Showing papers by "Oliver Ambacher published in 2018"

Elastic modulus and coefficient of thermal expansion of piezoelectric Al1−xScxN (up to x = 0.41) thin films

Abstract: Aluminum scandium nitride (Al1−xScxN with x = 0–0.41) thin films were deposited by reactive pulsed-DC magnetron sputtering on Si(001) and Al2O3(0001) substrates. X-ray diffraction indicated high degree of c-axis orientation in all the films, and based on pole figure measurements, epitaxial relationship could be defined as [101¯0]AlScN//[112¯0]sapphire and (0001)AlScN//(0001)sapphire in films deposited on Al2O3. Piezoelectric coefficient increased up to d33 = 31.6 pC/N in Al0.59Sc0.41N, which is 550% higher than for AlN. The biaxial elastic modulus and the in-plane coefficient of thermal expansion (CTE) as a function of Sc concentration were determined by thermal cycling method: biaxial elastic modulus decreased from 535 GPa in pure AlN to 269 GPa in Al0.59Sc0.41N and CTE was 4.65 × 10−6 K−1 for AlN and 4.29 × 10−6 K−1 for Al0.59Sc0.41N. Additionally, we observed an increase in CTE from 4.18 × 10−6 K−1 at 65 °C to up to 6.38 × 10−6 K−1 at 375 °C for Al0.68Sc0.32N. The experimentally determined CTE and elastic modulus allow a more precise design of Al1−xScxN-based frequency filters which are used in mobile communications and are important parameters for the prediction of device performance at elevated temperatures.

Monolithically integrated power circuits in high-voltage GaN-on-Si heterojunction technology

Abstract: This study presents monolithically integrated power circuits, fabricated in a high-voltage GaN-on-Si heterojunction technology. Different advanced concepts are presented and compared with solutions found in the literature. High switching transition slew rates are demonstrated by means of a monolithic power circuit with integrated gate driver. A highly linear temperature sensor is integrated in a GaN-high-electron-mobility transistor (HEMT) power device for the 600 V class and on-state resistance of 53 mΩ. An area-efficient HEMT structure with integrated freewheeling diodes is presented. This structure is applied in a monolithic multilevel converter chip, as well as in a 600 V class half-bridge chip. The multilevel chip is integrated by an advanced printed circuit board embedding technology and tested in inverter operation with a mains voltage output of 120 V RMS . The performance of the half-bridge is demonstrated in a synchronous buck converter operation from 400 to 200 V and with a switching frequency of 3 MHz.

Broadband High-Power W-Band Amplifier MMICs Based on Stacked-HEMT Unit Cells

Abstract: This paper reports on a broadband high-power amplifier (HPA) millimeter-wave integrated circuit (MMIC) covering the extended W-band (65–125 GHz). The MMIC is based on the Fraunhofer IAF 50-nm gate-length metamorphic high-electron-mobility transistor (mHEMT) technology. The HPA consists of two parallelized unit amplifiers. Each unit amplifier (UA) utilizes four stacked-HEMT unit power cells (UPCs) and four-way power combiners at the input and output. The UPCs stack four transistors with a gate width of $4\times 40~\mu \text{m}$ per HEMT. The UA achieves an average small-signal gain of 19.4 dB and an average saturated output power of 21.6 dBm at least from 70 to 110 GHz. The HPA yields an average small-signal gain of 16.8 dB and an average saturated output power of 22.5 dBm at least from 68 to 110 GHz. A peak output power of 24.1 dBm is achieved at an operating frequency of 75 GHz.

Highly Isolating and Broadband Single-Pole Double-Throw Switches for Millimeter-Wave Applications Up to 330 GHz

Abstract: This paper reports on the investigation, design, and fabrication of single-pole double-throw (SPDT) millimeter-wave integrated circuit (MMIC) switches for applications with an operating frequency of up to 330 GHz. An analysis of the well-established millimeter-wave (mmW) switch topology ( $\lambda /4$ -shunt SPDT switch) is done to achieve wideband performance and high isolation simultaneously. Using a standard technology parameter ( $R_{\mathrm{\scriptscriptstyle ON}}$ ), this theory allows a performance estimation of switch MMICs before a wafer run and can support the design of mmW switches. Based on this analysis, a novel switch topology, with an improved isolation, is introduced. Additionally, this paper demonstrates three SPDT switch MMICs (SPDTs 1–3), the first two of which are targeting the W-band, whereas the latter is targeting the H-band frequency range. All MMICs were fabricated on the Fraunhofer Institute for Applied Solid State Physics 50-nm gate length metamorphic high-electron-mobility transistor process. SPDT 1 achieves a bandwidth of 52–168 GHz. The average insertion loss (IL) and the isolation are 3.1 and 42.1 dB, respectively. The peak performance is 2.1 and 52 dB. SPDT 2 utilizes the novel switch topology and yields the average IL and isolation of 4.5 and 56.4 dB, respectively. The operating bandwidth is 75–170 GHz. The peak performance is 3 and 65 dB. The 1-dB compression points ( $P_{\mathrm {in1\,dB}}$ ) of SPDTs 1 and 2 are 19 dB and 14 dBm, respectively. SPDT 3 operates from 122 to 330 GHz and achieves the average IL and isolation of 2.2 and 17.4 dB, respectively. The peak performance is 1.5 and 22.8 dB.

Transfer of AlGaN/GaN RF-devices onto diamond substrates via van der Waals bonding

Abstract: We present a novel bonding process for gallium nitride-based electronic devices on diamond heat spreaders. In the proposed technology, GaN devices are transferred from silicon (Si) onto single (SCD) and polycrystalline diamond (PCD) substrates by van der Waals bonding. Load-pull measurements on Si and SCD heat spreaders at 3 GHz and 50 V drain bias show comparable power-added-efficiency and output power (Pout) levels. A thermal analysis of the hybrids was performed by comparison of 2 × 1mm2 AlGaN/GaN Schottky diodes on Si, PCD, and SCD, which exhibit a homogeneous field in the channel in contrast to gated transistors. Significantly different currents are observed due to the temperature dependent mobility in the 2DEG channel. These measurements are supported by a 3D thermal finite element analysis, which suggests a large impact of our transfer technique on the thermal resistance of these devices. In summary, we show a promising new GaN-on-diamond technology for future high-power, microwave GaN device applications.

Full W-Band GaN Power Amplifier MMICs Using a Novel Type of Broadband Radial Stub

Abstract: In this paper, we describe the design of the first reported full W-band (75–110 GHz) power amplifier (PA) monolithic microwave integrated circuits (MMICs) based on gallium nitride technology. The discussed MMICs come in two versions. Over a bandwidth (BW) of 70–110 GHz, the three-stage PA can deliver, on average, 25.6 dBm with a power-added efficiency (PAE) of 6.5%, while the four-stage PA is able to generate 27 dBm with a PAE of 6.1%. A peak output power of 28.6 dBm is achieved at 80 GHz with a PAE of 8.6%, which corresponds to a power density of 2.6 W/mm. The significant BW was achieved partially by incorporating a novel type of broadband radial stub into the design, which can provide nearly a twofold rejection-BW improvement over the conventional version. To the best of our knowledge, no other solid-state circuit can deliver such power levels over the complete W-band.

W-band SPDT switches in planar and tri-gate 100-nm gate-length GaN-HEMT technology

Abstract: This paper reports on W-band (75 to 110 GHz) single-pole double-throw (SPDT) switch millimeter-wave integrated circuits (MMICs) based on a planar and a tri-gate 100-nm gate-length GaN high-electron-mobility transistor (HEMT) technology. The SPDT switches utilize the well-established quarter-wave stub topology with shunt transistors. For an improved wideband performance, an optimized layout approach is used that connects the shorted stub for the compensation of the capacitance of a shunt transistor at the center of the transistor. The presented SPDT switch MMIC demonstrates for both technology versions a measured average insertion loss of 1.3 dB over the entire W-band with a peak insertion loss of 1.2 and 1.1 dB in the center of the W-band for planar and tri-gate HEMTs, respectively. The one-dB bandwidth is for both MMICs almost an octave. For an input power of at least 25 dBm, both SPDT switches do not show an indication of compression.

Avalanche multiplication in AlGaN-based heterostructures for the ultraviolet spectral range

Abstract: AlxGa1-xN based avalanche photodiodes grown on sapphire substrate with Al-contents of x = 0.65 and x = 0.60 have been examined under back- and frontside illumination with respect to their avalanche gain properties. The photodetectors suitable for the solar-blind ultraviolet spectral regime show avalanche gain for voltages in excess of 30 V reverse bias in the linear gain mode. Devices with a mesa diameter of 100 μm exhibit stable avalanche gain below the break through threshold voltage, exceeding a multiplication gain of 5500 at 84 V reverse bias. A dark current below 1 pA can be found for reverse voltages up to 60 V.

Analysis and Development of Submillimeter-Wave Stacked-FET Power Amplifier MMICs in 35-nm mHEMT Technology

Abstract: This paper reports on the first stacked field-effect transistor (stacked-FET) submillimeter-wave monolithic integrated circuit (S-MMIC) power amplifier cell operating at 0.3 THz and the first stacked-FET medium power amplifier (MPA) at 240 GHz. Both circuits are fabricated using a 35-nm InGaAs-on-GaAs metamorphic high electron mobility transistor (mHEMT) technology with grounded coplanar waveguide lines. In both cases, compactness and performance are enhanced thanks to the use of an in-house process, based on three-metallization layers, instead of the usual two-layer process. The single-stacked cell exhibits an ultrawide small-signal 3-dB relative bandwidth (RBW) of $\hbox{47.3}{\%}$ , with output power levels higher than 4.3 dBm from 280 to 308 GHz (9.5%). The MPA MMIC combining four triple-stacked mHEMT cells in parallel achieves a small-signal 3-dB RBW of 24.2%, 10.8 dBm of output power, and a power-added efficiency of 5.02%. These values outperform the state-of-the-art results of MPAs published within a comparable technology.

Graphene as an active virtually massless top electrode for RF solidly mounted bulk acoustic wave (SMR-BAW) resonators.

Abstract: Mechanical and electrical losses induced by an electrode material greatly influence the performance of bulk acoustic wave (BAW) resonators. Graphene as a conducting and virtually massless 2D material is a suitable candidate as an alternative electrode material for BAW resonators which reduces electrode induced mechanical losses. In this publication we show that graphene acts as an active top electrode for solidly mounted BAW resonators (BAW-SMR) at 2.1 GHz resonance frequency. Due to a strong decrease of mass loading and its remarkable electronic properties, graphene demonstrates its ability as an ultrathin conductive layer. In our experiments we used an optimized graphene wet transfer on aluminum nitride-based solidly mounted resonator devices. We achieved more than a triplication of the resonator's quality factor Q and a resonance frequency close to an 'unloaded' resonator without metallization. Our results reveal the direct influence of both, the graphene quality and the graphene contacting via metal structures, on the performance characteristic of a BAW resonator. These findings clearly show the potential of graphene in minimizing mechanical losses due to its virtually massless character. Moreover, they highlight the advantages of graphene and other 2D conductive materials for alternative electrodes in electroacoustic resonators for radio frequency applications.

Investigations of Active Antenna Doherty Power Amplifier Modules Under Beam-Steering Mismatch

Abstract: This letter investigates the performance of a Doherty power amplifier (DPA) in an $8\times 1$ active antenna transmitter module under an average voltage-standing-wave ratio of 4:1 induced by the array 1-D beam steering and mutual coupling. The GaN-based DPA and a $2\times 1$ segment of the array are verified via measurements at the operating frequency of 3.5 GHz. The co-design and co-simulation of both the antenna array and the behavioral model of the DPA show that the inherent back-off efficiency benefit—when compared to traditionally used class-AB and -B amplifiers in base stations—can be maintained when restricting the beam-steering angle, in this case $\theta \in $ [−45°, 45°], and operational bandwidth of the combined system.

70-116-GHz LNAs in 35-nm and 50-nm Gate-Length Metamorphic HEMT Technologies for Cryogenic and Room-Temperature Operation

Abstract: In this paper, the room-temperature and cryogenic performance of two low-noise amplifier (LNA) modules in the frequency range from 70 to 116 GHz is presented and analyzed. The investigation is based on LNA millimeter-wave integrated circuits using 35-nm and 50-nm gate-length metamorphic high-electron-mobility transistor technologies. At room temperature, the WM-2540 waveguide modules demonstrate an average noise temperature of 214 and 247 K over the 70–116-GHz frequency range. The lowest achieved noise temperatures are 171 and 196 K. When cooling the LNA modules to an ambient temperature of 6 K, the average noise temperatures improve to 30.1 and 31 K. The lowest achieved noise temperatures are 20.7 and 19.2 K. To the best of the authors' knowledge, the demonstrated LNAs yield the lowest published average noise temperatures at room temperature over the 70–116-GHz frequency range. Furthermore, the achieved cryogenic noise performance is among the best results published so far.

Multi-Stage Cascode in High-Voltage A1GaN/GaN-on-Si Technology

Abstract: This work investigates a new approach of a multistage cascode. The concept is applied as intrinsic structure in an A1GaN/GaN-on-Si technology. The fabricated device achieves an off-state voltage >600 V and an on-state resistance of 14 Ω mm. A special pull-down pin is connected to the source of the highest segments. This pin can be used for characterization and is intended to drive further stacked cascade segments. Thus, integrated multi-stage cascodes are found suitable as flexible device for high voltage applications.

Investigation of Temperature Characteristics and Substrate Influence on AlSeN-Based SAW Resonators

Abstract: It has been found that introducing Sc into AIN to form Al 1-x Sc x N can lead to a significant increase in piezoelectric response, making Al 1-x Sc x N a promising material for electroacoustic devices, such as frequency filters for mobile communications. 1 J.1m thick highly c-axis oriented Al 1-x Sc x N thin films were deposited on 100 mm Si(001)(x = 0, 0.14, 0.23, and 0.32)and on Al203(0001)(x = 0.14 and 0.23)substrates using reactive pulsed-DC magnetron sputtering and SAW resonators with wavelengths λ = 2–24 μm were fabricated. The effective electromechanical coupling k2eff of the resonators improved with increasing Sc concentration, for instance, the λ = 2 μm AIN (fs = 2.08 GHz)and Alo.68Sco.32N (fs = 1.83 GHz)resonators exhibited coupling of 0.6 % and 2.6 %, respectively. Furthermore, 3 % increase in resonant frequency and 9.6 % increase in coupling were observed when comparing λ = 2 μm resonators based on Al203 to those based on Si substrates. The temperature coefficient of frequency (TCF)was found to reach up to −27.4 ppm/K at x = 0.23 using Si substrates, while the equivalent resonators based on Al203 substrates exhibited lower TCF of −26.1 ppm/K. Thermal hysteresis errors were calculated to be ~1%. Employing Al203 substrates for λ = 2 μm Al 0.77 Sc 0.23 N-based resonators showed reduced TCF, higher resonant frequency, and higher coupling coefficient when compared to the same design resonators on Si substrates.

Temperature Cross-Sensitivity of AlN-Based Flexural Plate Wave Sensors

Abstract: In this paper, we investigate the temperature cross-sensitivity of aluminum nitride (AlN)-based flexural plate wave devices for sensing applications in contact with liquids. In our improved device topology, the interdigital transducers are designed as a buried electrode, and thus, are electrically shielded, enabling full immersion of the sensor into the liquid. The fabricated devices showed a mass sensitivity of 240 cm2/g for loading with deionized water which is in good agreement with the theoretical predictions. The temperature coefficient of frequency (TCF) was evaluated for devices with initially compressive and tensile stressed AlN layers. Devices with compressive film stress exhibited a TCF of −62 ppm/K to −28 ppm/K, while the devices with tensile film stress showed an increased TCF of −391 ppm/K to −72 ppm/K. It is shown that variations in in-plane tension are mainly accountable for the increased TCF, while the lower TCF is primarily caused by material softening.

A $G$ -Band Broadband Balanced Power Amplifier Module Based on Cascode mHEMTs

Abstract: The first full $G$ -band power amplifier module demonstrated in GaAs metamorphic high electron mobility transistor (mHEMT) technology has been developed for use as a driver amplifier in instrumentation. The circuit integrated in this module uses a compact balanced cascode topology and is based on a 35-nm mHEMT technology in a grounded coplanar waveguide environment. High compactness is achieved due to the use of a small ground-to-ground spacing of $14~\mu {\mathrm{ m}}$ and an advanced process with three metallization layers. The millimeter-wave integrated circuit exhibits a linear gain higher than 15.3 dB and return losses better than 10 dB from 118 to 236 GHz, which represents an ultrabroad relative bandwidth (RBW) of 67%. A peak output power of 10 dBm is achieved at 200 GHz. The WR-5 module uses broadband transitions, which show insertion losses of less than 1.5 dB. It demonstrates a small-signal gain that exceeds 13.4 dB in the whole $G$ -band (RBW ≥ 54%). Large-signal characterization exhibits power levels higher than 3.6 dBm from 130 to 210 GHz (RBW = 47%). This module achieves the highest bandwidth in $G$ -band and the highest output power when comparing it with modules based on similar technologies.

PCB-Embedding for GaN-on-Si Power Devices and ICs

Abstract: This paper investigates a printed circuit board (PCB)-embedding technology as packaging technology for 600 V class GaN-on-Si power devices and power integrated circuits (ICs). The electrical and thermal characteristics of PCB-embedding are investigated and compared to conventional assemblies based on wire-bonding and soldering.

A 300 GHz microstrip multilayered antenna on quartz substrate

Abstract: In this paper we demonstrate an antenna for 300 GHz. The antenna is processed on quartz and consists of 2 quartz layers with 2 metal layers each, which are separated by benzocyclobutene (BCB). The edge length is 2 millimeters. Design, simulation and the analysis is done in CST Microwave Studio. Measurement is done after packaging on a brass disk. This work shows the design details, simulated results and measurement data of the presented antenna.

Riemann-Pump based RF-Power DACs in GaN Technology for 5G Base Stations

Abstract: This paper introduces two realizations of a Riemann-Pump based digital transmitter geared towards software defined radio in 5G systems. In contrast to traditional digital-to-analog converter (DAC) concepts, the Riemann Pump exhibits a higher signal-to-noise ratio (SNR). Since the used 0.25 μm gallium nitride (GaN) technology has a transit frequency of 30 GHz, an oversampling ratio (OSR) of five for the targeted frequency range from DC to 6 GHz is enabled. As the Riemann Pump serves as a digital driver, a co-integration with a suitable wideband power amplifier is possible to reach high power levels. A hybrid assembly is compared to the fully integrated Riemann Pump circuit in GaN technology. The world’s first measurement data of a two bit resolution Riemann Pump in GaN technology are discussed. Radio frequency (RF) power in the watt level range is measured for frequencies between 0.5 GHz - 12 GHz into a capacitive load representing the input capacitance of a power amplifier.

A Novel Type of Broadband Radial Stub

Abstract: We report on a new type of broadband radial stub. It consists of two inclined half-stubs, which are separated by a small gap at their vertices. This radial stub can provide −15 dB rejection over almost 29% fractional bandwidth, which is an 86% improvement over the conventional radial stub. Using this stub in a double-sided (butterfly) configuration results in a −15 dB rejection bandwidth of about 61%, higher by a factor of two than in the case of a conventional butterfly stub. This stub was utilized in the DC-bias paths of a GaN power amplifier, which can provide more than 10 dB of gain between 60 and 110 GHz, resulting in the first-ever GaN power amplifier able to cover the whole W-band (75–110 GHz).

Determination of Elastic and Piezoelectric Properties of Al 0.84 Sc 0.16 N Thin Films

Abstract: In this work, bar-and disk-shaped resonators were fabricated using $\text{Al}_{0.84}\text{Sc}_{0.16}\text{N}$ thin film. Sensitivity analysis of the resonators' resonance frequencies with respect to the piezoelectric material parameters revealed that resonance analysis can be used for material parameter extraction. An optimization algorithm is employed to determine the piezoelectric material parameters $\text{s}_{11}^{E}, \ \text{s}_{12}^{E}$ , and $\text{d}_{31}$ of $\text{Al}_{0.84}\text{Sc}_{0.16}\text{N}$ from the length-and radial-extensional mode. Using the piezoelectric constitutive equations and combining the results obtained in this work with the results obtained in a previous work, the stress-charge form of elastic and piezoelectric parameters could be derived.

Towards Highly-Integrated High-Voltage Multi-MHz GaN-on-Si Power ICs and Modules

Abstract: This work investigates how integration of half-bridge transistors, drivers and freewheeling-diodes on a conductive Si-substrate influences DC-DC converter performance. Based on a lateral 600 V GaNtechnology, a 300-150V, 750W buck converter is built. Conventional separate substrate-to-source terminations of the high-side and low-side devices resulted in 97.3% maximum efficiency. A common semi-floating substrate termination as applicable to monolithic ICs reduced the maximum efficiency to 96.8%, but improved no-load losses. Capacitance measurements which include the substrate as terminal explain two opposite effects resulting from the common substrate: Increased Miller capacitance slows down hard-switching turn-on transitions; reduced effective switch-node capacitance speeds up resonant transitions. Operating the integrated GaN driver with external Si-CMOS pre-driver in burst operation up to 40MHz at 350 V shows potential for multi-MHz applications. While monolithic integration is beneficial to reduced parasitic inductances and assembly effort, a differentiated analysis considering different switching conditions is required to identify system-level advantages.

Instabilities by Parasitic Substrate-Loop of GaN-on-Si HEMTs in Half-Bridges

Abstract: The stability of GaN-on-Si HEMTs with substrate-to-source termination is analyzed in a high-voltage half-bridge. The work exposes that external substrate termination creates a parasitic substrate loop, which leads to unstable switching behavior under certain conditions. Stability analysis reveals that parasitic inductance in the substrate-loop alone is sufficient for instabilities, even with zero parasitic inductance in the gate- and power-loops. A systematic analytical stability analysis is carried out based on a small-signal equivalent circuit. The theory is verified by measurements using a PCB-embedded 600 V GaN HEMT with integrated gate driver. Adequate damping of the substrate loop resonance enables stable operation of the half-bridge module.

Dynamic Load Modulated Low-Voltage GaN PA Using Novel Low-Loss GaN Varactors

Abstract: This paper gives a first presentation of a dynamic load modulated (DLM) power amplifier (PA) employing novel AlGaN/GaN varactors. With an AlGaN/GaN HEMT this enables the highest operating frequency ever presented for a varactor-based GaN DLM PA. Despite the low drain-source voltage of 10 V, making the PA suitable for mobile applications, it shows a high drain efficiency of 61 % at a maximum output power of 24 dBm. At 6 dB output power back-off the PA still reaches 45 % efficiency. The achieved results demonstrate the promising potential of AlGaN/GaN varactors, and show the usability of AlGaN/GaN HEMT at low voltages.

Voltage- and Temperature-Dependent Degradation of AIN/GaN High Electron Mobility Transistors

Abstract: We investigated the voltage- and temperature -dependent degradation of AIN/GaN high electron mobility transistors with gate lengths of 70 nm and 100 nm. The devices under test were dc stressed in semi-on-state conditions at constant power dissipation of $4.5\displaystyle \frac {\mathrm {w}}{\mathrm {m}\mathrm {n}}$ for approximately 200h or until the drain saturation current $I_{\mathrm {D}\mathrm {S}\mathrm {S}}$ dropped by 10 %. To examine whether a Arrhenius like temperature acceleration and additionally voltage acceleration can be ascertained, the channel temperature and stress voltage were varied. In our tests, both acceleration factors could be confirmed and are shown to be interdependent. Arrhenius fits resulted in activation energies between 0.80 eV and 1.12 eV. A generalized Eyring approach is used to model the combined acceleration by temperature and voltage as well as their interdependency.

RF-Noise Modeling of InGaAs Metamorphic HEMTs and MOSFETs

Abstract: The mechanisms causing the RF-noise in InGaAs metamorphic HEMTs and MOSFETs have been investigated and compared. A small signal model for InGaAs- metamorphic HEMTs and InGaAs MOSFETs, including an accurate description of the RF-noise, is presented. The model is based on a distributed multiport-network approach, which is scalable in gate width, the number of gate-fingers and covers usual bias points used in amplifier circuits. The noise model is capable of analyzing the sources of noise in InGaAs HEMTs and MOSFETs and their impact on the overall device noise figure. The new extracted MOSFET model is verified on circuit level in the W-Band (75 to 110GHz).

Analysis of 4-way divider MMICs in GaAs technology for H-band applications

Abstract: A 4-way Wilkinson and a 4:1 Dolph-Chebyshev divider MMIC based on grounded coplanar waveguide technology on GaAs are presented, analyzed and compared. The Dolph-Chebyshev structure was simulated by using electromagnetic software, whereas the 4-way Wilkinson divider was simulated by a schematic CAD tool. Both dividers are processed and experimentally evaluated for operation in H-band (220–330 GHz). The 4-way Wilkinson MMIC achieves the best insertion losses, which are lower than 107 dB between 235 and 255 GHz (8 2 % of relative bandwidth, RBW). Input return losses better than 12 dB, an amplitude imbalance between outer and inner branches lower than 0.7 dB and a phase difference of (1.5 ± 0.1)° is achieved by the Dolph-Chebyshev from 235 to 257 GHz (RBW=8.9 %). The 4-way Wilkinson divider is very compact with a size of 0.09 × 0.22 mm2.

Spurious Mode Suppression in the Design of GCPW Submillimeter-wave Power Amplifiers

Abstract: This work investigates the problem of spurious mode propagation in a grounded coplanar waveguide (GCPW) environment at frequencies between 200 GHz and 335 GHz. Design strategies focused on minimizing undesired effects are explored through full-wave electromagnetic (EM) analysis and experimental results from different test structures. It is shown that a $\lambda/13$ distance between via-holes should be chosen to avoid unwanted resonances at these high frequencies. The critical role of via-holes is also demonstrated through the experimental comparison of two power splitters. Finally, the need of closed RF pads to avoid any propagation of parasitic modes is experimentally shown in an application example of a power amplifier (PA) cell based on a 35 nm GaAs metamorphic high electron mobility transistor (mHEMT) technology. The PA cell with closed pads achieves a 0.5 dB bandwidth of 28 % with small-signal gain levels larger than 5.2 dB.

State Dependency, Low-Frequency Dispersion, and Thermal Effects in Microwave III-V HEMTs

Abstract: An AlGaN/GaN HEMT- and an InAlAs/lnGaAs mHEMT technology, both with a gate-length of 100 nm, are investigated w.r.t. state dependency vs. average gate and drain voltages, and low-frequency (LF) dispersion, and the separation of thermal effects is demonstrated. Based on a comprehensive DC-CW and pulsed-RF small-signal characterization, it is shown that the GaN HEMT shows all three effects, while the mHEMT is nearly free of state dependency. The description of the LF dispersion using classical large-signal FET models is compared to the recently proposed integral-transform (ITF) model. A product separation approach for the thermal effects and the extraction of thermal parameters via simultaneous equations is described. A new formulation of the ITF model is capable of describing all three above effects in pulsed-RF and even in CW load-pull operation conditions.