Patent Description:
An audio device, such as an amplifier connected to a loudspeaker or a microphone, is associated with many physical properties, such as an on-resistance, a signal-to-noise ratio, SNR, consumption of electrical power/energy. These physical properties often include a compromise between high fidelity audio quality and the physical dimension of the audio device. With the presently rapid transition of consuming sound through ever smaller wireless earbuds pushes this compromise further, especially since wireless earbuds have a built-in battery to supply power to a plurality of loudspeakers, microphones, and relatively power draining wireless communication means. Hence, there is an ongoing need to reduce the on-resistance and increase the SNR, while at the same time keeping the dimensions of the audio device minimal as well as minimizing the power demand of the same.

For completion, the following two references point to relevant prior art:.

It is an object to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and at least partly solve the above-mentioned problem.

The invention is as set out in the appended set of claims.

According to a first aspect, there is provided a loudspeaker device. The loudspeaker device comprises a micro-electromechanical unit. The micro-electromechanical unit comprises: a diaphragm having a planar extension, the diaphragm comprising a semiconductor structure comprising a first and a second layer, each layer extending in the planar extension, wherein the first layer comprises Al(<NUM>-x)Ga(x)N, wherein <NUM> ≤ x ≤ <NUM>, and the second layer comprises 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 an extension transverse to the planar extension to thereby generate sound waves. A calibrating weight is arranged on a surface of the diaphragm. The micro-electromechanical unit further comprises a resonant box.

Elemental materials are referred to herein by their element symbol or abbreviation. For example, gallium nitride may typically be referred to as GaN and aluminium gallium nitride may be referred to as AlGaN. In general, layers or structures said to comprise a specific material or element, may be understood as at least partially comprising or substantially consisting of the specific material or element. The layers of the semiconductor structure may be understood as ordered in a bottom up order. In this context the term "on" refers to arranging layers or structures above or onto other layers or structures. The term "vertical" refers to the direction in which layers are arranged on each other. The vertical direction is considered perpendicular or normal to the top surface of the substrate, wherein the top surface may be considered to be substantially planar. The term "laterally" refers to any direction being perpendicular to the vertical direction.

The semiconductor structure is piezoelectrically actuable. Hence, a mechanical impact on the semiconductor structure may generate a voltage difference, and thereby an electrical current. Conversely, an applied voltage may cause a deformation or movement of the semiconductor structure. Hence, the diaphragm may be utilized for acoustics, such as being capable of generating or detecting pressure differences in air, i.e. sound waves. Sound waves as discussed herein are to be interpreted in their normal meaning, i.e., being periodic pressure differences with frequencies approximately in the range of <NUM>-<NUM><NUM>, and preferably in the range of <NUM>-<NUM>. Hence, the diaphragm may function as both a loudspeaker/microphone membrane and a solenoid. Traditionally, a solenoid, which has a working principle according to Lenz law, is an external device connected to the membrane to generate/capture movements of the membrane and thereby generate/capture sound waves. Herein, such a solenoid is redundant, thereby forming an integrated device with fewer parts. The micro-electromechanical unit may reduce the need of electrical power. Other advantages may include an increased signal-to-ratio, SNR, and a reduced on-resistance. Further, this may allow for a relatively compact size.

The terms oscillate and vibrate may be used interchangeably throughout.

The diaphragm may be circular in the planar extension, wherein a diameter of the diaphragm is between <NUM> and <NUM> micrometer, µm.

The semiconductor structure of the diaphragm may be a superlattice comprising GaN and Al(<NUM>-x)Ga(x)N layers, wherein <NUM> ≤ x ≤ <NUM>. The superlattice may comprise a plurality of heterostructure layers, wherein each heterostructure layer comprises one GaN layer and one Al(<NUM>-x)Ga(x)N layer. The superlattice may provide a p-type two-dimensional hole gas, 2DHG, providing a channel for conduction along an interface between two layers. The superlattice facilitate generating an increased piezoelectrical effect and hence improve the signal-to-noise ratio of a speaker device according to the invention. A membrane having relatively pronounced piezoelectric properties by a superlattice may facilitate the manufacturing of piezoelectric membranes beyond using rare earth metals such as scandium in scandium-doped AlN, i.e. ScAlN or lead-based materials such as lead zirconate titanate, PZT. Standard piezoelectric thin films used for MEMS are often polycrystalline. The superlattice considered herein is preferably crystalline; hence the piezoelectric effect is not deteriorated by possible defects from polycrystalline grains. Hence, the semiconductor structure provides for a more sensitive membrane to be used for, e.g. detecting smaller deformations of the membrane.

A periodicity of the superlattice may be between <NUM> and <NUM>.

A thickness of the diaphragm may be between <NUM> and <NUM> micrometer.

The micro-electromechanical unit may further comprise a backplate having a plurality of through-holes. The density of through-holes may be <NUM>-<NUM> holes per micron. The backplate is elevated relative to the diaphragm by a circumferential wall. Hence, a cavity is present between the diaphragm and the backplate. The cavity may function as a pressure chamber or a resonant box. The through-holes may be through-going openings oriented perpendicular to the planar extension of the diaphragm. The plurality of through-holes may facilitate acoustical properties of the diaphragm. Further, the cavity may facilitate amplifying pressure differences originating from movements of the diaphragm. The backplate may be made in aluminium nitride by sputtering e.g. magnetron sputtering. This may provide a backplate with a low residual stress on top of the AlGaN diaphragm during manufacturing of the micro-electromechanical unit. Alternatively, the backplate may be made in silicon nitride.

The electrode may be ring shaped and arranged at an edge of the diaphragm. This facilitate generation of a voltage induced by a deformation of the diaphragm. Conversely, a specific voltage may induce a relatively large deformation of the diaphragm. Hence, this arrangement provides for a more sensitive diaphragm.

The support structure may comprise a semiconductor layer structure forming a transistor. A layer structure may be understood as a layer being arranged above another layer in the vertical direction as well as the above layer sharing a physical interface with the below adjacent layer. Such a physical interface may be configured to provide conductive contact, i.e. allowing electron and/or hole transport across the physical interface. Conductive contact may refer to, e.g. ohmic contact, Schottky contact, or contact across a pn-junction or tunnel junction. The transistor may be a high-electron-mobility transistor, HEMT. HEMTs enable higher switching frequencies and improved high power characteristics compared to conventional metal oxide semiconductor field effect transistors, MOSFETs.

The semiconductor layer structure of the support structure may comprise a silicon base layer. The silicon base layer allows a thicker GaN to be deposited/grown thereon to improve crystal quality without manufacturing complications involved by formation of the crystalline GaN/AlGaN layer structure. Other materials may be possible such as silicon carbide, SiC. However, pure silicon is preferable as of its relatively low cost.

The semiconductor layer structure of the support structure may comprise a GaN layer and a Al(<NUM>-x)Ga(x)N layer, wherein <NUM> ≤ x ≤ <NUM>.

The semiconductor layer structure of the support structure may comprise an AIN layer. Such an AIN layer may be arranged above a silicon base layer and below any GaN and/or Al(<NUM>-x)Ga(x)N layers. Due to factors such as crystal lattice constants and thermal expansion coefficients being different for silicon and nitride materials, just forming a nitride layer onto a silicon layer will most often result in cracks, defects and overall poor crystal quality of the formed nitride layers due to, e.g. mismatching material properties. Hence, an AIN layer facilitate a smoother material transition between a silicon base layer and any GaN and Al(<NUM>-x)Ga(x)N layers, thereby providing an adequate electron or hole mobility through the support structure.

The micro-electromechanical unit may further comprise a calibrating weight arranged on a surface of the diaphragm. The calibrating weight may be ring-shaped, especially in the event the diaphragm is ring-shaped. The calibrating weight may be manufactured by a metal; preferably AlCu. The calibrating weight is arranged to adjust the mass of the diaphragm for tuning the acoustical properties of the diaphragm.

According to a second aspect, there is provided a loudspeaker device. The loudspeaker device comprising: a micro-electromechanical unit according to the first aspect as set out above; and circuitry configured to actuate the diaphragm of the micro-electromechanical unit.

The above-mentioned features in connection with the micro-electromechanical unit, when applicable, apply to this second aspect as well. In order to avoid undue repetition, reference is therefore made to the above. The circuitry may be comprised in a class D amplifier.

The circuitry may comprise: a driver; a comparator; and a first and a second GaN high electron mobility transistor, HEMT, connected to the driver, wherein the first and the second HEMT are electrically connected to the comparator.

The comparator may function as an ordinary comparator, e.g. an operational amplifier, by comparing an audio input signal to a high-frequency triangle wave to digitize the audio input signal. The result of the comparison is a digital copy signal from the analog audio input signal, wherein low-frequency components of the digital signal represent the audio input signal, while high-frequency signals are in general ignored. The output of the comparator drives the HEMT by an intermediately connected driver as is normal for, e.g. a class D amplifier. As set out above, the GaN HEMT transistor may provide higher switching frequencies and improved high power characteristics compared to conventional MOSFETs. Further, the electrical power demand may be reduced.

According to a third aspect, not covered by the scope of the claims, there is provided a microphone device comprising a micro-electromechanical unit according to the first aspect and circuitry configured to detect a movement of the diaphragm of the micro-electromechanical unit.

The microphone device may be a "reversed loudspeaker device". However, circuitry included in the microphone device may be of class A type since power efficiency may normally be of little concern in microphones.

The figures should not be considered limiting; instead they are used for explaining and understanding.

In connection with <FIG> there is schematically shown a cross-sectional profile of a micro-electromechanical unit <NUM>. The micro-electromechanical unit <NUM> comprises a diaphragm <NUM>. The diaphragm <NUM> has a planar extension. The planar extension may be an extension in a plane of a substrate <NUM> on which the micro-electromechanical unit <NUM> is attached. Hence, the planar extension and the plane of the substrate <NUM> may be substantially parallel. The diaphragm <NUM> comprises a semiconductor structure. The semiconductor structure comprises a first <NUM> and a second <NUM> layer. Notice that <FIG> shows, in a non-limiting way, a plurality of first and second layers. Each layer of the first <NUM> and the second <NUM> layer extends in the planar extension. The first layer <NUM> comprises aluminum gallium nitride, Al(<NUM>-x)Ga(x)N, wherein <NUM> ≤ x ≤ <NUM>. The value of x sets the bandgap of Al(<NUM>-x)Ga(x)N. The bandgap ranges from <NUM> eV (x=<NUM>) to <NUM> eV (x=<NUM>). The second layer comprises gallium nitride, GaN. The bandgap of GaN is thereby <NUM> eV. The micro-electromechanical unit <NUM> further comprises a support structure <NUM>. The support structure <NUM> supports the diaphragm <NUM>. Preferably, an average cross-sectional area of the support structure <NUM> taken in the planar extension is smaller than an area of the diaphragm <NUM>. The support structure <NUM> may elevate the diaphragm <NUM> from the plane of the substate <NUM>. Hence, a bottom surface <NUM> of the diaphragm <NUM> and a top surface <NUM> of the plane of the substrate <NUM> may be separated by a diaphragm-substrate distance D1 being the closest distance between the diaphragm <NUM> and the substrate <NUM>. A cavity <NUM> may thereby be present below the diaphragm <NUM>. Hence, the diaphragm <NUM> may be allowed to oscillate in an extension transverse to the planar extension, i.e., in the vertical direction. The term oscillate and, e.g., the term vibrate may be used interchangeably throughout. The diaphragm <NUM> may further be allowed to expand/contract in the vertical direction. The diaphragm <NUM> may be p doped to form a two-dimensional hole gas in the interface <NUM> between Al(<NUM>-x)Ga(x)N and GaN. This because of the slightly different lattice constant between Al(<NUM>-x)Ga(x)N and GaN, forming a potential well perpendicular to the interface <NUM> so small that only a small number of bound electron states may exist, thereby preventing electrons or holes moving perpendicular to the interface <NUM>. Electrons or holes may, however, still move in vertical direction (e.g., across a superlattice of heterostructures) if a sufficient bias voltage across the interface <NUM> is applied.

The micro-electromechanical unit <NUM> further comprises an electrode <NUM> connected to the diaphragm. Preferably, the electrode <NUM> is arranged on a surface <NUM> of diaphragm <NUM>. The electrode <NUM> may be directly attached on a top surface <NUM> of the diaphragm <NUM>. The electrode <NUM> may be manufactured by any adequate electrically conducting material. For instance, the electrically conducting material may be Ti, Al, Cu, Ni, and/or Au. Preferably, the electrically conducting material is a compound or alloy such as, e.g. aluminium copper, AlCu.

The semiconductor structure <NUM> of the diaphragm <NUM> has piezoelectrical properties. Hence, a mechanical deformation of the semiconductor structure of the diaphragm provide for electrically polarize the semiconductor structure. A non-zero voltage between different locations on the semiconductor structure may thereby be induced by the mechanical deformation <NUM>, <NUM>. This is schematically shown in <FIG>. The non-zero voltage may further induce an electrical current. As is normal for piezoelectrical material, the opposite is possible. That is, a bias-voltage applied between different locations may generate a mechanical deformation of the semiconductor <NUM> structure of the diaphragm. In the present situation, this effect may be achieved by charging the electrode <NUM>. The charging may generate a deformation <NUM>, <NUM> of the diaphragm <NUM> such that the diaphragm <NUM> contracts <NUM> or expands <NUM> in the vertical direction, i.e. in a direction substantially perpendicular to the planar extension of the diaphragm <NUM>. For instance, a positive bias-voltage applied on the electrode <NUM> may generate a contraction <NUM> of the diaphragm, whereas a negative bias-voltage applied on the electrode <NUM> may generate an expansion <NUM> of the diaphragm <NUM>.

The diaphragm <NUM> may be circular in the planar extension. This is shown in <FIG>. The geometry of the diaphragm may thereby be substantially circularly symmetric when seen from a direction perpendicular to the surface of the diaphragm <NUM>. The support structure <NUM> may thereby have a similar circular geometry, for supporting the diaphragm <NUM> in proximity of the outer edge <NUM> of the diaphragm <NUM>. Hence, the support structure <NUM> may be substantially circular cylindrical, the support structure <NUM> having an axial extension being perpendicular to the plane of the diaphragm (and the plane of the substrate). A thickness of a cylindrical wall of the support structure may be significantly smaller than a diameter of the diaphragm <NUM>. The diameter of the diaphragm <NUM> may be between <NUM> and <NUM>.

In the event the diaphragm <NUM> has a circular geometry, the electrode <NUM> may be ring shaped and arranged at an edge of the diaphragm. The electrode <NUM> may be simply connected. Hence, any rupture or insulating portion along the azimuthal direction of the ring-shaped electrode <NUM> may be absent.

The semiconductor structure <NUM> of the diaphragm <NUM> may be a superlattice <NUM> comprising GaN and Al(<NUM>-x)Ga(x)N, wherein <NUM> ≤ x ≤ <NUM>. This is schematically shown in <FIG>. The superlattice <NUM> may comprise a plurality of heterostructure layers <NUM>, <NUM>, wherein each heterostructure layer comprises one GaN layer <NUM> and one Al(<NUM>-x)Ga(x)N layer <NUM>. Hence, every second layer, 2i, may be a GaN layer <NUM>, and every layer in between respective pair of layers of GaN, <NUM>(i+<NUM>), may be an Al(<NUM>-x)Ga(x)N layer <NUM>. However, the number of layers need not be an even number. Should the number of layers be an odd number, the bottom layer and the top layer may have a similar chemical composition; either GaN or Al(<NUM>-x)Ga(x)N. The periodicity of the superlattice <NUM> may be between <NUM> and <NUM> nanometers, nm. The superlattice <NUM> may provide a plurality of planes, wherein, in respective plane a p-type two-dimensional hole gas, 2DHG, is present. The 2DHG thereby provides a channel for conduction along an interface <NUM> between two adjacent layers <NUM>, <NUM>. However, as set out above, electrons or holes may move between layers provided a sufficient bias voltage is applied between layers. The superlattice <NUM> may be capable of generating an increased piezoelectrical effect.

A cross-sectional profile of the superlattice <NUM> taken in a plane coinciding with a center line of the semiconductor structure <NUM>, i.e. parallel to the axial extension of the support structure <NUM>, need not be rectangular, as exemplified in <FIG>. Hence, a top layer <NUM> of the superlattice <NUM> may have a smaller area than a bottom layer <NUM> of the superlattice. By way of example, the superlattice <NUM> may be radially chamfered such that a circumferential edge portion of the diaphragm <NUM> has a thickness being smaller than a thickness of a center portion of the diaphragm. Preferably, the diaphragm <NUM> is chamfered such that a bottom surface and a top surface of the diaphragm <NUM> in vicinity of the circumferential edge are substantially parallel. On this top surface the electrode <NUM> may be attached. Likewise, a top surface and a bottom surface of a centered portion of the diaphragm may be substantially parallel. The center portion may be associated with an area of the diaphragm <NUM> being relatively large, such that, e.g., constituting half or more of an area of a total top surface <NUM> of the diaphragm <NUM>.

A thickness of the superlattice <NUM> may be between <NUM> and <NUM>. Hence, the superlattice <NUM> may comprise hundreds or even thousands of AlGaN/GaN heterostructure layers <NUM>,<NUM>. As set out above, the thickness may vary between, e.g. an edge portion <NUM> and a central portion <NUM> of the diaphragm <NUM>. Preferably, the thickness is larger in the central portion <NUM> than in the edge portion <NUM>. Further, the thickness may be radially dependent.

The micro-electromechanical unit may further comprise a backplate <NUM>. The backplate may comprise a plurality of through holes <NUM>. The backplate is elevated relative to the diaphragm by a circumferential wall <NUM>. The circumferential wall <NUM> is directly or indirectly, preferably directly, attached to the diaphragm <NUM>. Preferably, the circumferential wall <NUM> is attached to a circumference of the diaphragm <NUM>. Alternatively, the circumferential wall <NUM> may be attached to the support structure <NUM>. Hence, a cavity <NUM> is present between the diaphragm <NUM> and the backplate <NUM>. The cavity may be viewed as a pressure chamber or a resonant box. The through-holes <NUM> may be through-going openings oriented perpendicular to the planar extension of the backplate <NUM>. The plurality of through-holes <NUM> may be randomly distributed in the backplate <NUM>. Alternatively, the plurality of through-holes <NUM> may be orderly distributed such that the through-holes <NUM> themselves form a lattice when viewing the backplate <NUM> from a direction perpendicular to its top or bottom surface.

Now returning to the support structure <NUM> of the micro-electromechanical unit <NUM>. The support structure <NUM> may comprise a semiconductor layer structure forming a transistor. As set out above, the layer structure may be understood as a layer being arranged above another layer in the vertical direction as well as the above layer sharing a physical interface with the below adjacent layer. Such a physical interface may be configured to provide conductive contact, i.e. allowing electron and/or hole transport across the physical interface. Conductive contact may refer to, e.g. ohmic contact, Schottky contact or contact across a pn-junction or tunnel junction. The transistor may be a high-electron-mobility transistor, HEMT, which may allow relatively high switching frequencies and desirable high-power characteristics. This since conduction electron/holes propagate in two dimensions (2DEG/2DHG) along an interface between two layers as set out above, facilitating high electron/hole mobility, and/or other typical properties of 2DEGs or 2DHGs. Notice that electron or hole mobility is generally smaller between layers in the layer structure than in the plane of the layers.

The semiconductor layer structure of the support structure <NUM> may comprise a silicon base layer <NUM>. The silicon base layer <NUM> may be directly attached on the substrate <NUM>. Alternatively, the silicon base layer <NUM> may form part of the substrate <NUM> in that the silicon base layer <NUM> is a relatively large silicon wafer on which AIN may be grown (further discussed below). The substrate <NUM>, <NUM> may thereby comprise a silicon bulk material <NUM>, <NUM>. A top surface <NUM> of the silicon base layer <NUM> may be substantially planar. The vertical thickness of the silicon base layer may be in the range <NUM>-<NUM>, and more preferably in the range <NUM>-<NUM>. If not explicitly stated otherwise, thickness will henceforth refer to vertical thickness. The top surface <NUM> of the silicon base layer may have Miller indices (<NUM>). The silicon base layer <NUM> may have a diamond-cubic crystal structure.

The semiconductor layer structure of the support structure may comprise an aluminium nitride, AIN, layer <NUM>. The AIN layer <NUM> may preferably have a thickness in the range <NUM>-<NUM> and more preferably a thickness in the range <NUM>-<NUM>. The AIN layer may comprise vertical nanowire structures <NUM>. These nanowires <NUM> may preferably have a vertical length in the range <NUM>-<NUM> and more preferably a vertical length in the range <NUM>-<NUM>. The vertical nanowire structures <NUM> may preferably have a substantially circular or hexagonal lateral cross section profile. A diameter of such a nanowire may be in the range <NUM>-<NUM> and more preferably <NUM>-<NUM>. The nanowires <NUM> may be arranged in a repeating array pattern seen in the vertical direction where each nanowire <NUM> has four equidistant closest other nanowires. Alternatively, the repeating array pattern may have a square pattern. A distance between adjacent nanowires <NUM> may preferably be in the range <NUM>-<NUM>, and more preferably in the range <NUM>-<NUM>.

The semiconductor layer structure of the support structure <NUM> may further comprise a GaN layer <NUM> and a Al(<NUM>-x)Ga(x)N layer (not shown), wherein <NUM> ≤ x ≤ <NUM>. The Al(<NUM>-x)Ga(x)N layer may be situated between the GaN layer <NUM> and the diaphragm <NUM>. The layer arranged directly above the previously described AIN layer <NUM> may be the GaN layer <NUM>. The GaN layer <NUM> may preferably have a thickness in the range <NUM>-<NUM> and more preferably a thickness in the range <NUM>-<NUM>. The GaN layer <NUM> may be considered to laterally enclose, encapsulate, or encompass the vertical nanowire structures <NUM>, i.e. filling in the space between the vertical nanowire structures. The GaN layer <NUM> may further be considered to vertically enclose or encapsulate the vertical nanowire structures, i.e. extending vertically above and covering top portions of the vertical nanowire structures.

The GaN layer <NUM> may attach directly to the diaphragm <NUM>.

The micro-electromechanical unit <NUM> further comprises a calibrating weight <NUM> arranged on a surface of the diaphragm <NUM>. The calibrating weight <NUM> may be directly situated on the top surface of the diaphragm. In the event the diaphragm is circular, as exemplified above, the calibrating weight <NUM> may be ring-shaped. This particular example shape is shown in <FIG>. The calibrating weight <NUM> may be centrally located on the diaphragm. Hence, the centre of the diaphragm <NUM> and a centre of the ring-shaped calibrating weight <NUM> may substantially coincide. The calibrating weight <NUM> may be manufactured by a metallic compound or alloy such as, e.g. AlCu. Other materials may however be applicable.

In connection with <FIG>, a loudspeaker device <NUM> is schematically shown. The loudspeaker device <NUM> comprises a micro-electromechanical unit. The micro-electromechanical unit <NUM> is as described above. The loudspeaker device <NUM> further comprises circuitry <NUM> configured to actuate the diaphragm <NUM> of the micro-electromechanical unit <NUM>. The circuitry <NUM> may form part of a class D audio amplifier. Hence, the circuitry <NUM> may utilize pulse-width modulation, PWM, using a triangle- or a square wave oscillator. Traditional class D amplifiers comprise two output MOSFETs and an external lowpass filter to recover the amplified audio signal. However, herein the class D amplifier may be a filter-less amplifier. Further, a first and a second GaN high electron mobility transistor, HEMT, may replace the traditional MOSFETs. The HEMTs may operate as current steering switches by alternately connecting an output node to a supply voltage, Vdd, and ground. Hence, the resulting output is 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 triangle- or sawtooth wave oscillator. This may be referred to a natural sampling where the triangle wave oscillator may act as a sampling clock. The resulting duty cycle of the square wave is proportional to the level of the input signal. The GaN HEMTs may be arranged for a variable such a duty cycle. The diaphragm <NUM> of the micro-electromechanical unit <NUM> may function as a source for sound waves of the loudspeaker device <NUM>. Acoustical properties of the diaphragm <NUM> thereby depend on the semiconductor structures thereof, e.g. the number of layers in semiconductor structure of the diaphragm <NUM>, possible through-holes <NUM>, geometry in the planar extension, etc. The acoustical properties to be configured herein may, as traditionally, include relative intensity, vibrational frequency, or the like. The class D amplifier may be a full bridge class D amplifier. The circuitry <NUM> may comprise a comparator <NUM>. The comparator <NUM> may function as an ordinary comparator, e.g. an operational amplifier, by comparing an audio input signal to a high-frequency triangle wave to digitize the audio input signal. The result of the comparison is a digital copy signal from the analogue audio input signal, wherein low-frequency components of the digital signal represent the audio input signal, while high-frequency signals are in general ignored. The first and the second HEMT may be electrically connected to the comparator. The output of the comparator drives the HEMT by an intermediately connected driver as is normal for, e.g., a class D amplifier. The diaphragm <NUM> may in connection with the loudspeaker device <NUM> be an <NUM>-ohm speaker. The circuitry <NUM> may comprise a field-programmable gate array, FPGA, to handle the digital part of the device. The input audio may be a <NUM>-bit pulse-code modulated, PCM, signal with a kHz sample rate to be encoded into streams of pulses.

The driving of GaN HEMT devices requires care to be taken to the distances between the GaN HEMT devices gate contact and the driver. The GaN HEMT devices may be abutting the driver to provide a short and equal distance between several GaN HEMT devices in order to switch the GaN HEMT devices ON and OFF in an alternating fashion for a half-bridge or full-bridge operating at a high switching frequency.

In connection with <FIG>, there is schematically shown a microphone device <NUM>. The microphone device <NUM> comprises a micro-electromechanical unit <NUM>. The micro-electromechanical unit <NUM> is as described above. The micro-electromechanical unit <NUM> comprises circuitry <NUM>. The circuitry <NUM> is configured to detect a movement of the diaphragm <NUM> of the micro-electromechanical unit <NUM>. Hence, a mechanical movement of the diaphragm <NUM> induces a voltage difference which can be converted to electrical signals "backwards" in view of the features described in connection to the loudspeaker device above.

Accordingly, and in summary, as discussed above, there has been described a micro-electromechanical unit <NUM>. The micro-electromechanical unit <NUM> may e.g. be used in a loudspeaker or a microphone. The piezoelectrical properties of the diaphragm <NUM> of the micro-electromechanical unit <NUM> may turn a coil/solenoid redundant as an applied bias voltage may, by the diaphragm itself, be converted to a mechanical movement/deformation <NUM>, <NUM>, and vice versa.

Claim 1:
A loudspeaker device (<NUM>) comprising:
a micro-electromechanical unit (<NUM>) comprising:
a diaphragm (<NUM>) having a planar extension, the diaphragm (<NUM>) comprising a semiconductor structure (<NUM>) comprising a first (<NUM>) and a second (<NUM>) layer, each layer extending in the planar extension, wherein the first layer (<NUM>) comprises Al(<NUM>-x)Ga(x)N, wherein <NUM> ≤ x ≤ <NUM>, and the second layer (<NUM>) comprises GaN, wherein the diaphragm (<NUM>) is arranged to oscillate in an extension transverse to the planar extension to thereby generate sound waves, and wherein a calibrating weight (<NUM>) is arranged on a surface of the diaphragm (<NUM>),
a support structure (<NUM>) supporting the diaphragm (<NUM>), and
an electrode (<NUM>) connected to the diaphragm; and
a resonant box.