High Frequency Electronics Research Articles

Thermal engineering increases current density in AlGaN/GaN superlattice devices

Aluminum gallium nitride/gallium nitride multi-channel superlattice devices are receiving increasing attention as a new paradigm for driving the power density of gallium nitride based transistors toward their theoretical limit. However, the superior electrical performance of superlattice-based transistors is currently limited by excessive Joule-heating. This Letter evaluates what impact the number of superlattice channels and the buffer layer composition has on the reduction of the thermal resistance, i.e., Joule heating, of AlGaN/GaN superlattice devices. A record low thermal resistance (12.51 ± 0.34 K mm W−1) was measured via scanning thermal microscopy for non-castellated superlattice AlGaN/GaN devices with a 100 μm channel width. Overall, the use of a thin gallium nitride buffer layer, in place of a thick aluminum gallium nitride layer, reduced the buffer thermal resistance enabling the accommodation of more superlattice channels (10 vs 6), therefore augmenting the maximum power density of these devices. The superlattice device proposed here not only provides an enhanced thermal dissipation solution for high power density radio frequency electronics, but it also has the benefit of fewer fabrication steps in comparison with previously reported castellated multichannel devices.

Demystifying metal-assisted chemical etching of GaN and related heterojunctions

GaN and related semiconductors have become an increasingly prominent material for a wide range of active and passive devices from optoelectronics to high frequency and power electronics as well as photocatalysis. Regardless of the application, anisotropic etching is required for micro and nano structuring, currently performed by reactive ion etching (RIE). Alternately, metal-assisted chemical etching (MacEtch) is an open-circuit plasma-free anisotropic etching method that has demonstrated high aspect ratio device structures devoid of plasma-induced damage found in RIE. This paper presents an in-depth study of the ensemble electrochemical mechanisms that govern the photo-enhanced MacEtch process of GaN and related heterojunctions. Through in-depth experimental investigations, modeling and simulations, the effects of local cathode and anode design, energy-band alignments, and solution chemistry on MacEtch are correlated with the underlying electronic mechanisms of carrier generation, annihilation, transport, and extraction, establishing a fundamental framework for parametrized prediction of system behavior. These findings carry profound implications for tailored design of photoelectrochemical processes employed not just for uniformly etching wide/ultrawide bandgap materials but more broadly for semiconductor-based photocatalytic reactions in general. One-pot photo-enhanced MacEtching of AlInGaN multi-heterojunction device structures including superlattices and multi-quantum wells are demonstrated.

Diamond for High-Power, High-Frequency, and Terahertz Plasma Wave Electronics.

High thermal conductivity and a high breakdown field make diamond a promising candidate for high-power and high-temperature semiconductor devices. Diamond also has a higher radiation hardness than silicon. Recent studies show that diamond has exceptionally large electron and hole momentum relaxation times, facilitating compact THz and sub-THz plasmonic sources and detectors working at room temperature and elevated temperatures. The plasmonic resonance quality factor in diamond TeraFETs could be larger than unity for the 240-600 GHz atmospheric window, which could make them viable for 6G communications applications. This paper reviews the potential and challenges of diamond technology, showing that diamond might augment silicon for high-power and high-frequency compact devices with special advantages for extreme environments and high-frequency applications.

Heteroepitaxy of N-polar AlN on C-face 4H-SiC: Structural and optical properties

To date, it has remained challenging to achieve N-polar AlN, which is of great importance for high power, high frequency, and high temperature electronics, acoustic resonators and filters, ultraviolet (UV) optoelectronics, and integrated photonics. Here, we performed a detailed study of the molecular beam epitaxy and characterization of N-polar AlN on C-face 4H-SiC substrates. The N-polar AlN films grown under optimized conditions exhibit an atomically smooth surface and strong excitonic emission in the deep UV with luminescence efficiency exceeding 50% at room temperature. Detailed scanning transmission electron microscopy (STEM) studies suggest that most dislocations are terminated/annihilated within ∼200 nm AlN grown directly on the SiC substrate due to the relatively small (1%) lattice mismatch between AlN and SiC. The strain distribution of AlN is further analyzed by STEM and micro-Raman spectroscopy, and its impact on the temperature-dependent deep UV emission is elucidated.

An UMOSFET integrated with graded semi-super-junction and 3C/4H–SiC hetero-crystalline freewheeling junction

SiC trench metal-oxide-semiconductor field-effect transistor (UMOSFET) can play a promising candidate in high frequency and large power electronics. Herein, an UMOSFET inserted with n/p-/p/p-/n-type graded semi-super-junction (GSSJ) in drift region and 3C/4H–SiC hetero-crystalline freewheeling junction (HCFJ) connected to source electrode is proposed by the Silvaco TCAD, called as the 3C/4H–SiC-GSSJ-UMOSFET. The GSSJ can be regarded as two back-to-back n/p-type unilateral graded junctions to shrink the depletion zones in n-type pillars for reducing the specific on-resistance (Ron, sp) without degrading the breakdown voltage (VB), hence optimizes the trade-off between VB and Ron,sp. Further, the 3C/4H–SiC HCFJ can be conducted prior to the body junction (BJ) composed of 4H–SiC p-body and n-type current spreading layer, due to the turning-on voltage of HCFJ much lower than that of BJ, which benefits to freewheel the backward current while decrease the reverse recovery charges and time. Thus can improve the device performance.

Structure evolution and magnetization properties of FeSiBCCr amorphous alloys prepared by spark plasma sintering

FeSiBCCr bulk metallic glasses were prepared via spark plasma sintering, and the effects of holding time and heating rate on the structure and magnetization properties of powder compacts were investigated by SEM, TEM, XRD, DSC and VSM. The results show that when the holding time is increased from 10 min to 20 min, the internal porosity is reduced, and the amorphous fraction is decreased from 92.0 ± 1.7 % to 82.4 ± 4.3 % accompanied by the precipitation of fine nanocrystalline grains. When the holding time is increased to 30 min, the precipitation of the ferro-boron phase and growth of nanocrystalline grains lead to a significant deterioration of magnetization properties, although the compact density is significantly increased to 6.13±0.12 g/cm3. When the heating rate is increased from 20 K/min to 100 K/min, the activation energy of viscous flow is decreased and the compact density is improved due to the enhanced mass transfer. Moreover, the thermal stability of the amorphous material is also improved, effectively suppressing the grain coarsening and the precipitation of the ferro-boron phase. As a result, the saturation magnetic flux density is increased by 10 % and the coercivity is reduced by 83 %. The FeSiBCCr bulk metallic glasses prepared at a sintering temperature of 773 K, a heating rate of 100 K/min, and a holding time of 10 min contain an amorphous matrix with fine Fe2Si grains. And the compact has a density of 5.60±0.08 g/cm3, a saturation magnetic flux density of 1.03±0.03 T, and a coercivity of 15.9 ± 1.2 A/m, exhibiting promising magnetization properties and being expected to be used in medium and high frequency electronics.

Flexible polytetrafluoroethylene (PTFE) composite film with enhanced thermal conductivity and low interface thermal resistance for high frequency electronics

The significant degradation in mechanical properties and filling-gap performance of thermal interface materials (TIMs) under high filling ratios remained a prevalent problem. Here, a polytetrafluoroethylene (PTFE) based composite film with excellent flexibility at ultra-high hybrid fillers content (up to 90 wt%) was developed through the mechanical interlocking strategy. Due to the sufficient fibrosis of PTFE under shear stress, boron nitride (BN) and aluminum nitride (AlN) were firmly constrained inside the fiber cage. The hybrid fillers arranged into a seesaw-like structure during the calendering process, which facilitated the formation of thermal conductive network in both horizontal and vertical directions. Importantly, this unique embedded structure allowed for direct contact between the fillers and the heat source, resulting in a 90.45% decrease in interfacial thermal resistance. The resultant film possessed a high thermal conductivity (4.3 W/m·K, in-plane, 0.75 W/m·K, out-plane), low dielectric constant (3.1, 10 GHz), and excellent flexibility, making it suitable for application of interface heat dissipation in high-frequency electronic devices.

A Novel Printed Circuit Board Laminate for High Frequency Electronics

A novel PCB material that is advantaged at the 10-100+ GHz range has been developed. It consists of a 3 layer laminate, nominally with a total thickness of 140 microns, and individual laminates can be stacked to make multilayer boards. The outer two layers of the laminate consist of the polymer Topas, 50 microns thick and the middle layer is Corning’s ribbon alumina, 40 microns thick. For brevity, the laminate is called “TAT”. The permittivities of the Topas and alumina are 2.5 and 10 respectively, and that of the 50/40/50 TAT laminate is 3.2 in a direction transverse to the plane of the laminate. The individual loss tangents are about 0.0002, and the loss tangent of the TAT laminate has about the same value. Thus, TAT has significantly lower loss with a lower effective permittivity than other PCB materials currently available. These dielectric properties have been measured experimentally at 10 and 15 GHz. The permittivity and loss tangents can also be predicted accurately using extremely simple analytical formulae. This makes predicting the properties of TAT structures with different dimensions very easy. A TAT strip line has been modeled and found to have an effective permittivity of 2.3. The machining of TAT is somewhat incompatible with conventional PCB shop practice. However, laser drilling and cutting are perfectly practicable. In-house processes have been successfully developed, and an external vendor has also developed these capabilities. Copper sheet adheres strongly to the TAT, and the conductivity of the resulting traces is high. Boards for a 77 GHz radar demonstrate the manufacturability of TAT. Mechanical testing of TAT has been carried out, and the measured flexural modulus and stress at failure compare well with calculations. Environmental tests at 80 °C and 80% humidity indicate that TAT is quite robust. Moreover, it is quite possible that TAT itself may satisfy the UL94 criteria, since the alumina may act as a flame retardant. The feasibility of loading Topas with ceramic or fluoropolymer powders to reduce flammability has been investigated, and it has been found that the addition of these powders causes negligible dielectric loss. In summary, laminates of this type are plausible extreme low loss candidates for PCBs for very advanced applications such as 5G and 6G devices.

Utilization of hazardous red mud in silicone rubber/MWCNT nanocomposites for high performance electromagnetic interference shielding

With burgeoning technological advancement in telecommunication and high frequency electronics, its repercussion as electromagnetic pollution is alarmingly high. Hence, much effort has been made to supress this undesired phenomenon using efficient electromagnetic interference (EMI) shielding materials. Red mud, a hazardous by-product of alumina extraction is rich in oxides of iron, aluminium & titanium with ca. 30–60% of hematite, a magnetic lossy material that could be suitable for enhancing microwave absorption and EMI shielding performance of polymer nanocomposites. The impetus for this work was to utilize red mud as magnetically lossy filler in silicone rubber (SR)/multi-wall carbon nanotube (MWCNT) nanocomposites to function as efficient EMI shields with superior microwave absorption. Herein, taking the advantage of conductive percolated network of MWCNT in SR, red mud was incorporated with varying concentration of 5–20 wt% & compression moulded to form flexible sheets. These nanocomposite sheets exhibited a maximum electrical conductivity of 24 S/cm with superior EMI shielding effectiveness value of −83.4 dB in the X band (8.2–12.4 GHz) region making it a suitable candidate for commercial shielding applications. Moreover, sustainable use of a hazardous industrial waste to suppress EMI, which is yet another challenging invisible pollutant, is the key highlight of this work.

Hydrogen etching of 4H–SiC(0001) facet and step formation

SiC has attracted much interest due to its vast applications in high power, high frequency and radiation-hardened electronics at high temperatures. Although the growth and processing technology for SiC wafers has matured, scratches created during surface preparation will unavoidably deteriorate both device performances and the quality of materials which are fabricated on it. Hydrogen etching has been proposed as an effective approach to obtain ideal SiC surfaces with large atomically flat steps. Here, we reported a systematical investigation on the effects of processing parameters to the hydrogen etching of 4H–SiC (0001). Accordingly, a new two-step etching method was developed with a specifically designed crucible to take advantage of the found correlation between those parameters and the terrace features. The method demonstrates high efficiency to realize atomically uniform SiC surface morphology with more than 70% coverage of the sample surface. It offers a possible route for large scale industrial SiC wafer planarization.

Exploring structural, electronic, optical, magnetic, and thermoelectric properties of Pt doped and Pt-Cu/Au co-doped GaN

We present a computational research on investigating the structural, electronic, optical, magnetic, and thermoelectric properties of Pt doped and Pt-Cu/Au co-doped GaN using the Wien2k code. The PBE-GGA approximation is employed in Wien2k code using density functional theory. Study of electronic properties reveals that the d-states of Pt, Cu, and Au atoms participate actively in transferring electron from valence to conduction band and their addition as mono and co-doped formation turns the non-magnetic character of GaN to magnetic. A blueshift in absorption spectrum appear and absorption is more pronounced for Pt-Cu/Au co-doped GaN, suggesting the use of these materials for high energy UV optoelectronics, spintronics, photonic, and sensing devices. However, lower reflection and enhanced conductivity of Pt-Cu/Au co-doped materials in the UV region extends their potential applications in high frequency electronics. Moreover, higher ZT value of Pt@GaN suggest it a better thermoelectric source other than Pt-Cu/Au@GaN.

Graphene FET on Diamond for High-Frequency Electronics

Transistors operating at high frequencies are the basic building blocks of millimeter-wave communication and sensor systems. The high velocity and mobility of carriers in graphene can open way for ultra-fast group IV transistors with similar or even better performance than can be achieved with III-V based semiconductors. However, the progress of high-speed graphene transistors has been hampered due to fabrication issues, influence of adjacent materials, and self-heating effects. Here, we report a graphene field-effect transistor (FET) on a diamond substrate, with a $f_{max}$ up to 54 GHz for a gate length of 500 nm. The high thermal conductivity of diamond provides an efficient heat-sink, and its relatively high optical-phonon energy improves saturation velocity of carriers in the graphene channel. Moreover, we show that graphene FETs on diamond, with different gate lengths, exhibit excellent scaling behavior. These results indicate that graphene FETs on diamond technology can reach sub-terahertz frequency performance.

All Screen‐Printed, Polymer‐Nanowire Based Foldable Electronics for mm‐Wave Applications

AbstractWith the surge in devices for Internet of Things (IoT) applications, there is great interest in flexible electronics to be mass manufactured at lower costs. Screen‐printing is well‐known for mass manufacturing, however, this method has mostly focused on printing metallic patterns. Rare efforts have been devoted to print substrates for high frequency (mm‐wave) electronics, which requires low dielectric loss to ensure a decent system efficiency. This paper presents a novel screen‐printable composite ink comprising of acrylonitrile‐butadiene‐styrene and ceramic particles, through which, dielectric substrates with various thicknesses (down to few microns), lateral dimensions, and relative permittivities can be printed. A low dielectric loss of 0.0063 at 28 GHz (fifth generation (5G) communication band) makes the substrates suitable for mm‐wave electronics. A custom silver nanowires based screen‐printable ink is utilized for metallic printing to provide high conductivity (3.4 × 106 S m‐1) and stable electrical response under bent or folded conditions. As a proof of concept for fully printed mm‐wave electronics, a flexible quasi‐Yagi antenna operating at 5G band (26.5–29.5 GHz) is demonstrated that exhibits decent performance in flat as well as bent conditions, confirming the suitability of the material system and printing processes for mass production of IoT and wearable electronics.

Selectively Metalizable Stereolithography Resin for Three-Dimensional DC and High-Frequency Electronics via Hybrid Additive Manufacturing.

Direct manufacturing of customized end-use electronic products is becoming an emerging trend of additive manufacturing (AM). This highly demands the evolution of the conventional AM processes from simply building single-material parts to simultaneously delivering complex structures and end-use functionalities. In this work, we propose a novel hybrid additive manufacturing solution that combines stereolithography (SLA) three-dimensional (3D) printing and laser-activated electroless plating for the manufacture of 3D fully functional electronic products. With our newly developed functional SLA resin that can be 3D printed, laser-activated, and thereafter selectively metalized, high-resolution circuitry can be free-formly patterned on 3D structures. In virtue of high-performance electrical materials, this technology is capable of creating not only 3D direct-current (DC) electronics but also 3D high-frequency devices like microwave/millimeter-wave antennas, which cannot be fabricated via traditional printed circuit board (PCB) technology and not even by most AM processes. This study represents a significant advance in additive manufacturing technologies, and more importantly offers a unique opportunity for the mass customization of fully functional 3D electronic products, which shows great potentials in consumer electronics, communication engineering, and automobile and aerospace industries.

Graphene in 2D/3D Heterostructure Diodes for High Performance Electronics and Optoelectronics

AbstractDiodes made of heterostructures of the 2D material graphene and conventional 3D materials are reviewed in this manuscript. Several applications in high frequency electronics and optoelectronics are highlighted. In particular, advantages of metal–insulator–graphene (MIG) diodes over conventional metal–insulator–metal diodes are discussed with respect to relevant figures‐of‐merit. The MIG concept is extended to 1D diodes. Several experimentally implemented radio frequency circuit applications with MIG diodes as active elements are presented. Furthermore, graphene‐silicon Schottky diodes as well as MIG diodes are reviewed in terms of their potential for photodetection. Here, graphene‐based diodes have the potential to outperform conventional photodetectors in several key figures‐of‐merit, such as overall responsivity or dark current levels. Obviously, advantages in some areas may come at the cost of disadvantages in others, so that 2D/3D diodes need to be tailored in application‐specific ways.

The liquid exfoliation of graphene in polar solvents

Graphene is the most attractive 2D nanomaterial with interesting applications from smart coating to high frequency electronics. Its production through liquid exfoliation of graphite in suitable solvents is the most practical approach to prepare graphene dispersions. In recent years, the production of graphene through a short sonication time has attracted huge attention, trying to find a balance between the quality and cost. However, little is known about how the technical parameters (solvent type, graphite amount, sonication time, centrifugation time and centrifugation speed) affect the exfoliation process. By combining different spectroscopical (UV–vis, EDS, IR, XRD and Raman) and morphological (SEM and TEM) characterizations, statistical analysis, and ab initio simulations, we present a detailed experimental and theoretical investigation seeking to explain the liquid exfoliation of graphene in polar solvents at short sonication time (up to 7 h). In particular, we demonstrate that the short-time exfoliation process can only be optimized by dispersing graphene in dimethylformamide (DMF). Raman measurements show that edge-type defects are most noticeable when graphene is exfoliated in ethanol, which is corroborated by SEM analysis. TEM results show well-exfoliated graphene nanosheets in DMF, but depend on the centrifugation speed. Computational studies (considering one- and two-graphene layers) predict that the orientation of the molecule and the number of layers affect the final adsorption energy.

Enhanced piezoelectricity and electromechanical efficiency in semiconducting GaN due to nanoscale porosity

Electrical polarization phenomena in GaN are important as they have significant impact on the operation of modern day energy efficient lighting and are fundamental to GaN-based high power and high frequency electronics. Controlling polarization is beneficial for the optimization of these applications. GaN is also piezoelectric, and therefore mechanical stress and strain are possible handles to control its polarization. Nonetheless, polar semiconductors in general, and GaN in particular, are weak piezoelectric materials when compared to ceramics, and are therefore not considered for characteristic electromechanical applications such as sensing, actuation and mechanical energy harvesting. Here, we examine the effect of nanoscale porosity on the piezoelectricity of initially conductive GaN. We find that for 40% porosity, the previously conductive GaN layer becomes depleted, and exhibits enhanced piezoelectricity as measured using piezoresponse force microscopy, as well as by using a mechanical energy harvesting setup. The effective piezoelectric charge coefficient of the porous GaN, d33,eff, is found to be about 8 pm/V which is 2-3 times larger than bulk GaN. A macroscale device comprising a porous GaN layer delivered 100 nW/cm2 across a resistive load under a 150 kPa mechanical excitation. We performed finite element simulations to analyze the evolution of the piezoelectric properties with porosity. The simulations suggest that increased mechanical compliance due to porosity gives rise to the observed enhanced piezoelectricity in GaN. Furthermore, the simulations show that for stress-based excitations, the porous GaN electromechanical figure of merit is increased by an order of magnitude and becomes comparable to that of barium titanate piezoceramics. In addition, considering the central role played by GaN in modern electronics and optoelectronics, our study validates a very promising research direction when considering stress-based electromechanical applications which combine GaN's semiconducting and piezoelectric properties.

Catalyst free approach for the fabrication of CoN//Zn3N2 asymmetric configuration for highly efficient flexible supercapacitor

This report presents a single-step deposition of crystalline and catalyst-free cobalt nitride (CoN) and zinc nitride (Zn3N2) electrodes on a flexible current collector for highly efficient flexible supercapacitors. These proposed electrodes take full advantage of mechanical strength, electrochemical stability, and tremendous electrical conductivity with excellent adhesion to a flexible current collector and show high capacitive performance with outstanding cyclic life. The asymmetric supercapacitor (ASC) was constructed using CoN as a negative electrode and Zn3N2 as a positive electrode, assembled with 1 M Na2SO4 aqueous electrolyte soaked Whatman filter paper as the separator. This ASC exhibits a wider voltage window (up to 2 V), good capacitance (75.4 Fg−1), and high specific energy (42 Wh kg−1) with good capacitance retention (93.6% for the flat cell and 80.1% for the 80° bend cell) over 5000 charging discharging cycles. Therefore, this design of ASC potentially expands the performance of high frequency and flexible electronics.

High-Frequency Power Electronics at the Grid Edge: A Bottom-Up Approach Toward the Smart Grid

Currently, Solar Energy Systems are supplying 2% of U.S. electricity generation and their use is rapidly growing. Data centers make up 2% of the U.S. total energy consumption. By 2030, 40% of the overall worldwide energy consumption is expected to be in the form of electricity. The future development of transportation electrification and smart grids requires grid-scale energy storage. These emerging and high-impact applications locate in the distribution grid. They represent exciting opportunities in developing fundamentally new principles for high-performance power electronics that are smaller, smarter, more efficient, and capable of performing new functions (Figure 1).

Direct epitaxial nanometer-thin InN of high structural quality on 4H–SiC by atomic layer deposition

Indium nitride (InN) is a highly promising material for high frequency electronics given its low bandgap and high electron mobility. The development of InN-based devices is hampered by the limitations in depositing very thin InN films of high quality. We demonstrate growth of high-structural-quality nanometer thin InN films on 4H–SiC by atomic layer deposition (ALD). High resolution x-ray diffraction and transmission electron microscopy show epitaxial growth and an atomically sharp interface between InN and 4H–SiC. The InN film is fully relaxed already after a few atomic layers and shows a very smooth morphology where the low surface roughness (0.14 nm) is found to reproduce sub-nanometer surface features of the substrate. Raman measurements show an asymmetric broadening caused by grains in the InN film. Our results show the potential of ALD to prepare high-quality nanometer-thin InN films for subsequent formation of heterojunctions.