Loudspeaker with microelectromechanical unit - Patent Application 20070123633

Abstract

A loudspeaker device (200) is provided. The loudspeaker device (200) comprises a microelectromechanical unit (100). The microelectromechanical unit (100) comprises a diaphragm (10) having a planar extension. The diaphragm (10) comprises a semiconductor structure (20) including a first layer (22) and a second layer (24), each extending in the extension of the plane. The first layer (22) comprises Al(1-x)Ga(x)N, where 0.2≦x≦0.4, and the second layer (24) comprises GaN. The diaphragm (10) is arranged to oscillate in the extension transverse to the extension of the plane, thereby generating sound waves. A calibration weight (17) is arranged on a surface of the diaphragm (10). The microelectromechanical unit (214) further comprises a support structure (30) that supports the diaphragm (10), an electrode (50) disposed on the surface of the diaphragm (10), and a resonator box.

Description

[Technical field]

[0001] The present disclosure relates to loudspeakers comprising microelectromechanical units. [Background technology]

[0002] Audio devices such as amplifiers connected to loudspeakers or microphones have many physical characteristics such as on-resistance, signal-to-noise ratio (SNR), power / energy consumption, etc. These physical characteristics often involve a compromise between high fidelity audio quality and the physical dimensions of the audio device. This compromise is now even greater as the consumption of sound by ever smaller wireless earphones rapidly transitions, especially since the wireless earphones have built-in batteries to power multiple loudspeakers, microphones, and relatively power-hungry wireless communication means. Thus, there is a current need to reduce the on-resistance and increase the SNR while simultaneously minimizing the dimensions of audio devices and their power requirements. Summary of the Invention [Problem to be solved by the invention]

[0003] The object is to mitigate, alleviate or eliminate one or more of the above deficiencies and disadvantages in the art singly or in any combination, and to at least partially resolve the above problems. [Means for solving the problem]

[0004] The invention is as set out in the accompanying claims.

[0005] According to a first aspect, a loudspeaker device is provided. The loudspeaker device comprises a microelectromechanical unit. The microelectromechanical unit comprises a diaphragm having a planar extension and comprising a semiconductor structure including a first layer and a second layer, each layer extending in the extension of the plane, the first layer comprising Al(1-x)Ga(x)N, 0.2≦x≦0.4, and the second layer comprising GaN, a support structure supporting the diaphragm, and an electrode connected to the diaphragm. Preferably, the electrode is arranged on a surface of the diaphragm. The diaphragm is arranged to oscillate in the extension transverse to the extension of the plane, thereby generating sound waves. A calibration weight is arranged on the surface of the diaphragm. The microelectromechanical unit further comprises a resonance box.

[0006] Elemental materials are referred to herein by their elemental symbol or abbreviation. For example, gallium nitride may be typically referred to as GaN, and aluminum gallium nitride may be referred to as AlGaN. In general, a layer or structure considered to include a particular material or element may be understood to at least partially include or consist essentially of the particular material or element. The layers of a semiconductor structure may be understood to be ordered in bottom-to-top order. In this context, the term "on" refers to the placement of a layer or structure above or onto another layer or structure. The term "vertical" refers to the direction in which layers are placed relative to one another. The vertical direction may be considered to be perpendicular or normal to the top surface of the substrate, which may be considered to be substantially planar. The term "lateral" refers to any direction perpendicular to the vertical direction.

[0007] The semiconductor structure is piezoelectrically actuable. Thus, a mechanical impact on the semiconductor structure can generate a voltage difference, thereby generating an electric current. Conversely, an applied voltage can cause deformation or movement of the semiconductor structure. Thus, the diaphragm may be utilized for acoustic effects, such as those capable of generating or detecting a pressure difference in air, i.e., a sound wave. Sound waves as discussed herein should be interpreted in their ordinary sense, i.e., a periodic pressure difference having a frequency in the range of about 5-100000 Hz, preferably in the range of 20-20000 Hz. Thus, the diaphragm can function both as a loudspeaker / microphone membrane and as a solenoid. Traditionally, a solenoid has an operating principle according to Lenz's law, being an external device connected to the membrane to generate / capture the movement of the membrane, thereby generating / capturing the sound wave. Herein, such a solenoid is redundant, thereby forming an integrated device with fewer members. The microelectromechanical unit can reduce the need for power. Other advantages include an increased signal-to-noise ratio SNR, and a reduced on-resistance. This may also allow for a relatively compact size.

[0008] The terms oscillating and vibrating may be used interchangeably throughout this specification.

[0009] The diaphragm may be circular in the extension of the plane, the diameter of the diaphragm being between 1 and 800 micrometers (μm).

[0010] The semiconductor structure of the diaphragm may be a superlattice including GaN and Al(1-x)Ga(x)N layers, with 0.2≦x≦0.4. The superlattice may include multiple heterostructure layers, each including one GaN and one Al(1-x)Ga(x)N layer. The superlattice may provide a p-type two-dimensional hole gas 2DHG to provide a channel for conduction along the interface between the two layers. The superlattice may facilitate the generation of an increased piezoelectric effect, thus improving the signal-to-noise ratio of the speaker device according to the invention. A film having relatively pronounced piezoelectric properties due to the superlattice may facilitate the fabrication of the piezoelectric film, compared to the case of using rare earth metals such as scandium in scandium-doped AlN, i.e., scandium in ScAlN, or lead-based materials such as lead zirconate titanate PZT. In many cases, standard piezoelectric thin films used in MEMS are polycrystalline. The superlattices considered herein are preferably crystalline, so that the piezoelectric effect is not degraded by possible defects from polycrystalline grains. Thus, the semiconductor structure provides a more sensitive membrane that can be used to detect, for example, smaller deformations of the membrane.

[0011] The periodicity of the superlattice may be from 2 to 6 nm.

[0012] The thickness of the diaphragm may be 0.1 to 5 micrometers.

[0013] The microelectromechanical unit may further comprise a backplate having a plurality of through holes. The density of the through holes may be 0.1 to 0.6 holes per micron. The backplate is raised relative to the diaphragm by a circumferential wall. Thus, a cavity exists between the diaphragm and the backplate. The cavity may function as a pressure chamber or a resonating box. The through holes may be through openings oriented perpendicular to the extension of the plane of the diaphragm. The plurality of through holes may enhance the acoustic properties of the diaphragm. Furthermore, the cavity may facilitate the amplification of the pressure difference resulting from the movement of the diaphragm. The backplate may be made of aluminum nitride by sputtering, for example magnetron sputtering. This may provide a backplate with low residual stress on top of the AlGaN diaphragm during fabrication of the microelectromechanical unit. Alternatively, the backplate may be made of silicon nitride.

[0014] The electrodes may be ring-shaped and may be arranged on the edge of the diaphragm. This facilitates the generation of a voltage induced by the deformation of the diaphragm. Conversely, a specific voltage can induce a relatively large deformation of the diaphragm. This arrangement therefore provides a more sensitive diaphragm.

[0015] The support structure may include a semiconductor layer structure forming a transistor. A layer structure may be understood as a structure in which one layer is vertically disposed on another layer and the upper layer shares a physical interface with an adjacent layer below. Such physical interface may be configured to provide a conductive contact, i.e., to allow electron and / or hole transport across the physical interface. A conductive contact may refer to, for example, an ohmic contact, a Schottky contact, or a contact at a pn junction or a tunnel junction. The transistor may be a high electron mobility transistor HEMT. A HEMT allows for higher switching frequencies and improved high power characteristics compared to a conventional metal oxide semiconductor field effect transistor MOSFET.

[0016] The semiconductor layer structure of the support structure may include a silicon substrate, which allows thicker GaN to be deposited / grown thereon, improving crystal quality without the manufacturing complexities associated with forming a crystalline GaN / AlGaN layer structure. Other materials such as silicon carbide SiC may also be possible. However, pure silicon is preferred due to its relatively low cost.

[0017] The semiconductor layer structure of the supporting structure may include a GaN layer and an Al(1-x)Ga(x)N layer, where 0.2≦x≦0.4.

[0018] The semiconductor layer structure of the support structure may include an AlN layer. Such an AlN layer may be disposed on a silicon substrate and below a GaN layer and / or an Al(1-x)Ga(x)N layer. In most cases, due to factors such as different crystal lattice constants and thermal expansion coefficients for silicon and nitride materials, simply forming a nitride layer on top of a silicon layer would result in cracks, defects, and overall poor crystal quality in the formed nitride layer, e.g., due to mismatch in material properties. Thus, the AlN layer facilitates a smoother material transition between the silicon substrate and the GaN and Al(1-x)Ga(x)N layers, thereby providing sufficient electron or hole mobility through the support structure.

[0019] The microelectromechanical unit may further comprise a calibration weight arranged on the surface of the diaphragm. The calibration weight may be ring-shaped, especially when the diaphragm is ring-shaped. The calibration weight may be made of metal, preferably AlCu. The calibration weight is arranged to adjust the mass of the diaphragm to adjust the acoustic properties of the diaphragm.

[0020] According to a second aspect, there is provided a loudspeaker device comprising a microelectromechanical unit according to the first aspect above and a circuit configured to actuate the diaphragm of the microelectromechanical unit.

[0021] The features above relating to the microelectromechanical unit apply equally to this second aspect, where applicable, and therefore in order to avoid undue repetition, please refer to the above. The circuit may be configured in a class D amplifier.

[0022] The circuit may include a driver, a comparator, and a first GaN high electron mobility transistor HEMT and a second GaN high electron mobility transistor HEMT connected to the driver, the first HEMT and the second HEMT electrically connected to the comparator.

[0023] The comparator can function as a normal comparator, e.g., an operational amplifier, by comparing the audio input signal with a high-frequency triangular wave and digitizing the audio input signal. The result of the comparison is a digital copy of the analog audio input signal, where the low-frequency components of the digital signal represent the audio input signal and the high-frequency signals are largely ignored. The output of the comparator drives a HEMT with a driver connected in between, e.g., as is typical for a class-D amplifier. As mentioned above, GaN HEMT transistors can offer higher switching frequencies and improved high-power characteristics compared to conventional MOSFETs. Furthermore, power requirements may be reduced.

[0024] According to a third aspect, there is provided a microphone device comprising a microelectromechanical unit according to the first aspect and circuitry configured to detect movement of a diaphragm of the microelectromechanical unit.

[0025] The microphone device may be an "inverted loudspeaker device", but since power efficiency is generally of little concern in microphones, the circuitry contained within the microphone device may be of Class A type.

[0026] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

[0027] Therefore, it is to be understood that the present invention is not limited to the specific components of the device described or the operation of the method described, since such devices and methods may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. It should be noted that, as used in this specification and the appended claims, the articles "a", "an", "the" and "said" are intended to mean that one or more of an element may be present, unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and so forth. Furthermore, words such as "comprises", "includes", "including" and similar expressions do not exclude other elements or steps.

[0028] These and other aspects of the present invention will now be described in more detail with reference to the accompanying figures, which should not be considered limiting, but rather are used for purposes of explanation and understanding, and in which like reference numbers refer to similar elements throughout the specification. [Brief description of the drawings]

[0029] [Figure 1] FIG. 2 is a schematic diagram illustrating a cross-sectional profile of a microelectromechanical unit. [Figure 2A-2B] FIG. 2A is a schematic diagram illustrating a cross-sectional profile of a microelectromechanical unit exposed by a bias voltage. [Diagram 3] FIG. 2 is a diagram illustrating a top view of a diaphragm. [Figure 4]1A-1C are schematic diagrams illustrating alternative geometries of the diaphragm; [Diagram 5] FIG. 1 shows a highly schematic diagram of a loudspeaker device. [Figure 6] FIG. 1 shows a highly schematic diagram of a microphone device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The present invention will now be described in more detail hereinafter with reference to the accompanying drawings, in which presently preferred embodiments of the invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein, but rather these embodiments are provided for thoroughness and completeness, and to fully convey the scope of the invention to those skilled in the art.

[0031] With reference to FIG. 1, a cross-sectional profile of a microelectromechanical unit 100 is shown diagrammatically. The microelectromechanical unit 100 comprises a diaphragm 10. The diaphragm 10 has a planar extension. The planar extension may be an extension in the plane of a substrate 110 to which the microelectromechanical unit 100 is attached. The planar extension and the plane of the substrate 100 may therefore be substantially parallel. The diaphragm 100 comprises a semiconductor structure. The semiconductor structure comprises a first layer 22 and a second layer 24. It should be noted that FIG. 1 shows a non-limiting representation of a plurality of first and second layers. Each of the first layer 22 and the second layer 24 extends in a planar extension. The first layer 22 comprises aluminum gallium nitride Al(1-x)Ga(x)N, with 0.2≦x≦0.4. The value of x sets the band gap of Al(1-x)Ga(x)N. The band gap is in the range of 3.4 eV (x=1) to 6.2 eV (x=0). The second layer includes gallium nitride GaN. Thereby, the band gap of GaN is 3.4 eV. The microelectromechanical unit 100 further comprises a support structure 30. The support structure 30 supports the diaphragm 10. Preferably, the average cross-sectional area of ​​the support structure 30 that occupies within the extension of the plane is smaller than the area of ​​the diaphragm 10. The support structure 30 can elevate the diaphragm 10 from the plane of the substrate 110. Thus, the bottom surface 12 of the diaphragm 10 and the top surface 112 of the plane of the substrate 110 may be separated by a diaphragm-substrate distance D1, which is the closest distance between the diaphragm 10 and the substrate 110. Thereby, a cavity 40 may be present below the diaphragm 10. Thus, the diaphragm 10 may be enabled to oscillate within the extension transversely, i.e. perpendicularly, to the extension of the plane. The terms oscillating, e.g., vibrating, may be used interchangeably throughout this specification. The diaphragm 10 may further be allowed to expand / contract in the vertical direction. The diaphragm 10 may be p-doped to form a two-dimensional hole gas at the interface 23 between Al(1-x)Ga(x)N and GaN. This creates a potential well perpendicular to the interface 23 that is small enough that only a few bound electronic states can exist, due to the slight difference in lattice constants between Al(1-x)Ga(x)N and GaN, thereby preventing electrons or holes from moving perpendicular to the interface 23.However, if a sufficient bias voltage is applied to interface 23, the electrons or holes will still move vertically (eg, across the heterostructure superlattice).

[0032] The microelectromechanical unit 100 further comprises an electrode 50 connected to the diaphragm. Preferably, the electrode 50 is disposed on the surface 14 of the diaphragm 10. The electrode 50 may be directly attached on the top surface 50 of the diaphragm 10. The electrode 50 may be made of any material having sufficient electrical conductivity. For example, the conductive material may be Ti, Al, Cu, Ni, and / or Au. Preferably, the conductive material is a compound or alloy, such as, for example, aluminum copper AlCu.

[0033] The semiconductor structure 20 of the diaphragm 10 has piezoelectric properties. Thus, mechanical deformation of the semiconductor structure of the diaphragm provides an electrical polarization of the semiconductor structure. Thereby, a non-zero voltage between different locations of the semiconductor structure may be induced by the mechanical deformation 80, 82. This is shown diagrammatically in Fig. 2A and Fig. 2B. A non-zero voltage may further induce a current. As is typical for piezoelectric materials, the reverse is also possible: a bias voltage applied between different locations may generate a mechanical deformation of the semiconductor structure 20 of the diaphragm. In this situation, this effect may be achieved by charging the electrode 50. The charging may generate deformations 80, 82 of the diaphragm 10, thus causing a contraction 80 or expansion 82 of the diaphragm 10 in the vertical direction, i.e. substantially perpendicular to the extension of the plane of the diaphragm 10. For example, a positive bias voltage applied to the electrode 50 may generate a contraction 80 of the diaphragm, and a negative bias voltage applied to the electrode 50 may generate an expansion 82 of the diaphragm 10.

[0034] The diaphragm 10 may be circular in its plane extension. This is shown in FIG. 3. The geometry of the diaphragm may thereby be substantially circularly symmetric when viewed in a direction perpendicular to the surface of the diaphragm 10. The support structure 30 may thereby have a similar circular geometry in order to support the diaphragm 10 near its outer edge 16. The support structure 30 is thus substantially cylindrical, the support structure 30 having an axial extension perpendicular to the plane of the diaphragm (and to the plane of the substrate). The thickness of the cylindrical wall of the support structure may be significantly smaller than the diameter of the diaphragm 10. The diameter of the diaphragm 10 may be between 1 and 50 μm.

[0035] If the diaphragm 10 has a circular geometry, the electrodes 50 may be ring-shaped and may be located at the edge of the diaphragm. The electrodes 50 may be simply connected. Thus, there may not be any breaks or separations along the azimuthal direction of the ring electrodes 50.

[0036] The semiconductor structure 20 of the diaphragm 10 may be a superlattice 20 including GaN and Al(1-x)Ga(x)N, with 0.2≦x≦0.4. This is shown diagrammatically in FIG. 1. The superlattice 20 may include multiple heterostructure layers 22, 24, each including one GaN layer 22 and one Al(1-x)Ga(x)N layer 24. Thus, every second layer 2i may be a GaN layer 22, and every layer 2(i+1) between each pair of GaN layers may be an Al(1-x)Ga(x)N layer 24. However, the number of layers need not be an even number. If the number of layers is an odd number, the bottom and top layers may have similar chemical compositions, GaN or Al(1-x)Ga(x)N. The periodicity of the superlattice 20 may be 2 to 6 nanometers (nm). The superlattice 20 may provide multiple planes, in each of which a p-type two-dimensional hole gas 2DHG is present. The 2DHG thereby provides a channel for conduction along the interface 23 between two adjacent layers 22, 24. However, as noted above, electrons or holes can move between the layers, provided a sufficient bias voltage is applied between the layers. The superlattice 20 may be capable of producing an enhanced piezoelectric effect.

[0037] As illustrated in FIG. 4, the cross-sectional profile of the superlattice 20 in a plane coincides with the centerline of the semiconductor structure 20, i.e., parallel to the axial extension of the support structure 30, and does not have to be rectangular. Thus, the top layer 18 of the superlattice 20 can have a smaller area than the bottom layer 19 of the superlattice. As an example, the superlattice 20 can be radially chamfered, so that the thickness of the periphery of the diaphragm 10 is smaller than the thickness of the central portion of the diaphragm. Preferably, the diaphragm 10 is chamfered so that the bottom and top surfaces of the diaphragm 10 near the periphery are substantially parallel. On this top surface, an electrode 50 can be attached. Similarly, the top and bottom surfaces of the central portion of the diaphragm can also be substantially parallel. The central portion can involve a relatively large area of ​​the diaphragm 10, and thus constitute, for example, more than half the area of ​​the entire top surface 14 of the diaphragm 10.

[0038] The thickness of the superlattice 20 may be between 0.1 and 5 μm. Thus, the superlattice 20 may include hundreds or even thousands of AlGaN / GaN heterostructure layers 22, 24. As mentioned above, the thickness may vary, for example, between the edge 11 and the central portion 13 of the diaphragm 10. Preferably, the thickness is greater at the central portion 13 than at the edge 11. Furthermore, the thickness may depend in the radial direction.

[0039] The microelectromechanical unit may further comprise a back plate 60. The back plate may include a plurality of through holes 15. The back plate is elevated with respect to the diaphragm by a circumferential wall 62. The circumferential wall 62 is attached to the diaphragm 10 directly or indirectly, preferably directly. Preferably, the circumferential wall 62 is attached to the circumference of the diaphragm 10. Alternatively, the circumferential wall 62 is attached to the support structure 30. Thus, a cavity 70 exists between the diaphragm 10 and the back plate 60. The cavity may be seen as a pressure chamber or a resonance box. The through holes 15 may be through-openings oriented perpendicular to the extension of the plane of the back plate 10. The plurality of through holes 15 are randomly distributed in the back plate 10. Alternatively, the plurality of through holes 15 may be orderly distributed such that the through holes 15 themselves form a lattice when the back plate 60 is viewed perpendicular to its top or bottom surface.

[0040] Returning now to the support structure 30 of the microelectromechanical unit 100, the support structure 30 may include a semiconductor layer structure forming a transistor. As mentioned above, a layer structure may be understood as a structure in which one layer is vertically disposed on top of another layer, and the upper layer shares a physical interface with the adjacent layer below. The physical interface may thus be configured to provide a conductive contact, i.e., to allow electron and / or hole transport across the physical interface. The conductive contact may refer to, for example, an ohmic contact, a Schottky contact, or a contact at a pn junction or a tunnel junction. The transistor may be a high electron mobility transistor HEMT, which may enable relatively high switching frequencies and desirable high power characteristics. This facilitates high electron / hole mobility, and / or other typical characteristics of a 2DEG or 2DHG, since the electron / hole conduction propagates in two dimensions (2DEG / 2DHG) along the interface between the two layers as mentioned above. It is noted that the electron or hole mobility is generally smaller between layers in the layer structure than within the plane of the layers.

[0041] The semiconductor layer structure of the support structure 30 may include a silicon base layer 32. The silicon base layer 32 may be directly attached onto the substrate 110. Alternatively, the silicon base layer 32 may be a relatively large silicon wafer on which AlN may be grown (discussed further below), and thus the silicon base layer 32 may form part of the substrate 110. The substrate 32, 110 may thereby include a silicon bulk material 32, 110. The top surface 33 of the silicon base layer 32 may be substantially planar. The vertical thickness of the silicon base layer may be in the range of 100 to 1000 μm, more preferably in the range of 275 to 525 μm. Unless expressly stated otherwise, hereinafter thickness refers to the vertical thickness. The top surface 33 of the silicon base layer may have Miller indices (111). The silicon base layer 32 may have a diamond cubic crystal structure.

[0042] The semiconductor layer structure of the support structure may include an aluminum nitride AlN layer 34. The AlN layer 34 may preferably have a thickness in the range of 100-500 nm, more preferably in the range of 200-300 nm. The AlN layer may include vertical nanowire structures 35. These nanowires 35 may preferably have a vertical length in the range of 50-500 nm, more preferably in the range of 150-250 nm. The vertical nanowire structures 35 may preferably have a substantially circular or hexagonal transverse cross-sectional profile. The diameter of such nanowires may be in the range of 5-50 nm, more preferably in the range of 10-30 nm. The nanowires 35 may be arranged in a vertically repeating array pattern, with each nanowire 35 having four equidistant nearest other nanowires. Alternatively, the repeating array pattern may have a rectangular pattern. The distance between adjacent nanowires 35 may be preferably in the range of 10 to 500 nm, more preferably in the range of 50 to 200 nm.

[0043] The semiconductor layer structure of the support structure 30 may further include a GaN layer 36 and an Al(1-x)Ga(x)N layer (not shown), where 0.2≦x≦0.4. The Al(1-x)Ga(x)N layer may be disposed between the GaN layer 36 and the diaphragm 10. The layer disposed directly above the aforementioned AlN layer 34 may be the GaN layer 36. The GaN layer 36 may have a thickness preferably in the range of 100-500 nm, more preferably in the range of 200-300 nm. The GaN layer 36 may be considered to laterally encapsulate, encapsulate, or surround the vertical nanowire structures 35, i.e., fill the space between the vertical nanowire structures. The GaN layer 36 may further be considered to vertically encapsulate or encapsulate the vertical nanowire structures, i.e., extend vertically above the vertical nanowire structures and cover the tops of the vertical nanowire structures.

[0044] The GaN layer 36 may be attached directly to the diaphragm 10 .

[0045] The microelectromechanical unit 100 may further include a calibration weight 17 disposed on the surface of the diaphragm 10. The calibration weight 17 may be disposed directly on the top surface of the diaphragm. As exemplified above, if the diaphragm is circular, the calibration weight 17 may also be ring-shaped. This particular exemplary shape is shown in FIG. 3. The calibration weight 17 may be disposed centrally on the diaphragm. Thus, the center of the diaphragm 10 and the center of the ring-shaped calibration weight 17 may substantially coincide. The calibration weight 17 may be manufactured by a metal compound or alloy, for example, AlCu. However, other materials may also be applicable.

[0046] With reference to FIG. 5, a loudspeaker device 200 is shown in schematic form. The loudspeaker device 200 comprises a microelectromechanical unit. The microelectromechanical unit 100 is as described above. The loudspeaker device 200 further comprises a circuit 210 configured to actuate the diaphragm 10 of the microelectromechanical unit 100. The circuit 210 may form part of a class D audio amplifier. Thus, the circuit 210 may utilize a pulse width modulation PWM, using a triangular or square wave oscillator. A conventional class D amplifier comprises two output MOSFETs and an external low pass filter to recover the amplified audio signal. However, in the present specification, the class D amplifier may be a filter-less amplifier. Furthermore, first and second GaN high electron mobility transistors HEMTs may replace the conventional MOSFETs. The HEMTs may operate as current steering switches by alternately connecting the output nodes to a supply voltage Vdd and ground. The resulting output is therefore a high frequency square wave. The output square wave may be pulse width modulated by the input audio signal. This may be achieved by comparing the input audio signal to an internally generated triangular or sawtooth oscillator. This may be referred to as natural sampling, where a triangular oscillator may act as the sampling clock. The duty cycle of the resulting square wave is proportional to the level of the input signal. GaN HEMTs may be arranged for such a variable duty cycle. The diaphragm 10 of the microelectromechanical unit 100 may act as the acoustic wave source of the loudspeaker device 200. Thereby, the acoustic properties of the diaphragm 10 depend on its semiconductor structure, for example the number of layers in the semiconductor structure of the diaphragm 10, possible through holes 15, the geometry of the planar extension, etc. The acoustic properties configured herein may include relative intensity, vibration frequency, etc. as in the conventional manner. The class D amplifier may be a full bridge class D amplifier. The circuit 210 may comprise a comparator 212. The comparator 212 may function as a conventional comparator, for example an operational amplifier, by comparing the audio input signal with a high frequency triangle wave and digitizing the audio input signal.The result of the comparison is a digital copy of the analog audio input signal, with the low frequency components of the digital signal representing the audio input signal and the high frequency signals being largely ignored. The first and second HEMTs may be electrically connected to a comparator. The output of the comparator drives the HEMTs by a driver connected in between, as is typical for example for a class D amplifier. The diaphragm 10 may be connected to a loudspeaker device 200, which is an 8Ω speaker. The circuit 210 may comprise a field programmable gate array FPGA to handle the digital part of the device. The input audio may be a 24-bit Pulse Code Modulated PCM signal, encoded into a stream of pulses at a kHz sample rate.

[0047] Driving GaN HEMT devices requires that attention be paid to the distance between the GaN HEMT device gate contacts and the driver, which can provide short and equal distances between several GaN HEMT devices abutting the driver to alternately switch the GaN HEMT devices ON and OFF for half-bridge or full-bridge operation at high switching frequencies.

[0048] With reference to Fig. 6, a microphone device 300 is shown diagrammatically. The microphone device 300 comprises a microelectromechanical unit 100. The microelectromechanical unit 100 is as described above. The microelectromechanical unit 100 comprises a circuit 310. The circuit 310 is configured to detect a movement of the diaphragm 10 of the microelectromechanical unit 100. Thus, the mechanical movement of the diaphragm 10 induces a voltage difference, which may be converted into an electrical signal in the "reverse direction" having regard to the features described in relation to the loudspeaker device above.

[0049] Thus, in summary, as discussed above, a microelectromechanical unit 100 has been described. The microelectromechanical unit 100 may be used, for example, in a loudspeaker or microphone. The piezoelectric properties of the diaphragm 10 of the microelectromechanical unit 100 may make a coil / solenoid redundant, since an applied bias voltage may be converted into mechanical movement / deformation 60, 62 by the diaphragm itself, and vice versa.

[0050] Those skilled in the art will appreciate that the present invention is in no way limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

[0051] For example, it will be readily appreciated that the microelectromechanical units may be used in many different devices, such as pressure sensors, flow sensors, and cantilever-based sensors for biomarkers.

[0052] Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. A microelectromechanical unit, comprising: a diaphragm having a planar extension and comprising a semiconductor structure including a first layer and a second layer, each layer extending within the planar extension, the first layer including Al(1−x)Ga(x)N with 0.2 ≦ x ≦ 0.4, the second layer including GaN, the diaphragm being arranged to oscillate in a transverse direction with respect to the planar extension within the extension, thereby generating a sound wave, and a calibration weight being arranged on the surface of the diaphragm; a support structure for supporting the diaphragm; and an electrode connected to the diaphragm, the microelectromechanical unit; a resonance box; A loudspeaker device comprising.

2. The loudspeaker device according to claim 1, wherein the diaphragm is circular within the planar extension and the diameter of the diaphragm is 1 to 50 micrometers.

3. The loudspeaker device according to claim 1, wherein the semiconductor structure of the diaphragm is a superlattice including a GaN layer and an Al(1−x)Ga(x)N layer, with 0.2 ≦ x ≦ 0.

4.

4. The loudspeaker device according to claim 3, wherein the periodicity of the superlattice is 2 to 6 nm.

5. The loudspeaker device according to claim 1, wherein the thickness of the diaphragm is 0.1 to 5 μm.

6. The loudspeaker device according to claim 1, wherein the resonance chamber comprises a back plate having a plurality of through holes.

7. The loudspeaker device according to claim 1, wherein the electrode is ring-shaped and is arranged at the edge of the diaphragm.

8. The loudspeaker device according to claim 1, wherein the support structure includes a semiconductor layer structure forming a transistor.

9. The loudspeaker device according to claim 8, wherein the semiconductor layer structure of the support structure includes a silicon-based layer.

10. The loudspeaker device according to claim 8, wherein the semiconductor layer structure of the support structure includes a GaN layer and an Al(1−x)Ga(x)N layer, with 0.2 ≦ x ≦ 0.

4.

11. The loudspeaker device according to claim 8, wherein the semiconductor layer structure of the support structure includes an AlN layer.

12. The loudspeaker device according to claim 1, further comprising a calibration weight arranged on the surface of the diaphragm.

13. A circuit configured to operate the diaphragm of the microelectromechanical unit The loudspeaker device according to claim 1, further comprising

14. wherein the circuit comprises a driver, a comparator, a first GaN high electron mobility transistor (HEMT) and a second GaN high electron mobility transistor (HEMT) that are in contact with the driver, the loudspeaker device according to claim 13, wherein the first HEMT and the second HEMT are electrically connected to the comparator.

Images