High electron mobility transistor device having aluminum-doped buffer layer

Abstract

To provide a high electron mobility transistor device having an aluminum-doped buffer layer that eliminates leakage and thus provides improved linearity.SOLUTION: A high electron mobility transistor (HEMT) device 10 includes a substrate and an epitaxial layer 14 covering the substrate that includes a buffer layer having a dopant 30 comprising aluminum, the concentration of aluminum within the buffer layer being between 0.5% and 3%. The epitaxial layer further includes a channel layer covering the buffer layer and a barrier layer covering the channel layer. A gate contact is disposed on a surface of the epitaxial layer. A source contact and a drain contact are also 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.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 on December 17, 2020, the disclosure of which is hereby incorporated by reference in its entirety.

[0002] This disclosure relates to high electron mobility transistor (HEMT) devices having buffer layers that provide improved linearity.

Background Art

[0003] The radio frequency (RF) spectrum is becoming increasingly congested. Therefore, it is highly desirable to increase the linearity of amplifiers to share the congested RF spectrum. At least one problem that reduces the linearity of amplifiers is the unwanted intrusion of silicon or oxygen during the epitaxial growth of gallium nitride buffers, which are typically undoped gallium nitride buffers that are normally n - type. n - type gallium nitride buffers typically result in leaky gallium nitride high electron mobility transistor devices with very poor RF performance. There is a need for high electron mobility transistor devices having buffer layers that eliminate leakage and thus provide improved linearity.

Summary of the Invention

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

[0005] In other embodiments, for additional advantages, any of the embodiments described herein may be combined individually or together, as well as various other distinct embodiments and features described herein. Unless otherwise indicated herein, any of the various features and elements disclosed herein may be combined with one or more other disclosed features and elements.

[0006] Those skilled in the art will understand the scope of this disclosure and recognize additional embodiments after reading the following "Modes for Carrying Out the Invention" in conjunction with the accompanying drawings.

[0007] The accompanying drawings incorporated herein and forming part thereof illustrate several aspects of the present disclosure and, together with the description, serve to illustrate the principles of the present disclosure. [Brief explanation of the drawing]

[0008] [Figure 1] This is a cross-sectional view of a high electron-mobility transistor (HEMT) element structured in accordance with this disclosure. [Figure 2] This is a cross-sectional view of an exemplary embodiment of a HEMT element, in which the buffer layer is laterally divided into an undoped region and a first doped region having a dopant containing aluminum. [Figure 3] This 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] This is a band diagram of a typical semiconductor structure for a field-effect transistor. [Figure 5] This is a typical deep trap profile for a gallium nitride (GaN) buffer using iron traps. [Figure 6A] This shows the drain current dynamics of the HEMT element when it is operated as a gate voltage controlled switch. [Figure 6B] This is a cross-sectional view of a HEMT element, where the traps in the buffer are not filled and are represented by white circles. [Figure 6C]This is a cross-sectional view of an HEMT element, where the buffer is filled with traps and represented by black circles. [Figure 7A] This graph shows the quiescent drain current dynamics of a HEMT device using a GaN buffer with a 2% diluted Al dopant. [Figure 7B] This table summarizes the static drain current dynamics shown in the graph in Figure 7A. [Figure 8A] This graph shows the dynamics of a similarly structured device having an iron deep trap instead of the aluminum dopant of the HEMT device of this disclosure. [Figure 8B] This table summarizes the static drain current dynamics shown in the graph in Figure 8A. [Figure 9] An amplifier is shown that utilizes a pair of HEMT elements as an amplifying element configured to amplify a radio frequency signal in accordance with this disclosure. [Modes for carrying out the invention]

[0009] The embodiments described below represent the necessary information to enable those skilled in the art to practice the embodiments and represent the best mode of practice. By reading the following description in reference to the accompanying drawings, those skilled in the art will understand the concepts of this disclosure and recognize applications of these concepts that are not covered in detail herein. It should be understood that these concepts and applications fall within the scope of this disclosure and the accompanying 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 solely to distinguish one element from another. For example, without departing from the scope of this disclosure, the first element may be referred to as the second element, and similarly, the second element may be referred to as the first element. As used herein, the term "and / or" includes any and all combinations of one or more of the listed items relating to the subject.

[0011] When an element such as a layer, region, or substrate is described as being "on top of" or "extending upward" of another element, it will be understood that there may be elements directly on top of, extending directly upward, or intervening elements. In contrast, when an element is described as being "directly on top of" or "extending directly upward" of another element, there are no intervening elements. Similarly, when an element such as a layer, region, or substrate is described as "covering" or "extending over" another element, it will be understood that there may be elements that directly cover or extend over the other element, or intervening elements. In contrast, when an element is described as "directly covering" or "extending over" another element, there are no intervening elements. Furthermore, when an element is described as being "connected" or "bonded" to another element, it will be understood that there may be elements that can directly connect to, be bonded to, or intervening elements. In contrast, when an element is said to be "directly connected" or "directly joined" to another element, there is no intermediary element.

[0012] Relative terms such as "lower side" or "upper side," "top" or "bottom," or "horizontal" or "vertical" may be used herein to describe the relationship between one element, layer, or region and another when illustrated in a figure. It will be understood that these terms and the terms considered above are intended to encompass different orientations of the elements, in addition to the orientation shown in the figure.

[0013] The terms used herein are intended solely to describe specific embodiments and are not intended to limit the disclosure. When used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context explicitly indicates otherwise. When used herein, the terms “comprises,” “comprising,” “includes,” and / or “including” specify the presence of the described feature, integer, step, operation, element, and / or component, 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 those generally understood by those skilled in the art to which this disclosure belongs. Terms used herein should be construed to have meanings consistent with their meanings in the context of this specification and the related art, and it will be further understood that they should not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0015] Embodiments are described herein with reference to schematic drawings of embodiments of the present disclosure. Therefore, the actual dimensions of layers and elements may differ, and deformations from the exemplary shapes are expected as a result of manufacturing techniques and / or tolerances. For example, areas illustrated or described as squares or rectangles may have rounded or curved features, and areas shown as straight lines may have some irregularities. Thus, the areas illustrated in the figures are schematic, and their shapes are not intended to illustrate the exact shapes of areas of elements and are not intended to limit the scope of the present disclosure. Additionally, the size of structures or areas may be exaggerated relative to other structures or areas for illustrative purposes, and are therefore provided to illustrate general structures of the subject matter, and may or may not be drawn to scale. Common elements between figures may be indicated herein by common element numbers and may not be described again thereafter.

[0016] Undoped gallium nitride (GaN) buffers are typically n-type due to undesirable 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 the leakage of the GaN buffer, device engineers introduce deep traps during growth. The two most common deep level 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 abruptly switched. This results in adverse consequences such as having to add more deep level traps to the buffer than necessary. 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 growth conditions including temperature, growth rate, and the V / III ratio. If the optimized growth window for achieving the highest crystal quality of the gallium nitride buffer is relatively small, controlling carbon doping to the desired level becomes very difficult.

[0018] Deep level traps have positive characteristics such as suppressing leakage between the drain and source sides of high electron mobility transistor devices, which results in generating higher performance RF devices. Deep level traps have negative characteristics such as slow detrapping when receiving receive events and transmit events, which creates undesirable dynamics in the quiescent drain current state of the device.

[0019] The dynamics in receive and transmit switching events are particularly adverse in the context of time division duplex base station applications because of the dynamic change in the mutual conductance that the device experiences when its operation progresses and reaches the steady-state quiescent drain current. These dynamic changes in mutual conductance complicate the application of digital predistortion algorithms used to linearize gallium nitride amplifiers. This results in high electron mobility transistor devices with difficult linearization and thus limited information transmission capabilities.

[0020] Finally, the number of deep-level traps in the buffer layer and the detrapping times associated with them determine how a particular device can be used. In the case of applications that do not involve transmission and reception switching, deep-level traps of iron are preferred because they decouple the growth temperature and quality of the buffer layer from the amount of insulation required in the buffer layer. In the case of applications that involve transmission and reception switching, carbon-doped buffer layers are preferred because they do not require a large number of deep-level traps (instantaneous switching during growth) and do not introduce detrapping dynamics (very slow detrapping times) that interfere with the baseband signal being transmitted, and because they suppress low quality and require fewer controllable buffer layers.

[0021] In the present disclosure, a new insulating buffer having the temperature decoupling and high controllability of iron, as well as the abrupt switching of growth and slow detrapping time of carbon, is disclosed.

[0022] Specifically, a replacement for typical carbon or iron deep-level traps is fabricated that has aluminum-oxygen complexes and other native defects generated through doping of the buffer using diluted aluminum dopants. The main advantages of using aluminum instead of carbon or iron are as follows.

[0023] Similar to carbon, the aluminum-oxygen complex is a very slow trap, which prevents aliasing of any detrapping dynamics and information dynamics. Also, similar to carbon, the profile of the aluminum-oxygen complex can be changed abruptly, thereby eliminating the need to add more traps than are 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 by the flow of trimethylaluminum decouples the intrusion of deep-level traps from the growth conditions of the buffer, and thus the quality of the entire buffer is maintained.

[0024] In short, the replacement of carbon and iron with aluminum produces low-concentration deep-level trap elements with excellent transmit / receive switching characteristics controlled through a more manufacturable growth process, without using growth conditions to control the number of traps.

[0025] Figure 1 is a cross-sectional view of a high electron-mobility transistor (HEMT) element 10 structured according to this disclosure. The HEMT element 10 comprises a substrate 12 and an epitaxial layer 14 covering the substrate 12. The epitaxial layer 14 includes a buffer layer 16, a channel layer 18 covering the buffer layer 16, and a barrier layer 20 covering the channel layer 18. The HEMT element 10 also includes a gate contact 22 located on the surface 24 of the epitaxial layer 14. It further includes a source contact 26 located covering the surface 24 of the epitaxial layer 14 and a drain contact 28 located on the surface 24 of the epitaxial layer 18, the source contact 26 and the drain contact 28 being spaced apart from and away from the gate contact 22. In exemplary embodiments, the buffer layer 16 is made of gallium nitride having an aluminum-containing dopant 30. In some exemplary embodiments, the concentration of aluminum in the buffer layer 16 due to the dopant 30 is 0.5% to 3%. In some other exemplary embodiments, the concentration of aluminum in the buffer layer 16 with dopant 30 is 0.5% to 0.75%. In yet another exemplary embodiment, the concentration of aluminum in the buffer layer 16 with dopant 30 is 0.75% to 1.25%. In yet another exemplary embodiment, the concentration of aluminum in the buffer layer 16 with dopant 30 is 1.25% to 2%. In yet another exemplary embodiment, the concentration of aluminum in the buffer layer 16 with dopant 30 is 2% to 3%.

[0026] In some exemplary embodiments, as shown in Figure 2, the buffer layer 16 is laterally divided into an undoped region 32 and a first doped region 34 having an aluminum-containing dopant 30. In other exemplary embodiments, as shown in Figure 3, the buffer layer 16 has a second doped region 36 having a non-aluminum dopant 38, represented by the letter X in Figure 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 of a typical semiconductor structure for a field-effect transistor. Note the energy of the unintentionally present donor states. The black lines represent 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. Otherwise, the buffer will become conductive and unsuitable for RF applications.

[0028] Figure 5 shows 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 iron concentration cannot decrease rapidly. The ideal trap profile, shown by the dashed line in Figure 5, has a steep profile. This steeper profile can be achieved using carbon or the aluminum-oxygen composite present in the aluminum-diluted GaN buffer. However, using carbon requires changes to the growth conditions, which, among other things, affects the breakdown of field-effect transistors built into the buffer.

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

[0030] As shown in Figure 6A, as the gate voltage increases from state (1) to state (2), the drain current flowing from the drain to the source increases. Since the trap takes a certain amount of time to detrap, the increase to the steady state (3) is not instantaneous. It is this dynamic fluctuation of the drain current related to the receive-transmit switching that makes digital pre-distortion linearization difficult. If there are too many traps, such as in a 4% diluted Al dopant in a GaN buffer, the drain current starting point in state (2) is very low, making it difficult to linearize the device.

[0031] Figure 7A is a graph showing the quiescent drain current dynamics of HEMT element 10 using a GaN buffer with a 2% diluted Al dopant when subjected to receive-transmit dynamics over 5ms at a drain voltage (VD) of -4V to -10V gate voltage (VG). The table in Figure 7B summarizes the quiescent drain current dynamics shown in Figure 7A. For comparison, Figure 8A is a graph showing the dynamics of a similarly structured element having an iron deep trap instead of the aluminum dopant of HEMT element 10. Those skilled in the art will recognize that the aluminum-doped buffer exhibits significantly improved dynamics. The table in Figure 8B summarizes the quiescent drain current dynamics shown in Figure 8A.

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

[0033] For additional advantages, it is conceivable that any of the embodiments described herein, as well as the various distinct embodiments and features described herein, may be combined. Unless otherwise indicated herein, any of the various embodiments disclosed herein may be combined with one or more other disclosed embodiments.

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

Claims

1. A non-layered substrate; an epitaxial layer overlying the non-layered substrate, a buffer layer disposed directly on a surface of the unlayered substrate, the buffer layer having a dopant comprising aluminum that forms an aluminum-oxygen complex within the buffer layer, the concentration of aluminum within 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. A non-layered substrate; an epitaxial layer overlying the non-layered substrate, a buffer layer laterally divided into a first doped region having an aluminum dopant concentration of between 0.5% and 3% that forms an aluminum-oxygen complex, a second doped region disposed directly on the surface of the unlayered substrate and having a non-aluminum dopant, and a third doped region disposed over the first doped region and being an undoped region; 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.

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.