. The buffer layer 2 and the filler 16 are thus lattice-matched to each other, so that the filler 16 can easily be deposited.
In the conventional devices, high-resistance Si substrates having resistivities of 102 to 104 Ωcm in order to optimize high-frequency characteristics. In the present embodiment, the region below the transistor, the circuit portion and the wiring portion is filled with a material having a high insulating property. Therefore, even if the resistivity of the Si substrate 1 in the other region is low, it does not affect the high-frequency characteristics of the semiconductor device. As a result, even when the resistivity of the Si substrate 1 is equal to or lower than 104 Ωcm, high-frequency characteristics as good as those in a case where a semi-insulating substrate is used can be obtained.
A semiconductor device used as a high-output amplifier is ordinarily made to operate at a high operating voltage equal to or higher than 10 V in order to obtain a high output. Therefore, a p-type Si substrate 1 may alternatively be used. Use of a p-type Si substrate 1 enables holes generated between the gate and the drain between which a high electric field is applied to be drawn out from the p-type Si substrate 1 below the source electrode 10 to the back surface through a short distance. Holes generated in the transistor at the time of application of the high electric field can thus be inhibited from staying in the epitaxially grown crystal layer below the gate electrode 9 in particular. As a result, the reliability can be improved uniformly with respect to the active layer region 6.
Forming of the through hole 15 by etching the Si substrate 1 using the nitride buffer layer 2 as a stopper layer is easier to perform.
Since an increase in the capacitance Cds between the drain electrodes 11 and the substrate back surface is a cause of degradation of high-frequency characteristics, part of the Si substrate 1 below the drain electrodes 11 may be removed and replaced with the filler 16. The high-frequency characteristics can be improved thereby, as are those in the first embodiment. While the thickness of the AlN layer deposited as filler 16 is about several micrometers in the first embodiment, the thickness of the AlN layer in the present embodiment can be reduced relative to that in the first embodiment. Forming of the filler 16 is facilitated thereby.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
The entire disclosure of Japanese Patent Application No. 2013-197261, filed on Sep. 24, 2013 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-197261 | Sep 2013 | JP | national |
