High electron mobility transistor element including aluminum-doped buffer layer

Abstract

To provide a high electron mobility transistor element including an aluminum-doped buffer layer.SOLUTION: A HEMT element 10 includes a substrate 12, and an epitaxial layer 14 including a buffer layer 16 including a dopant containing aluminum and covering the substrate. The buffer layer has an aluminum concentration of 0.5 to 3%. The epitaxial layer further includes a channel layer 18 covering the buffer layer, and a barrier layer 20 covering the channel layer. A gate contact 22 is disposed on a surface 24 of the epitaxial layer. A source contact 26 and a drain contact 28 are also disposed on the surface of the epitaxial layer. The source contact and the drain contact are apart from each other and from the gate contact.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of Provisional Patent Application No. 63 / 126,837, filed December 17, 2020, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The present disclosure relates to high electron mobility transistor devices having buffer layers that provide improved linearity. [Background technology]

[0003] The radio frequency (RF) spectrum is becoming increasingly crowded. Therefore, to share the crowded RF spectrum, it is highly desirable to increase the linearity of amplifiers. At least one problem that reduces the linearity of amplifiers is undoped gallium nitride buffers, which are usually n-type, due to undesired intrusion of silicon or oxygen during the epitaxial growth of the gallium nitride buffer. N-type gallium nitride buffers typically result in leaky gallium nitride high electron mobility transistor elements with very poor RF performance. There is a need for high electron mobility transistor elements with a buffer layer that eliminates leakage and therefore provides improved linearity. Summary of the Invention

[0004] A high electron mobility transistor (HEMT) device is disclosed. The HEMT device includes a substrate and an epitaxial layer overlying the substrate including a buffer layer having a dopant including aluminum, the concentration of aluminum in the buffer layer being between 0.5% and 3%. The epitaxial layer further includes a channel layer overlying the buffer layer and a barrier layer overlying the channel layer. A gate contact is disposed on a surface of the epitaxial layer. Source and drain contacts are also disposed on the surface of the epitaxial layer, the source and drain contacts being spaced apart from the gate contact and from each other.

[0005] In another embodiment, any of the above-described embodiments may be combined individually or together, and / or with various separate embodiments and features described herein, for additional advantages. Unless otherwise indicated herein to the contrary, any of the various features and elements disclosed herein may be combined with one or more of the other disclosed features and elements.

[0006] Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description in conjunction with the accompanying drawings.

[0007] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate several aspects of the disclosure and, together with the description, serve to explain the principles of the disclosure. [Brief description of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view of a high electron mobility transistor (HEMT) device structured in accordance with the present disclosure. [Diagram 2] 1 is a cross-sectional view of an exemplary embodiment of a HEMT device in which a buffer layer is laterally divided into an undoped region and a first doped region having a dopant comprising aluminum. [Diagram 3] FIG. 13 is a cross-sectional view of another exemplary embodiment, in which the buffer layer has a second doped region having a non-aluminum dopant, such as iron or carbon. [Figure 4] FIG. 1 is a band diagram of a typical semiconductor structure of a field effect transistor. [Diagram 5] Typical deep trap profile of a gallium nitride (GaN) buffer using iron traps. [Figure 6A] The drain current dynamics of a HEMT device when operated as a gate voltage controlled switch are shown. [Figure 6B] FIG. 1 is a cross-sectional view of a HEMT device in which the traps in the buffer are not filled and are represented by the open circles. [Figure 6C]FIG. 2 is a cross-sectional view of a HEMT device in which the traps in the buffer are filled and represented by the black circles. [Figure 7A] 1 is a graph showing the quiescent drain current dynamics of a HEMT device using a GaN buffer with 2% diluted Al dopant. [Figure 7B] 7B is a table summarizing the quiescent drain current dynamics shown in the graph of FIG. 7A. [Figure 8A] 11 is a graph showing the dynamics of a similarly structured device having iron deep traps instead of the aluminum dopants of the HEMT device of the present disclosure. [Figure 8B] 8B is a table summarizing the quiescent drain current dynamics shown in the graph of FIG. 8A. [Figure 9] 1 illustrates an amplifier in which a pair of HEMT devices are utilized as amplifying elements configured to amplify radio frequency signals in accordance with the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The embodiments described below represent the necessary information to enable those skilled in the art to practice the embodiments and show the best mode for practicing the embodiments. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the present disclosure and recognize applications of these concepts that are not specifically addressed herein. These concepts and applications should be understood to be within the scope of the present disclosure and the appended claims.

[0010] In this specification, terms such as first, second, etc. may be used to describe various elements, but it will be understood that these elements should not be limited by these terms. These terms are used only to distinguish one element from another element. For example, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element, without departing from the scope of the present disclosure. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0011] When an element, such as a layer, region, or substrate, is referred to as being "on" or extending "up" another element, it will be understood that it may be directly on or extending directly onto the other element, or that intervening elements may also be present. In contrast, when an element is referred to as being "directly on" or extending "up" another element, there are no intervening elements. Similarly, when an element, such as a layer, region, or substrate, is referred to as "over" or extending "over" another element, it will be understood that it may directly overlay or extend directly over the other element, or that intervening elements may also be present. In contrast, when an element is referred to as "directly over" or extending "directly over" another element, there are no intervening elements. It will also be understood that when an element is referred to as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or that intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.

[0012] Relative terms such as "lower" or "upper," or "top" or "lower," or "horizontal" or "vertical" may be used herein to describe the relationship of one element, layer, or region to another element, layer, or region when illustrated in the figures. It will be understood that these terms, and those discussed above, are intended to encompass different orientations of the element in addition to the orientation depicted in the figures.

[0013] The terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural, unless the context clearly dictates otherwise. It will be further understood that as used herein, the terms "comprises", "comprising", "includes" and / or "including" specify the presence of stated features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0014] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by those skilled in the art to which this disclosure belongs. Terms used herein should be interpreted to have a meaning that is not inconsistent with their meaning in the context of this specification and related technology, and it will be further understood that they will not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0015] The embodiments are described herein with reference to schematic diagrams of embodiments of the present disclosure. Thus, the actual dimensions of layers and elements may vary and variations from the illustrated shapes are expected as a result of manufacturing techniques and / or tolerances. For example, regions illustrated or described as square or rectangular may have rounded or curved features, and regions shown as straight lines may have some irregularities. Thus, the regions illustrated in the figures are schematic, and their shapes are not intended to illustrate the exact shape of the regions of the elements and are not intended to limit the scope of the present disclosure. Additionally, the size of structures or regions may be exaggerated relative to other structures or regions for illustrative purposes, and thus are provided to show the general structure of the subject matter, and may or may not be drawn to scale. Common elements between the figures may be indicated by common element numbers herein and may not be described again thereafter.

[0016] Undoped gallium nitride (GaN) buffers are typically n-type due to the undesired intrusion of silicon (Si) or oxygen (O), two known shallow donors, during epitaxial growth. N-type GaN buffers result in leaky GaN field effect transistor (FET) devices with very poor radio frequency (RF) performance.

[0017] To reduce leakage in GaN buffers, device engineers introduce deep traps during growth. The two most common deep traps used for this purpose during epitaxial growth are carbon and iron. For example, iron is a relatively fast trap, but exhibits growth memory, i.e., the incorporation of iron in the buffer cannot be switched abruptly. This has the detrimental consequence of having to add more deep traps to the buffer than is needed. In contrast, carbon is a slower trap. Carbon does not exhibit growth memory, but the amount of carbon incorporation in the buffer is controlled by the growth conditions, including temperature, growth rate, and V / III ratio. Given that the optimized growth window for achieving the highest crystal quality of gallium nitride buffers is relatively small, controlling carbon doping to the desired level becomes very difficult.

[0018] Deep traps have positive properties such as suppressing leakage between the drain and source sides of high electron mobility transistor devices, which produces higher performance RF devices, and they have negative properties such as slow de-trapping when subjected to receive and transmit events, which creates undesirable dynamics of the quiescent drain current state of the device.

[0019] The dynamics in receive and transmit switching events are particularly detrimental in the context of time-domain duplexed base station applications because of the dynamic changes in transconductance that the elements undergo as their operation progresses and a steady-state quiescent drain current is reached. These dynamic changes in transconductance complicate the application of digital predistortion algorithms used to linearize gallium nitride amplifiers. This results in high electron mobility transistor elements that are difficult to linearize and therefore have limited information transmission capabilities.

[0020] Ultimately, the number of deep traps in the buffer layer and their associated detrapping times will determine how a particular device can be used. For applications that do not involve transmit and receive switching, deep traps are preferred because iron deep traps decouple the growth temperature and quality of the buffer layer from the amount of insulation required for the buffer layer. For applications that involve transmit and receive switching, carbon doped buffer layers are preferred because they do not require a large number of deep traps (instantaneous switching during growth) and do not result in detrapping dynamics (very slow detrapping times) that interfere with the baseband signal being transmitted, resulting in less poor quality and less controllable buffer layers.

[0021] In this disclosure, a new insulating buffer is disclosed that has temperature decoupling and high controllability of iron, and abrupt switch-on growth and slow detrapping times of carbon.

[0022] Specifically, replacements for typical carbon or iron deep level traps are created with aluminum-oxygen complexes and other native defects generated through doping of the buffer with diluted aluminum dopants. The main advantages of using aluminum instead of carbon or iron are:

[0023] Like carbon, the aluminum-oxygen complex is a very slow trap, which prevents any aliasing of the detrapping and information dynamics. Also, like carbon, the profile of the aluminum-oxygen complex can be changed rapidly, thereby eliminating the need to add more traps than typically used. In contrast to carbon, the control of aluminum-oxygen can be achieved through the flow of trimethylaluminum without changing other growth conditions. The control of aluminum-oxygen through the flow of trimethylaluminum decouples the deep level trap entry from the growth conditions of the buffer, and therefore the quality of the overall buffer is preserved.

[0024] Briefly, the exchange of aluminum for carbon and iron produces low concentration deep level trap elements with excellent transmit / receive switching characteristics that are controlled through a more manufacturable growth process that does not use growth conditions to control the number of traps.

[0025] 1 is a cross-sectional view of a high electron mobility transistor (HEMT) device 10 structured in accordance with the present disclosure. The HEMT device 10 has a substrate 12 and an epitaxial layer 14 overlying the substrate 12. The epitaxial layer 14 includes a buffer layer 16, a channel layer 18 overlying the buffer layer 16, and a barrier layer 20 overlying the channel layer 18. The HEMT device 10 also includes a gate contact 22 disposed on a surface 24 of the epitaxial layer 14. The epitaxial layer 18 further includes a source contact 26 disposed over the surface 24 of the layer 14 and a drain contact 28 disposed on the surface 24 of the epitaxial layer 18, the source contact 26 and the drain contact 28 being spaced apart from the gate contact 22 and from each other. In an exemplary embodiment, the buffer layer 16 is made of gallium nitride having a dopant 30 including aluminum. In some of the exemplary embodiments, the concentration of aluminum in the buffer layer 16 from the dopant 30 is between 0.5% and 3%. In some other of the exemplary embodiments, the concentration of aluminum in the buffer layer 16 from the dopant 30 is between 0.5% and 0.75%. In still other of the exemplary embodiments, the concentration of aluminum in the buffer layer 16 from the dopant 30 is between 0.75% and 1.25%. In still other of the exemplary embodiments, the concentration of aluminum in the buffer layer 16 from the dopant 30 is between 1.25% and 2%. In further exemplary embodiments, the concentration of aluminum in buffer layer 16 from dopant 30 is between 2% and 3%.

[0026] In some exemplary embodiments, as depicted in FIG. 2, the buffer layer 16 is laterally divided into an undoped region 32 and a first doped region 34 having a dopant 30 comprising aluminum. In other exemplary embodiments, as depicted in FIG. 3, the buffer layer 16 has a second doped region 36 having a non-aluminum dopant 38, represented by the letter X in FIG. 3. In these exemplary embodiments, the undoped region 32 is adjacent to the channel layer 18, and the first doped region 34 is sandwiched between the undoped region 32 and the second doped region 34. In some exemplary embodiments, the non-aluminum dopant 38 is carbon. In other exemplary embodiments, the non-aluminum dopant 38 is iron.

[0027] Figure 4 shows the band diagram for a typical semiconductor structure of a field effect transistor. Note the energy of the unintentionally present donor states. The black lines represent the intentionally added deep traps that isolate the buffer. The number of intentionally added deep traps must be greater than the number of unintentionally present donor state deep traps. If not, the buffer will be conductive and cannot be used for RF applications.

[0028] FIG. 5 is a typical deep trap profile of a GaN buffer using iron traps, represented by the solid line profile. Due to the growth memory effect, the concentration of iron cannot drop sharply. The ideal trap profile, shown by the dashed line in FIG. 5, has a steep profile. This steeper profile can be achieved by carbon or by using aluminum-oxygen complexes present in the aluminum-diluted GaN buffer. However, the use of carbon requires a change in the growth conditions, which among other things affects the breakdown of field effect transistors built on the buffer.

[0029] Figure 6A shows the drain current dynamics of the HEMT device 10 when operated as a gate voltage controlled switch as shown in Figures 6B and 6C. When the gate voltage is at deep pinch-off, no drain current is circulating through the channel layer 18. All traps (black circles) in the buffer 16 are filled as shown in Figure 6C.

[0030] As shown in FIG. 6A, when the gate voltage is increased from state (1) to state (2), the drain current increases from the drain to the source. The increase to steady state (3) is not instantaneous because the traps take some time to detrap. It is this dynamic variation of the drain current with respect to the receive-transmit switch that makes digital predistortion linearization difficult. If there are too many traps, such as 4% diluted Al dopant in the GaN buffer, the drain current onset point for state (2) is very low and the device is difficult to linearize.

[0031] FIG. 7A shows the results of a MOSFET with 2% diluted Al dopant when subjected to receive-transmit dynamics for 5 ms from −4 V to −10 V gate voltage (VG) at a drain voltage (VD) of 48 V. FIG. 7B is a graph showing the quiescent drain current dynamics of the HEMT device 10 using a GaN buffer with a GaN dopant. The table in FIG. 7B summarizes the quiescent drain current dynamics depicted in FIG. 7A. For comparison, FIG. 8A is a graph showing the dynamics of a similarly structured device with iron deep traps instead of the aluminum dopants of the HEMT device 10. Those skilled in the art will recognize that the aluminum doped buffer exhibits greatly improved dynamics. The table in FIG. 8B summarizes the quiescent drain current dynamics depicted in FIG. 8A.

[0032] 9 shows an amplifier 40 in which a pair of HEMT elements 10-1 and 10-2 are utilized as amplifying elements configured to amplify an RF signal. In this exemplary embodiment, the amplifier is a Doherty amplifier with the HEMT element 10-1 acting as a carrier amplifier and the HEMT element 10-2 acting as a peak amplifier. In this exemplary embodiment, the pair of HEMT elements 10-1 and 10-2 are coupled between an input quadrature coupler 42 and an output quadrature coupler 44. A radio frequency signal input terminal 46 (RFIN) is coupled to a first port P1 of the input quadrature coupler 42, and an input termination impedance 48 is coupled to a second port P2 of the quadrature coupler 42. An input termination impedance 50 is coupled to a third port P3 of the output quadrature coupler 44, and a radio frequency signal output terminal 52 (RFOUT) is coupled to a fourth port P4 of the output quadrature coupler 44.

[0033] It is contemplated that any of the above-described aspects and / or various separate aspects and features described herein may be combined to additional advantage. Unless otherwise indicated herein, any of the various embodiments disclosed herein may be combined with one or more of the other disclosed embodiments.

[0034] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure, and all such improvements and modifications are considered to be within the scope of the concepts disclosed herein and the following claims.

Claims

1. A substrate; an epitaxial layer overlying the substrate, a buffer layer having a dopant comprising aluminum, the concentration of aluminum in the buffer layer being between 0.5% and 3%; a channel layer covering the buffer layer; a barrier layer covering the channel layer; an epitaxial layer comprising: a gate contact disposed on a surface of the epitaxial layer; a source contact disposed over the surface of the epitaxial layer; a drain contact disposed on the surface of the epitaxial layer, the source contact and the drain contact being spaced apart from the gate contact and from each other; Equipped with High electron mobility transistor (HEMT) device.

2. the buffer layer comprises gallium nitride; The HEMT device according to claim 1 .

3. the concentration of aluminum in the buffer layer is 0.5% to 0.75%; The HEMT device according to claim 1 .

4. the concentration of aluminum in the buffer layer is between 0.75% and 1.25%; The HEMT device according to claim 1 .

5. the concentration of aluminum in the buffer layer is between 1.25% and 2%; The HEMT device according to claim 1 .

6. the concentration of aluminum in the buffer layer is between 2% and 3%; The HEMT device according to claim 1 .

7. the buffer layer being laterally divided into a doped region having a dopant comprising aluminum and an undoped region; The HEMT device according to claim 1 .

8. the concentration of aluminum in the doped region is between 0.5% and 0.75%; The HEMT device according to claim 7.

9. the concentration of aluminum in the doped region is between 0.75% and 1.25%; The HEMT device according to claim 7.

10. the concentration of aluminum in the doped region is between 1.25% and 2%; The HEMT device according to claim 7.

11. the concentration of aluminum in the doped region is between 2% and 3%; The HEMT device according to claim 7.

12. The buffer layer is a first doped region having a dopant comprising said aluminum; a second doped region having a non-aluminum dopant; Undoped region and is divided horizontally into The HEMT device according to claim 1 .

13. the concentration of aluminum in the first doped region is between 0.5% and 0.75%; The HEMT device according to claim 12.

14. the concentration of aluminum in the first doped region is between 0.75% and 1.25%; The HEMT device according to claim 12.

15. the concentration of aluminum in the first doped region is between 1.25% and 2%; The HEMT device according to claim 12.

16. the concentration of aluminum in the first doped region is between 2% and 3%; The HEMT device according to claim 12.

17. the second doped region is doped with carbon; The HEMT device according to claim 12.

18. the second doped region is doped with iron; The HEMT device according to claim 12.

19. the undoped region is adjacent to the channel layer; the first doped region is sandwiched between the undoped region and the second doped region; The HEMT device according to claim 12.

20. the HEMT device comprises an amplifier configured to amplify a radio frequency signal; The HEMT device according to claim 1 .

21. the amplifier is a Doherty amplifier; The HEMT device according to claim 20.

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