Doherty amplifiers made of group III nitrides using different epitaxial structures
Doherty amplifiers with distinct epitaxial structures for main and peaking amplifiers improve efficiency and reduce thermal management needs, addressing variability and manufacturing challenges.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- MACOM TECH SOLUTIONS HLDG INC
- Filing Date
- 2024-06-21
- Publication Date
- 2026-07-08
Smart Images
Figure 2026522640000001_ABST
Abstract
Description
Technical Field
[0001] This application claims priority to U.S. Utility Patent Application No. 18 / 213,682, filed Jun. 23, 2023, the entire content of which is incorporated herein by reference.
[0002] This application generally relates to Doherty amplifiers including Group III nitride materials, and more particularly, to the characteristics of the epitaxial structure of Group III nitride materials.
Background Art
[0003] A high electron mobility transistor (HEMT) is a type of field effect transistor (FET) having a low noise figure at microwave frequencies. HEMTs are used in radio frequency (RF) circuits as both digital switches and current amplifiers, and high performance at very high frequencies is required. HEMTs employ a heterojunction—a junction between materials having different bandgaps. HEMTs have been fabricated from several materials including silicon (Si), gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs), and gallium nitride (GaN) and aluminum gallium nitride (AlGaN).
[0004] Si has a relatively low electron mobility (e.g., 1450 cm2 / V-s). This results in a high source resistance, and the high source resistance limits the HEMT gain. GaAs has a higher electron mobility (e.g., 6000 cm2 / V-s), and thus a lower source resistance, thereby enabling higher gain at high frequencies. However, GaAs has a bandgap of only 1.42 eV at room temperature and a small breakdown voltage, thereby limiting high power performance at high frequencies.
[0005] Group III nitrides have a larger band gap compared to these other semiconductor materials, making them suitable for higher power and higher frequency applications. While GaN is particularly important, generally, Group III nitride heterojunctions for HEMTs can be formed from two-component, three-component, or four-component alloys of Group III metals and nitrogen. This configuration can be represented as AlxInyGa1-x-yN, where 0<=x<=1 and 0<=y<=1, i.e., any combination of some or all of aluminum, indium, and gallium mixed with nitrogen. In particular, the densities of the various alloys may be modified to control the semiconductor properties. For example, aluminum increases the band gap of GaN, while indium decreases it.
[0006] GaN has a bandgap of 3.36 eV and relatively high electron mobility (e.g., 2019 cm² / Vs). Therefore, GaN HEMTs offer high power and high-temperature operation at high frequencies, making them well-suited for wireless communications, radar, defense applications, and other uses. In GaN HEMTs, a heterojunction is formed at the boundary between a GaN layer and, for example, an AlGaN layer. As used herein, AlGaN is an abbreviation for the chemical formula AlxGa1-xN, for 0 ≤ x ≤ 1, meaning that the concentration of Al in the alloy may vary. The AlGaN layer may further have a concentration gradient, where the concentration of Al atoms in the lattice varies as a function of depth.
[0007] In a heterojunction between a GaN layer and an AlGaN layer, the difference in bandgap energy between the higher bandgap AlGaN and GaN creates a two-dimensional electron gas (2DEG) within the smaller bandgap GaN, which has higher electron affinity. The 2DEG has a very high electron concentration. Furthermore, the Al moiety within the AlGaN layer generates piezoelectric charges at the interface, transferring electrons to the 2DEG within the GaN layer, enabling high electron mobility. For example, the sheet density in the 2DEG of an AlGaN / GaN HEMT can exceed 10¹³ cm⁻². The high carrier concentration and high electron mobility in the 2DEG create large transconductance, resulting in high performance for HEMTs at high frequencies.
[0008] In practical applications, GaN HEMTs require various circuits containing a variety of components such as capacitors, inductors, and resistors. For example, a HEMT used as an amplifier generally requires RF filters at the amplifier input, output, or both, to optimize the amplifier's operation over the desired frequency range and to perform impedance matching with the connected circuitry.
[0009] One particular type of amplifier is known as a Doherty amplifier. An example of a Doherty amplifier 10 is schematically represented in Figure 1. The Doherty amplifier 10 comprises a power divider 12, a phase shifter 14, input matching stages 16a, 16b, first and second transistors 18a, 18b, output matching stages 20a, 20b, an impedance inverter 22, and a transformer 24.
[0010] The power divider 12 receives the RF input signal (RF INThe signal is split into a first signal and a second signal (for example, using a quadrature coupler). The first transistor 18a operates on the first signal and is used for most signal amplification. In this respect, the first transistor 18a is often referred to as the “main” or “carrier” amplifier stage. The second transistor 18b operates on the second signal and is used to amplify the signal peak. Thus, the second transistor 18b is often referred to as the “sub” or “peak” amplifier stage. The more general terms “first” and “second” amplifiers are used herein.
[0011] A key feature of the Doherty amplifier is the output connection of the first and second amplifiers via an impedance inverter 22, which is often implemented using a quarter-wavelength transmission line and frequently has a 90-degree phase shift. At low input signal power levels, the second amplifier is inactive and effectively open-circuit. The system impedance is reduced at the output of the second amplifier by the output matching network 24. This impedance is inverted to a much higher impedance by the impedance inverter 22, presenting a high output impedance to the first amplifier 18a and improving its efficiency. As the second amplifier begins to amplify the signal peak, its increasing output current (added to the output current of the first amplifier) increases the voltage across the load impedance, and the impedance inverter 22 presents this load impedance to the first amplifier as a lower impedance. The lower impedance allows the output power of the first amplifier to increase as the input signal power increases. This is known as load modulation, and as a result, the Doherty amplifier 10 exhibits high efficiency over the entire range of input signal power.
[0012] Efficient and effective construction of Doherty amplifiers is crucial for optimizing performance. In particular, simplifying the manufacturing of integrated circuits, as well as improving their yield and reliability, can lead to cost reductions and higher integration. To enhance reliability and simplicity, main amplifiers and peaking amplifiers are traditionally constructed using the same manufacturing process (for example, using die-cutting from the same wafer) because this tends to reduce variability between amplifiers.
[0013] The background section of this document is provided to place embodiments of the invention in a technical and operational context and to help those skilled in the art understand their scope and practicality. The methods described in the background section are not necessarily previously devised or performed, but are performable. Unless explicitly stated otherwise, claims herein are not considered prior art simply by their inclusion in the background section. [Overview of the Initiative]
[0014] The following is a simplified summary of the disclosure to provide a basic understanding to those skilled in the art. This summary is not a comprehensive overview of the disclosure and is not intended to identify important / crucial elements of the embodiments of the invention or to precisely outline the scope of the invention. The sole purpose of this summary is to present some of the concepts disclosed herein in a simplified form as an introduction to the more detailed description to be presented later.
[0015] Embodiments of the present disclosure generally relate to Doherty amplifiers comprising amplifier dies having different epitaxial structures. For example, the amplifier dies may be from different Group III nitride epiwafers and have different epitaxial structures. This is in contrast to conventional Doherty amplifiers, which, for example, involve amplifier dies that use the same epitaxial structure and are typically produced from the same epiwafer, in order to further reduce variability.
[0016] In particular, one or more embodiments include a Doherty amplifier comprising a main amplifier and a peaking amplifier, which are electrically connected to the same input signal source and have different epitaxial structures of a group III nitride material.
[0017] In some embodiments, the epitaxial structure of the peaking amplifier provides the peaking amplifier with a higher power density than the epitaxial structure of the main amplifier.
[0018] In some embodiments, the epitaxial structure of the main amplifier provides the main amplifier with a higher gain than the epitaxial structure of the peaking amplifier.
[0019] In some embodiments, the epitaxial structure of the main amplifier provides the main amplifier with higher transconductance than the epitaxial structure of the peaking amplifier.
[0020] In some embodiments, the epitaxial structure of the peaking amplifier allows the peaking amplifier to have a higher maximum current than the epitaxial structure of the main amplifier.
[0021] In some embodiments, the epitaxial structure of the main amplifier provides the main amplifier with more linear amplification than the epitaxial structure of the peaking amplifier.
[0022] In some embodiments, the group III nitride material includes AlGaN.
[0023] In some embodiments, the epitaxial structure of the main amplifier and the epitaxial structure of the peaking amplifier have different polarities. In some such embodiments, the different polarities include GaN polarity, nitrogen polarity, and / or semipolarity.
[0024] In some embodiments, the main amplifier and / or peaking amplifier further include a dielectric intermediate layer.
[0025] Other embodiments are directed to a method of forming a Doherty amplifier. The method includes forming a main amplifier and a peaking amplifier that include Group-III nitride transistors having different epitaxial structures from different epitaxial wafers such that the Group-III nitride transistors of the main amplifier and the Group-III nitride transistors of the peaking amplifier have different epitaxial structures. The method further includes dicing the epitaxial wafers to produce respective amplifier dies each including a main amplifier and a peaking amplifier. The method further includes mounting the amplifier dies on a common heat sink. The method further includes electrically connecting the main amplifier and the peaking amplifier to a common input signal source.
[0026] In some embodiments, the epitaxial structure of the peaking amplifier provides the peaking amplifier with a higher power density than the epitaxial structure of the main amplifier.
[0027] In some embodiments, the epitaxial structure of the main amplifier provides the main amplifier with a higher gain than the epitaxial structure of the peaking amplifier.
[0028] In some embodiments, the epitaxial structure of the main amplifier provides the main amplifier with a higher transconductance than the epitaxial structure of the peaking amplifier.
[0029] In some embodiments, the epitaxial structure of the peaking amplifier enables the peaking amplifier to have a higher maximum current than the epitaxial structure of the main amplifier.
[0030] In some embodiments, the epitaxial structure of the main amplifier provides the main amplifier with more linear amplification than the epitaxial structure of the peaking amplifier.
[0031] In some embodiments, the Group-III nitride material includes AlGaN.
[0032] In some embodiments, the epitaxial structure of the main amplifier and the epitaxial structure of the peaking amplifier have different polarities. In some such embodiments, the polarity includes GaN polarity, nitrogen polarity, and / or semipolarity.
[0033] In some embodiments, the main amplifier and / or peaking amplifier further include a dielectric intermediate layer.
[0034] Of course, those skilled in the art will understand that this embodiment is not limited to the above context or examples, and will also understand further features and advantages upon reading the following detailed description and looking at the accompanying drawings.
[0035] Hereafter, the present invention will be described more fully with reference to the accompanying drawings illustrating embodiments of the invention. However, the present invention should not be construed as being limited to the embodiments expressed herein. Rather, these embodiments are provided so that this disclosure is thorough and comprehensive and so as to fully convey the scope of the invention to those skilled in the art. Throughout, similar numbers refer to similar elements. [Brief explanation of the drawing]
[0036] [Figure 1] This block diagram shows an example of a Doherty amplifier. [Figure 2] This block diagram shows an example of a Doherty amplifier according to one or more embodiments of the present disclosure. [Figure 3A] This is a schematic side view illustrating an example of a group III nitride transistor having a different epitaxial structure according to one or more embodiments of the present disclosure. [Figure 3B] This is a schematic side view illustrating an example of a group III nitride transistor having a different epitaxial structure according to one or more embodiments of the present disclosure. [Figure 3C]This is a schematic side view illustrating an example of a group III nitride transistor having a different epitaxial structure according to one or more embodiments of the present disclosure. [Figure 4] This is a schematic top view of a group III nitride transistor according to one or more embodiments of the present disclosure. [Figure 5] This is a flowchart illustrating a method for forming a Doherty amplifier according to one or more embodiments of the present disclosure. [Modes for carrying out the invention]
[0037] The embodiments described below represent the information necessary to enable a person skilled in the art to practice the embodiments and illustrate the best way to practice them. A person skilled in the art will understand the concepts of this disclosure upon reading the following description in consideration of the accompanying drawings, and will also understand that the uses of these concepts are not specifically described herein. It should be understood that these concepts and uses fall within the scope of this disclosure and the accompanying claims.
[0038] While terms such as "first," "second," etc., may be used herein to describe various elements, 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, the first element may be called the second element, and similarly, without departing from the scope of this disclosure, the second element may be called the first element. As used herein, the term "and / or" includes any and all combinations of one or more of the related enumerated items.
[0039] When an element such as a layer, region, or substrate is described as being "on top of" another element or extending "on top of" another element, it will be understood that the element may be directly above another element or may extend directly above another element, or there may be further intervening elements. In contrast, when an element is described as being "directly above" another element or extending "directly above" another element, there are no intervening elements. Similarly, when an element such as a layer, region, or substrate is described as being "above" another element or extending "above" another element, it will be understood that the element may be directly above another element or may extend directly above another element, or there may be further intervening elements. In contrast, when an element is described as being "directly above" another element or extending "directly above" another element, there are no intervening elements. When an element is said to be “connected” or “joined” to another element, it will be understood that the element can be directly connected to or joined to other elements, or that there may be an intervening element. In contrast, when an element is said to be “directly connected” or “directly joined” to another element, there is no intervening element.
[0040] Relative terms such as "below," "above," "upper," "lower," "horizontal," or "up and down" may be used herein to describe the relationship of one element, layer, or region to another, as shown in the figures. It will be understood that these terms and the content discussed above are intended to encompass different orientations of the device, in addition to the orientation depicted in the figures.
[0041] The terminology used herein is intended to describe only specific embodiments and is not intended to limit the disclosure. When used herein, the singular forms “a,” “an,” and “the” are intended to include the plural form as well, unless the context explicitly indicates otherwise. When used herein, the terms “comprises,” “comprising,” “includes,” and / or “including” express the presence of the described features, integers, steps, actions, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, actions, elements, components, and / or groups thereof.
[0042] Unless otherwise specified, all terms used herein (including technical and scientific terms) have the same meaning as they would be generally understood by those skilled in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted in a way that is consistent with their meanings in the context of this specification and in the relevant technical field, and will not be interpreted in an idealized or overly formal sense unless explicitly stated herein to do so.
[0043] Figure 2 illustrates a simplified example of a Doherty amplifier according to one or more embodiments of the present disclosure. As shown in Figure 2, the first and second amplifiers (i.e., transistors 18a and 18b) are mounted on a common heatsink 94. According to some embodiments, the approximate dimensions of each amplifier are 1 to 6 mm × 0.8 to 1.4 mm. Each amplifier is connected (for example, by bond wires, as shown) to its respective input circuits 91a and 91b. Either or both of the input circuits 91a and 91b may include, for example, a phase shifter 14, an input matching stage 16, and / or a harmonic termination circuit. Furthermore, it should be understood that the amplifier may comprise a single-stage or multi-stage transistor 18. In addition, the Doherty amplifier may have multiple peaking paths with additional transistors. In this embodiment, for simplicity, the amplifiers are shown as single-stage transistors 18a and 18b, respectively.
[0044] Each of the input circuits 91a and 91b receives an RF input signal (RF IN The input signal is provided by a power divider 12 connected to an input signal source that provides the input signal.
[0045] Each of the transistors 18a and 18b is connected to their respective output circuits 92a and 92b (for example, by bond wires as shown). Either or both of the output circuits 92a and 92b may include, for example, an output matching stage 20 and / or a harmonic termination circuit. Each of the output circuits 92a and 92b combines their respective signals into a combined RF output signal (RF) of the Doherty amplifier 10. OUTThe power is supplied to the power combiner 93 which generates the power. According to some embodiments, each of the transistors 18a and 18b uses the same drain bias voltage of 50V. That said, the transistors 18a and 18b may have different maximum current capacities because, in some embodiments, they are constructed with Group III nitride transistors having different epitaxial structures and / or using different wafer manufacturing processes. For example, the Doherty amplifier 10 may include respective dies for transistors 18a and 18b cut from different Group III nitride epiwafers having different epitaxial structures.
[0046] Although Figure 2 only shows the first amplifier 18a and the second amplifier 18b, it should be understood that other examples of the Doherty amplifier 10 may include one or more further amplifiers. Each further amplifier may operate as an additional peaking amplifier configured to amplify signal peaks. Each further amplifier may be implemented in a similar manner to the second amplifier 18b, accepting the same input signal from an input signal source and having its output power combined with other output signals in the power combiner 93.
[0047] Figure 3A schematically shows an exemplary cross-section of a Group III nitride HEMT or transistor 18 according to one or more embodiments of the present disclosure. The transistor 18 includes a substrate 110 formed from, for example, silicon carbide (SiC), silicon, or sapphire. In fact, embodiments of this application can utilize any suitable substrate. For example, the substrate 110 may be formed from a SiC wafer on which many transistors 18 are formed. The wafer is then separated into individual dies. It will be understood that each transistor 18 depicted in Figures 3A-C is a unit cell of the die, and multiple unit cells are formed in parallel and diced to form individual transistors, as shown in Figure 4, which will be discussed below.
[0048] The transistor 18 may further include a nucleation layer 120 deposited on the substrate 110. The nucleation layer 120 may be formed from, for example, aluminum nitrate (AlN).
[0049] The transistor 18 further includes a GaN channel layer 130 deposited on the nucleation layer 120 (or, alternatively, directly on the substrate 110). The amplifier 18 further includes a barrier layer 150 deposited on the channel layer 130. The barrier layer 150 may include, for example, a GaN alloy with aluminum (AlGaN).
[0050] A heterojunction is formed at the boundary between the channel layer 130 and the barrier layer 150. The difference in bandgap energy between the higher bandgap AlGaN and the lower bandgap GaN creates a two-dimensional electron gas (2DEG) 140 in the GaN, which has a higher electron affinity. Furthermore, the Al portion within the AlGaN layer generates piezoelectric charges at the interface, moving electrons into the 2DEG 140 within the GaN layer, enabling high electron mobility. For example, the sheet density in the 2DEG 140 of an AlGaN / GaN HEMT can exceed 10¹³ cm⁻². The high carrier concentration and high electron mobility in the 2DEG 140 produce large transconductance, resulting in high performance for the HEMT at high frequencies. Such performance can be particularly useful in applications such as RF. Therefore, in some useful embodiments, the transistor 18 may be included in an RF Doherty amplifier.
[0051] The barrier layer 150 may include doping 160 of n-type material in injection areas on the upper surface of the barrier layer 150 to facilitate electrical connectivity between the barrier layer 150 and a plurality of contacts formed on the barrier layer 150, which are spaced laterally apart from each other. The contacts may include, for example, a drain contact 1005 and a gate contact 1010. The material of the gate contact 1010 may be selected based on the composition of the barrier layer 150 and, in some embodiments, may be a Schottky contact.
[0052] The source contact 1015 may also be formed on the barrier layer 150 on the opposite side of the drain contact 1005 from the gate contact 1010. The source contact 1015 may be coupled to a reference signal, such as the ground voltage. Coupling to the reference signal may be provided by a via 1025 extending from the bottom surface of the substrate 110 through the substrate 1022 (not just any intermediate layer) to the top surface of the barrier layer 150. The via 1025 may expose the bottom surface of the resistive portion 1015a of the source contact 1015. In this way, a signal coupled to the back metal layer 1035 beneath the substrate 110 may be electrically connected to the source contact 1015.
[0053] The transistor 18 may include a first insulating layer 1050 and a second insulating layer 1055. The first insulating layer 1050 may be in contact with the upper surface of the barrier layer 150. The second insulating layer 1055 may be formed on the first insulating layer 1050. It will be further understood that in some embodiments, three or more insulating layers may be included. The first insulating layer 1050 and the second insulating layer 1055 can function as passivation layers for the transistor 18.
[0054] The source contact 1015, drain contact 1005, and gate contact 1010 may be formed within the first insulating layer 1050. In some embodiments, at least a portion of the gate contact 1010 is on the first insulating layer 1050. In some embodiments, the gate contact 1010 may be formed as a T-gate and / or gamma gate, the formation of which is discussed as an example in U.S. Patents 8,049,252, 7,045,404, and 8,120,064, the disclosures of which are incorporated herein by reference in their entirety. The second insulating layer 1055 may be formed on the first insulating layer 1050, as well as on portions of the drain contact 1005, gate contact 1010, and source contact 1015.
[0055] In some embodiments, the field plate 1060 is formed on the second insulating layer 1055. At least a portion of the field plate 1060 may be on the gate contact 1010. At least a portion of the field plate 1060 may be on the portion of the second insulating layer 1055 between the gate contact 1010 and the drain contact 1005. The field plate and techniques for forming the field plate are discussed as an example in U.S. Patent No. 8,120,064, the disclosure of which is incorporated herein by reference in its entirety.
[0056] The metal contacts 1065 may be disposed within the second insulating layer 1055. The metal contacts 1065 can provide interconnections between the drain contacts 1005, gate contacts 1010, and source contacts 1015 and other components of the amplifier 18. Each of the metal contacts 1065 can directly contact each of the drain contacts 1005 and / or each of the source contacts 1015.
[0057] The group III nitride materials in the first and second amplifiers 18a and 18b have different epitaxial structures in at least one respect. Depending on the embodiment, the different epitaxial structures may take the form of different epitaxial layers, such as different number of layers and corresponding layers having different compositions (e.g., a barrier layer containing an AlGaN layer with 25% Al versus a barrier layer containing an AlGaN layer with 35% Al), different doping levels, and / or different thicknesses; different polarity devices, such as N-polar GaN versus Ga-polar GaN; different doping levels, grading, and / or profiles; and different injection regions, levels, depths, and / or profiles. The different epitaxial structures do not include variations based on production tolerances, such as plus or minus 10%.
[0058] Figure 3B schematically shows another exemplary cross-section of the transistor 18 according to one or more embodiments of the present disclosure. The epitaxial structure of the group III nitride material (AlGaN in this example) of the transistor 18 in Figure 3B differs from the epitaxial structure in Figure 3A. In this particular example, the layer thicknesses vary. Furthermore, although not shown in the figure, the alloy concentration inside the barrier layer 150 may differ in some embodiments. Additionally or alternatively, the group III nitride material of the first amplifier 18a and the group III nitride material of the second amplifier 18b may differ. In one such example, one of the amplifiers may contain AlGaN and the other may contain AlN.
[0059] Additionally or alternatively, the doping 160 in the barrier layer may be different. In the example in Figure 3B, the barrier layer 150 and a portion of the channel layer 130 include doping 160 (e.g., using an n-type dopant) to improve conductivity. In contrast to the relatively shallow doped region in Figure 3A, the doping 160 in Figure 3B extends into the channel layer 130 and includes 2DEG140 formed at the heterojunction between the channel layer 130 and the barrier layer 150.
[0060] The epitaxial structures of the first transistor 18a and the second transistor 18b may vary in other ways, either additionally or alternatively. For example, the group III nitride materials of the first and second transistors 18a and 18b may have different crystal orientations. In one such example, the AlGaN of the barrier layer 130 of the amplifiers 18a and 18b may have different polarities (e.g., GaN polarity, nitrogen polarity, semipolarity, nonpolarity). Additionally or alternatively, the group III nitride materials of the first and second transistors 18a and 18b may have different polytypes (e.g., 3C, 2H, 4H, 6H).
[0061] Due to the different epitaxial structures of transistors 18a and 18b, they have different characteristics from each other. This allows, for example, a main amplifier to be designed with characteristics that are advantageous for relatively continuous signal amplification, while a peaking amplifier may be designed with characteristics that are advantageous for amplifying peaks.
[0062] For example, the Doherty amplifier 10 can often operate efficiently because the main amplifier itself can provide a sufficient amount of gain for operation for a significant portion of the time and only activate the additional peaking amplifier circuit when needed. Under such circumstances, given that the main amplifier is configured to amplify the signal almost continuously, the main amplifier may have stricter heat dissipation requirements than the peaking amplifier, which is not operated as frequently.
[0063] In view of these potential thermal constraints, embodiments of the present disclosure may include a main amplifier having a lower power density than the peaking amplifier. Additionally or alternatively, the main amplifier may occupy a larger space than the peaking amplifier. Such features can reduce thermal management requirements, for example, by allowing the main amplifier to dissipate heat more aggressively relative to the peaking amplifier. For example, by using a main amplifier that does not concentrate heat accumulation and / or increases surface contact with the heatsink 94, heat can be effectively dissipated despite the nearly constant operation of the main amplifier.
[0064] In contrast, given that peaking amplifiers are configured to amplify signals more intermittently, the only advantage they may have is lower thermal management needs. Therefore, peaking amplifiers can have higher power density and / or occupy less space than the main amplifier. Occupying less space and / or having higher power density means that peaking amplifiers may require less material to operate, and thus, among other advantages, may be cheaper to produce.
[0065] Another difference between the epitaxial structure of the main amplifier and the epitaxial structure of the peaking amplifier in some embodiments is that the epitaxial structure of the main amplifier provides the main amplifier with a higher gain than the epitaxial structure of the peaking amplifier. This can be advantageous considering that the main amplifier may generally play a role in providing all of the gain of the output signal, at least for a portion of the time, whereas the peaking amplifier only plays a role in providing an amount of gain that enhances peaks higher than those already provided by the main amplifier.
[0066] Another difference between the epitaxial structure of the main amplifier and the epitaxial structure of the peaking amplifier in some embodiments is that the epitaxial structure of the main amplifier provides the main amplifier with higher transconductance than the epitaxial structure of the peaking amplifier. Generally, the higher the transconductance of a device, the greater the gain that the device can deliver (all other factors are constant). Therefore, in some embodiments, the higher transconductance enabled by the epitaxial structure of the main amplifier can be used to achieve the aforementioned higher gain.
[0067] Another difference between the epitaxial structure of a peaking amplifier and the epitaxial structure of a main amplifier in some embodiments is that the epitaxial structure of the peaking amplifier allows the peaking amplifier to have a higher maximum current than the epitaxial structure of the main amplifier. In fact, certain embodiments of the peaking amplifier can have a maximum current capacity of any amount from 1.2 to 4 times that of the main amplifier due to the difference in epitaxial structure. In some embodiments, this higher maximum current works in conjunction with the aforementioned higher power density, for example, by having a more compact design that results in lower electrical resistance.
[0068] Additionally or alternatively, the epitaxial structure of the main amplifier can provide the main amplifier with more linear amplification than the epitaxial structure of the peaking amplifier. Generally speaking, the more linear the amplification, the less efficient the amplifier becomes. In this respect, the main amplifier can provide higher linearity, while the peaking amplifier can provide efficient operation when required. By operating in conjunction, the main amplifier and the peaking amplifier can find a favorable balance between output linearity and efficiency.
[0069] According to certain embodiments, the group III nitride material of amplifiers 18a and 18b is gallium nitride (GaN). The epitaxial structure of each amplifier 18a and 18b may have any suitable polytype. That said, in some embodiments, the epitaxial structures of the amplifiers may be of different polytypes. For example, in some embodiments, the epitaxial structure of the main amplifier may be the 3C polytype of the group III nitride material. Additionally or alternatively, the epitaxial structure of the peaking amplifier may be the 2H polytype of the group III nitride material.
[0070] As described above, some embodiments of the Doherty amplifier 10 include one or more amplifiers comprising a transistor 18 that includes an AlN intermediate layer 190 as part of the barrier layer 150. An example of such a transistor 18 is illustrated in Figure 3C.
[0071] A Doherty amplifier 10 according to one or more embodiments described herein may be particularly useful in a variety of high-performance applications. Such high-performance applications may include, among others, RF (for example, in cellular base station radio), aerospace and defense communications, and / or radar.
[0072] Figure 4 is a top view of the amplifier die 1000, which includes multiple unit cells 1016 (depicted as transistors 18 in Figures 3a-c). The dielectric layers that isolate the various conductive elements of the uppermost metallized layer 180 from each other are not shown in Figure 4 for the sake of simplicity.
[0073] To increase output power and current handling capability, and as described above with respect to Figures 3A-C, the transistor 18 is implemented in a "unit cell" configuration, where a number of individual "unit cell" transistors 18 are electrically arranged in parallel. The transistor 18 may also be implemented as a single "die" containing a number of unit cells.
[0074] As shown in Figure 4, the amplifier die 1000 comprises a plurality of unit cell transistors 1016, each containing a gate finger 1052, a drain finger 1054, and a source finger 1056 (shown in cross-section as transistor 18 in Figures 3a-c). The gate finger 1052 is electrically connected to a common gate bus 1046, and the drain finger 1054 is electrically connected to a common drain bus 1048. The gate bus 1046 is electrically connected to a gate terminal 1042 (for example, via conductive vias extending upward from the gate bus 1046), which is implemented as a gate bond pad, and the drain bus 1048 is electrically connected to a drain terminal 1044 (for example, via conductive vias extending upward from the drain bus 1048), which is implemented as a drain bond pad. The source finger 1056 is electrically connected to the source terminal via a plurality of conductive source vias 1066, the plurality of conductive source vias 1066 extending through the semiconductor layer structure 1030. The conductive source vias 1066 may include metal-plated vias that extend entirely through the semiconductor layer structure 1030.
[0075] In line with the above, Figure 5 illustrates a method 200 for forming a Doherty amplifier 10. As shown in Figure 5, method 200 includes forming a main amplifier and a peaking amplifier, including Group III nitride transistors having different epitaxial structures from different epiwafers, such that the Group III nitride transistors of the main amplifier and the Group III nitride transistors of the peaking amplifier have different epitaxial structures (block 210). Method 200 further includes dicing the epiwafer to produce separate amplifier dies 1000, each containing the main amplifier and the peaking amplifier, respectively, of Group III nitride transistors having different epitaxial structures (block 220). Method 200 further includes mounting the amplifier dies 1000 on a common heatsink 94 (block 230). Method 200 further includes electrically connecting the main amplifier and the peaking amplifier to a common RF input signal source (block 240). The main amplifier and the peaking amplifier, as well as their epitaxial structures, may have any of the attributes or characteristics described above. It should be understood that the main amplifier includes a first group III nitride transistor having a first epitaxial structure, and that the main amplifier may include multiple transistor stages. The peaking amplifier includes a second group III nitride transistor having a second epitaxial structure different from the first epitaxial structure, and the peaking amplifier may include multiple transistor stages. Further peaking paths are possible.
[0076] The present invention may, of course, be carried out in ways other than those specifically described herein, without departing from the essential characteristics of the invention. These embodiments should be considered illustrative in all respects, not limiting, and all modifications that fall within the meaning and scope of the appended claims and their equivalents are intended to be included therein. While the steps of various processes or methods described herein may be shown and described as being in a sequential or transient order, such steps of a process or method are not limited to being performed in any particular sequence or order unless otherwise indicated. Indeed, such steps of a process or method may generally be performed in various different sequences and orders, while still falling within the scope of the invention.
Claims
1. A Doherty amplifier (10) comprising a main amplifier (18a) and a peaking amplifier (18b) electrically connected to the same input signal source and having different epitaxial structures of a group III nitride material.
2. The Doherty amplifier according to claim 1, wherein the epitaxial structure of the peaking amplifier (18b) provides the peaking amplifier (18b) with a higher power density than the epitaxial structure of the main amplifier (18a).
3. The Doherty amplifier according to claim 1 or 2, wherein the epitaxial structure of the main amplifier (18a) provides the main amplifier (18a) with a higher gain than the epitaxial structure of the peaking amplifier (18b).
4. The Doherty amplifier according to any one of claims 1 to 3, wherein the epitaxial structure of the main amplifier (18a) provides the main amplifier (18a) with a higher transconductance than the epitaxial structure of the peaking amplifier (18b).
5. The Doherty amplifier according to any one of claims 1 to 4, wherein the epitaxial structure of the peaking amplifier (18b) enables the peaking amplifier (18b) to have a higher maximum current than the epitaxial structure of the main amplifier (18a).
6. The Doherty amplifier according to any one of claims 1 to 5, wherein the epitaxial structure of the main amplifier (18a) provides the main amplifier (18a) with more linear amplification than the epitaxial structure of the peaking amplifier (18b).
7. The Doherty amplifier according to any one of claims 1 to 6, wherein the group III nitride material includes aluminum gallium nitride (AlGaN).
8. The Doherty amplifier according to any one of claims 1 to 7, wherein the epitaxial structure of the main amplifier (18a) and the epitaxial structure of the peaking amplifier (18b) have different polarities.
9. The Doherty amplifier according to claim 8, wherein the different polarities include GaN polarity, nitrogen polarity, and / or semipolarity.
10. The Doherty amplifier according to any one of claims 1 to 9, wherein the main amplifier (18a) and / or the peaking amplifier (18b) further include a dielectric intermediate layer.
11. A method (200) for forming a Doherty amplifier (10), (210) Forming the main amplifier (18a) and the peaking amplifier (18b) to include the group III nitride transistors having different epitaxial structures from different epiwafers, such that the group III nitride transistors of the main amplifier (18a) and the group III nitride transistors of the peaking amplifier (18b) have different epitaxial structures, Dicing the epiwafer (220) to produce amplifier dies including the main amplifier (18a) and the peaking amplifier (18b), Mounting the amplifier die on a common heatsink (230), A method (200) comprising electrically connecting the main amplifier (18a) and the peaking amplifier (18b) to a common input signal source (240).
12. The method according to claim 11, wherein the epitaxial structure of the peaking amplifier (18b) provides the peaking amplifier (18b) with a higher power density than the epitaxial structure of the main amplifier (18a).
13. The method according to claim 11 or 12, wherein the epitaxial structure of the main amplifier (18a) provides the main amplifier (18a) with a higher gain than the epitaxial structure of the peaking amplifier (18b).
14. The method according to any one of claims 11 to 13, wherein the epitaxial structure of the main amplifier (18a) provides the main amplifier (18a) with a higher transconductance than the epitaxial structure of the peaking amplifier (18b).
15. The method according to any one of claims 11 to 14, wherein the epitaxial structure of the peaking amplifier (18b) enables the peaking amplifier (18b) to have a higher maximum current than the epitaxial structure of the main amplifier (18a).
16. The method according to any one of claims 11 to 15, wherein the epitaxial structure of the main amplifier (18a) provides the main amplifier (18a) with more linear amplification than the epitaxial structure of the peaking amplifier (18b).
17. The method according to any one of claims 11 to 16, wherein the group III nitride material includes aluminum gallium nitride (AlGaN).
18. The method according to any one of claims 11 to 17, wherein the epitaxial structure of the main amplifier (18a) and the epitaxial structure of the peaking amplifier (18b) have different polarities.
19. The method according to claim 18, wherein the different polarities include GaN polarity, nitrogen polarity, and / or semipolarity.
20. The method according to any one of claims 11 to 19, wherein the main amplifier (18a) and / or the peaking amplifier (18b) further include a dielectric intermediate layer.