Backing materials with tailored acoustic properties are beneficial for miniaturized ultrasonic transducer design. Whereas piezoelectric P(VDF-TrFE) films are common elements in high-frequency (>20 ...MHz) transducer design, their low coupling coefficient limits their sensitivity. Defining a suitable sensitivity-bandwidth trade-off for miniaturized high-frequency applications requires backings with impedances of >25 MRayl and strongly attenuating to account for miniaturized requirements. The motivation of this work is related to several medical applications such as small animal, skin or eye imaging. Simulations showed that increasing the acoustic impedance of the backing from 4.5 to 25 MRayl increases transducer sensitivity by 5 dB but decreases the bandwidth, which nevertheless remains high enough for the targeted applications. In this paper, porous sintered bronze material with spherically shaped grains, size-adapted for 25-30 MHz frequency, was impregnated with tin or epoxy resin to create multiphasic metallic backings. Microstructural characterizations of these new multiphasic composites showed that impregnation was incomplete and that a third air phase was present. The selected composites,
and
, at 5-35 MHz characterization, produced attenuation coefficients of 1.2 and >4 dB/mm/MHz and impedances of 32.4 and 26.4 MRayl, respectively. High-impedance composites were adopted as backing (thickness = 2 mm) to fabricate focused single-element P(VDF-TrFE)-based transducers (focal distance = 14 mm). The center frequency was 27 MHz, while the bandwidth at -6 dB was 65% for the
-based transducer. We evaluated imaging performance using a pulse-echo system on a tungsten wire (diameter = 25 μm) phantom. Images confirmed the viability of integrating these backings in miniaturized transducers for imaging applications.
This paper presents a novel synthesis for bandwidth switchable bandpass filters using Semi-conductor Distributed Doped Areas (ScDDAs) as active elements. A co-design method is proposed with a global ...and simultaneous conception for the active and the passive parts of the switchable filters. The ScDDAs, integrated in the silicon substrate, are able to commute from half-wavelength open-ended stubs to quarter-wavelength short-circuited ones. This co-design method offers a great flexibility and allows to integrate the active elements directly in the substrate, therefore avoiding any soldering of components. The synthesis is developed for the two-states of the active elements and applied, as a proof of concept, to a four-pole bandwidth switchable bandpass filter. This filter operates at 5 GHz with a 50 % bandwidth in the OFF-state (when the stubs are terminated by an open-circuit) and with a 70 % bandwidth in the ON-state (when the stubs are short-circuited). For this filter, the synthesis is detailed in the two-states allowing to choose the two desired bandwidths. A good fitting is obtained for these results proving the viability of such an approach.
This paper deals with the synthesis of high-magnetization porous silicon-based nanocomposites. Using well-controlled organometallic synthesis of ferromagnetic FeCo nanoparticles, the impregnation of ...mesoporous silicon has been performed by immersion of porous silicon in a colloidal solution. The technique was optimized by controlling the temperature, the immersion duration, and the solvent nature. The characterization of the nanocomposites showed a homogeneous filling of the pores and a high magnetization of 135 emu/cm
. Such composites present a great interest for many applications including data storage, medical instrumentations, catalysis, or electronics.
AlN nucleation layer is the key issue for the performance of GaN high frequency telecommunication and power switching systems fabricated after heteroepitaxy on Silicon or Silicon Carbide. In this ...work, we demonstrate and explain both the low level and the origin of propagation losses in GaN/3C–SiC/Si High Electron Mobility Transistors (HEMTs) at microwaves frequencies, in view of providing efficient circuits. First, it is shown that the use of 3C–SiC as an intermediate layer between the Si substrate and the GaN epitaxial layer drastically decreases RF propagation losses. Using Secondary Ion Mass Spectroscopy (SIMS) measurements, we demonstrate that dopant in-diffusion (both Al and Ga) into the 3C–SiC pseudo-substrate remains confined beneath the interface. Furthermore, by combining scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM), the 2D profile shows the presence of a slightly conductive zone beneath the AlN/3C–SiC interface that is highly limited (less than 50 nm) whatever the growth conditions of the (Al, Ga)N layers on 3C–SiC explaining the low propagation losses obtained for such devices. This behavior differs from the one previously observed for GaN growth on Si substrate. This work demonstrates the importance and efficiency of the 3C–SiC intermediate layer when used as a pseudo-substrate increasing not only the crystalline quality of the subsequent (Al, Ga)N layers but also permits to achieve high potential GaN power devices as it is crucial.
Abstract
AlN nucleation layers are the basement of GaN-on-Si structures grown for light-emitting diodes, high frequency telecommunication and power switching systems. In this context, our work aims ...to understand the origin of propagation losses in GaN-on-Si High Electron Mobility Transistors at microwaves frequencies, which are critical for efficient devices and circuits. AlN/Si structures are grown by Metalorganic Vapor Phase Epitaxy. Acceptor dopant in-diffusion (Al and Ga) into the Si substrate is studied by Secondary Ion Mass Spectroscopy and is mainly located in the first 200 nm beneath the interface. In this region, an acceptor concentration of a few 10
18
cm
-3
is estimated from Capacitance–Voltage (C–V) measurements while the volume hole concentration of several 10
17
cm
-3
is deduced from sheet resistance. Furthermore, the combination of scanning capacitance microscopy and scanning spreading resistance microscopy enables the 2D profiling of both the
p
-type conductive channel and the space charge region beneath the AlN/Si interface. We demonstrate that samples grown at lower temperature exhibit a
p
-doped conductive channel over a shallower depth which explains lower propagation losses in comparison with those synthesized at higher temperature. Our work highlights that this
p
-type channel can increase the propagation losses in the high-frequency devices but also that a memory effect associated with the previous sample growths with GaN can noticeably affect the physical properties in absence of proper reactor preparation. Hence, monitoring the acceptor dopant in-diffusion beneath the AlN/Si interface is crucial for achieving efficient GaN-on-Si microwave power devices.
This letter presents a novel 5-GHz transmitter/ receiver (TX/RX) single-pole double-throw (SPDT) switch designed on a silicon substrate. The novelty of this SPDT switch is the possibility it offers ...to simultaneously design the transmission lines and the active elements, without packaging and consequently its parasitic effects and frequency limitations. The device is intended to be easily integrated into a system with a more complex function, e.g., with amplifiers and antennas. The codesign method offers great flexibility in the positioning and sizing of the active elements, i.e., semiconductor distributed doped areas (ScDDAs), which are integrated n + pp + junctions, in order to obtain the best possible performances in both aspects. A demonstrator provides proof of the concept. In RX-mode, the insertion loss (IL) is 0.9 dB with isolation (ISO) higher than 30 dB. In TX-mode, IL is 2.38 dB and ISO is higher than 43 dB.
This letter presents a continuously tunable resonator with a novel triangular doped area on a silicon substrate. The resonator and its tunable element (an n + pp + junction) are co-designed in the ...same flow, leading to new tradeoffs and novel behaviors. The co-design flexibility allows the active area to be drawn with any doping shape. The base of the triangular doped area is located at the end of a stub. Consequently, when the junction is forward biased with a low bias voltage, the resonator is ended by a short circuit at the triangle base. Then, by increasing the voltage, the short-circuit plan moves continuously until it reaches the top of the triangle, thereby decreasing the length of the short-circuited stub. Besides, a demonstrator validates the concept and offers a continuous resonant frequency tunability of 50%, from 2.2 to 3.3 GHz, with a bias voltage from 0 to −1.3 V.
Anodization of silicon carbide (SiC) in hydrofluoric acid (HF) solutions is a promising way to etch this material which is very resistant against traditional chemical etching methods. Moreover, it ...has been shown that several reproducible porous SiC morphologies can be performed varying anodization conditions (current density, electrolyte composition, UV lighting) and/or substrate properties (doping type and level). This paper proposes a state of the art of porous SiC etching in GREMAN and a presentation of the morphologies achievable using anodization in HF based electrolytes.
In the present work, we report the high performance of zinc oxide (ZnO) nanosheet (NS) based source-gated transistors (SGTs) with asymmetric Schottky source and ohmic drain contacts: low saturation ...drain-source voltages (~2V) in the output scans (even at high gate voltages), high current on/off ratio (>107) and low off-currents (0.1pA). For a deeper understanding of the device mechanism and charge transport at metal–semiconductor contact interface, temperature dependent current–voltage studies have been performed. They revealed that the device operation can be ascribed to 3 main processes: i) the reverse biased Schottky source contact, which essentially controls charge carrier injection in to the NS channel, ii) effective manipulation of the source barrier by the gate field and iii) modulation of the depletion region beneath the source contact. These results are likely to improve the future generations of the ZnO based SGTs which offer several advantages for thin-film transistor design, including low power dissipation, small signal amplification, and as active load for the electronic circuits.
•Single-crystalline ZnO Nanosheets on r-plane Saphire•High performance FET using single ZnO Nanosheet (Mobility=50cm2/Vs)•High performance source-gated transistors: low saturation voltages (~2VD)•Low temperature processes for the device fabrication
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