Barium Strontium Titanate Varactors Research Articles

Tunable microstrip bandpass filter with constant absolute bandwidth using BST varactors and digitally tunable capacitors

Abstract In this paper, two second-order electronically tunable bandpass filters are presented. The filters are implemented in microstrip technology using barium–strontium–titanate (BST) varactors and digitally tunable capacitors (DTC) for tuning the frequency response of the bandpass filters. The filter realized using BST varactors has a 35% tuning range from 900 MHz to 1.275 GHz with an insertion loss variation from 3.1 to 2.6 dB. The absolute bandwidth is nearly constant over the entire tuning range, varying from 64 to 72 MHz (around ±5% variation). The filter realized using DTCs also has a 36% tuning range from 850 MHz to 1.225 GHz with an insertion loss variation from 3.1 to 1.5 dB. The absolute bandwidth is constant over the tuning range, varying from 88 to 98 MHz (around ±5% variation). The bandpass filters are tuned using a single control signal. The tunable bandpass filters are proposed for use in reconfigurable radios.

Suppression of Acoustic Resonances in BST-Based Bulk-Ceramic Varactors by Addition of Magnesium Borate

This work presents a method for reducing acoustic resonances in ferroelectric barium strontium titanate (BST)-based bulk ceramic varactors, which are capable of operation in high-power matching circuits. Two versions of parallel-plate varactors are manufactured here: one with pure BST and one with 10 vol-% magnesium borate, Mg3B2O6 (MBO). Each varactor includes a 0.85-mm-thick ferroelectric layer. Acoustic resonances that are present in the pure BST varactor are strongly suppressed in the BST-MBO varactor and, hence, the Q-factor is increased over a wide frequency range by the addition of small amounts of a low-dielectric-constant (LDK) MBO. Although the tunability is reduced due to the presence of non-tunable MBO, the increased Q-factor extends the varactor’s availability for low-loss and high-power applications.

Monolithic IPD-MEMS Technology and Its Applications to High-<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math> </inline-formula> Switched Capacitor Banks and Variable True-Time-Delay Networks

A low-loss monolithic integrated passive device-microelectromechanical systems (IPD-MEMS) technology process on an alumina substrate is proposed. Two applications are investigated both theoretically and experimentally using the proposed IPD-MEMS process: 1) a high-quality-factor switched capacitor bank, and 2) a variable true-time-delay (TTD) network with a fine resolution. The measured IPD-MEMS capacitor bank exhibits a variable capacitance of 0.63–3.8 pF with $Q$ -values between 100 and 300 at 1 GHz. A low-loss highly miniature IPD-MEMS digital delay line, with a delay ranging between 40 and 120 ps, is measured. A maximum 1-dB insertion loss is achieved for this digital TTD network of up to 2 GHz with a footprint of 2 mm $\times1.3$ mm. To realize a fine delay step resolution, a barium–strontium–titanate varactor is added to this IPD-MEMS digital design. A continuous maximum group delay between 70 and 148 ps with a fine resolution of 0.6 ps is achieved with an overall size of 2.9 mm $\times1.6$ mm.

Varactor-Based Passive Ripple Filter With Automatic Tuning for Low-Noise Power-Supply Systems

Use of parallel resonance to notch filter the fundamental ripple frequency on a switching regulator output offers advantages in the combined metric of size and power compared to traditional low-pass solutions. Availability of barium strontium titanate varactors with capacitance of several nanofarads now makes automatic tuning of parallel resonance alignment with the ripple fundamental a feasible alternative. A phase-lock automatic-tuning circuit is demonstrated. The effective use of this technique as the power-supply filter used to power a 16 bit ADC is shown. The suitability of the resulting filter and regulator to full surface mount encapsulation is demonstrated.

Reconfigurable Bandstop and Bandpass Filters With Wideband Balun Using IPD Technology for Frequency Agile Applications

A tunable bandstop filter (BSF) with a wideband balun using integrated passive devices (IPDs) technology for multichip module is proposed. The compact tunable BSF uses barium strontium titanate (BST) varactors demonstrating a tuning range of 55% from 1.3 to 2.3 GHz with a 20-dB rejection level. The performance of the filter is optimized by applying asynchronous biases to the BST varactors, leading to bandwidth and rejection level improvement. High-power test results show that the filter exhibits a third-order input intercept point of more than 40 dBm. An IPD wideband balun (0.5–2.5 GHz with ±1 dB amplitude imbalance and 180° ± 20°) using a wideband phase inverter is also presented. The design considerations of the wideband phase inverter (1.5–8 GHz) are addressed in this paper. Design and measured results are also presented for a fixed IPD bandpass filter, and an IPD bandpass filter with a tunable transmission zero to demonstrate the reconfigurability in frequency agile applications.

High Tolerance of BST Thin Film Based Varactor Under Neutron Irradiation

We report neutron irradiation resistance of the barium strontium titanate (BST) thin-film-based varactor under different neutron fluences. BST thin films were deposited on sapphire substrate by RF magnetron sputtering and gold interdigital capacitor (IDC) electrodes were patterned with the photolithography technique. Capacitance-voltage ( $C$ – $V$ ) and leakage current characteristics were measured at several neutron fluences. A small change in capacitance is observed at different fluences. The changes are attributed to the radiation-induced defect dipoles and other charged defects, which serve as effective domain wall pinning sites. We supplemented this macroscopic $C$ – $V$ behavior of BST varactor with investigation through X-ray diffraction and atomic force microscopy for their structural and morphological changes, respectively. This paper shows that BST varactor can be used in neutron environment due to its high tolerance.

Inside view

Sebastian Preis of the Ferdinand-Braun-Institute in Germany tells us about his group's work on high-electron mobility-transistors (HEMT) and varactors. The work was in collaboration with researchers at Darmstadt University of Technology. The transistor module mounted to a test fixture A high-electron mobility-transistor (HEMT), also known as heterojunction FET (HFET), is a type of transistor especially suited for high frequency power and low-noise applications. The gallium nitride (GaN) HEMT used here is a specially designed version. Based on the high bandgap of the GaN material used, as well as a high electron velocity, its benefits are high breakdown voltage, high output power, high maximum transition and robustness in terms of channel temperature. Varactors are variable capacitors – they can be tuned by applying a certain voltage. Depending on the technology, this can be some volts, as in case of silicon based semiconductor varactor diodes, but also kV for thick barium-strontium-titanate (BST) ceramic varactors. Already the concepts of realising a varactor differ a lot. Established are varactor diodes based on silicon but also compound semiconductors, such as SiGe, GaAs and GaN. Inherently, diodes suffer from their nonlinear characteristic. Micro-electro-mechanical-systems (MEMS) are introduced as very linear counterpart, but are limited in switching speed and power handling capability. Ceramic based varactors are similar to common multilayer capacitors with high dielectric constant ceramic material. BST varactors make use of the usually unwanted property of electric field dependent dielectric constant. Furthermore, prior to the present generation of metal-insulator-metal (MIM) BST varactors, the power handling capability of ceramic varactors for RF and microwave operation was limited to some watts, but this has been greatly improved now. Another problem is the thermal capacitance drift of BST varactors. The BST ceramic does not only change its dielectric constant in an electric field, but also if the temperature changes. Some other groups use the BST material with different doping as temperature sensors. The deployed MIM BST varactor generation is capable of handling power levels of at least 50 W. In fact, the power handling capability is a function of the surface area and is limited by the increasing capacitance of larger varactors. The whole module, thanks to the varactors, provides an electronically tuneable transistor with more than 20 W output power. The module can be used like conventional transistors, but it can be adapted to varying situations – such as changing load conditions or different operation frequency – by simply adapting the varactors’ control voltages. Thick-film metal-insulator-metal barium-strontium-titanate varactors in a GelPak Such a transistor can be used in a load modulated amplifier, which adapts its optimum load line to the actual required output power and therefore operates always in a very energy efficient mode. Thanks to the BST technology, the varactors power consumption is negligible. Besides this, base station amplifiers can be built, which can be operated at different frequencies. Also, space-born modules can be envisaged. The operator just needs to adjust the varactor control voltage to switch the frequency band. In the frame of the project, the European space agency (ESA) will test the modules, manufactured at Ferdinand-Braun-Institute (FBH). Further, the varactors and modules will be improved in terms of performance and maturity. BST varactors can be used to realise tuneable oscillators, mixers, and also passive circuits like phase shifters and filters. In general, this could link the analogue RF to the digital signal processing world. By simply changing a voltage, an RF system can be switched to operate in a different frequency band, on a different power level, while maintaining efficiency. Analogue RF systems have to become not only smaller but also smarter, i.e., easier to adjust remotely. Electronically tuneable capacitors, such as BST based varactors, are ideal for this purpose and will gain importance in the near future. First, big suppliers, such as ST Microelectronics, will start to distribute BST varactors commercially, together with GaN HEMT technology high performance transmitters. Space applications might change to this combination. Even in the telecom market, some applications such as point to point links could benefit from GaN and BST. I would like to see an integration of BST on wafer level into GaN transistor processes. This would solve several problems like the heat removal from the BST varactors and the parasitic circuit elements introduced due to varactor mounting. GaN MMICs incorporating BST varactors would allow smart devices, such as frequency agile MMIC amplifiers, adjustable mixers, oscillators, etc.

Reconfigurable package integrated 20 W RF power GaN HEMT with discrete thick‐film MIM BST varactors

A novel package integrated solution for gallium nitride high-electron-mobility transistors with an electronically two-dimensional reconfigurable L-section matching network is presented. Thick-film barium-strontium-titanate (BST) varactors are used to realise a tunability of the load impedance of 1:2 in resistive and 1:1.5 in reactive parts, respectively, applying tuning voltages of up to 200 V. The tuneable module achieves a peak output power of 43.2 dBm (20.9 W). Comparison with simulated results using field simulations and a large-signal model shows good agreement in terms of output power and power added efficiency and thus proves the concept of using metal–insulator–metal BST varactors for reconfigurability.

A Tunable Bandstop Filter Using Thin Film Barium-Strontium-Titanate Varactors

AbstractFerroelectric barium-strontium-titanate (BST) varactors are becoming extremely important particularly for frequency-agile RF and microwave circuits. Tunable bandstop filters are desired to eliminate unwanted signals for several communication systems. In this paper, a compact tunable bandstop filter (BSF) based on a single hairpin resonator, using ferroelectric BST varactors, is also presented. The frequency response of proposed BSF can be adjusted by increasing or decreasing the length, width, and space for the coupled line hairpin resonator. One pair of variable capacitors has been incorporated to the filter for tuning and miniaturization purposes. The notch frequency of BSF can be tuned from 705 MHz to 1.035 GHz with a tuning voltage of 6 Volts utilizing BST varactors.

LOW THIRD-ORDER INTERMODULATION DISTORTION IN Ba<sub>0.6</sub>Sr<sub>0.4</sub>TiO<sub>3</sub> THIN FILM INTERDIGITAL CAPACITORS

A new method for the theoretical analysis of the thirdorder intermodulation distortion (IM3) in barium strontium titanate (BST) thin film interdigital capacitors (IDCs) on r-plane sapphire substrates is presented. Two circuit topologies - the dual and series dual BST varactor circuit - are proposed and their theoretical models along with simulated and measured results are presented. Low IM3 is demonstrated and experimentally verified. By proper selective biasing, very low nulls are observed in both dual and series dual BST varactor circuit topologies which indicate minimum distortion. The measured first nulls are achieved at ±13V and ±20V for the dual and series dual topologies respectively. These results demonstrate the potential of incorporating these highly linear BST varactors in silicon on sapphire (SoS) applications.

Integration of BST varactors with surface acoustic wave device by film transfer technology for tunable RF filters

This paper presents a film transfer process to integrate barium strontium titanate (BST) metal–insulator–metal (MIM) structures with surface acoustic wave (SAW) devices on a lithium niobate (LN) substrate. A high-quality BST film grown on a Si substrate above 650 °C was patterned into the MIM structures, and transferred to a LN substrate below 130 °C by Ar-plasma-activated Au–Au bonding and the Si lost wafer process. Simple test SAW devices with the transferred BST variable capacitors (VCs) were fabricated and characterized. The resonance frequency of a one-port SAW resonator with the VC connected in series changed from 999 to 1018 MHz, when a dc bias voltage of 3 V was applied to the VC. Although the observed frequency tuning range was smaller than expected due to the degradation of BST in the process, the experimental result demonstrated that a tunable SAW filter with the transferred BST VCs was feasible.

Beam Steering Transmitarray Using Tunable Frequency Selective Surface With Integrated Ferroelectric Varactors

A tunable frequency selective surface (FSS) with beam steering capability is presented. The FSS is used as a transmitarray with a bandpass characteristic in Ku-band. The periodic sub-wavelength (λ0/25) unit-cells are composed of capacitive and inductive structures creating a bandpass for an incident wave. By patterning the capacitive elements on a screen-printed barium-strontium-titanate (BST) thick-film ceramic, the resonant frequency of the FSS can be tuned. This technology offers a simple and cost effective way for integrating varactors into the FSS and is particularly attractive for microwave circuits with a high varactor density. A prototype is fabricated including two capacitive layers with an overall size of 40 mm × 40 mm. Each layer includes a total of 1600 integrated BST varactors that are fabricated by using a patterning and metallization process. A transmitted wave passing through the FSS will experience a phase shift, which can be tuned by tuning the passband. This allows steering the transmitted wave to a certain direction by applying a phase gradient along the FSS interface. A phase shift range of 360° can be covered by cascading several FSS panels. The prototyped tunable FSS demonstrates the feasibility of the proposed technology and its potential for beam steering.

A Monolithic AlGaN/GaN HEMT VCO Using BST Thin-Film Varactor

A monolithic AlGaN/GaN HEMT voltage-controlled oscillator (VCO) using a barium-strontium-titanate (BST) thin-film varactor is presented. The fabrication process of the VCO is fully compatible with the standard process of GaN HEMT monolithic microwave integrated circuits (MMICs). An improved equivalent circuit model of the fabricated BST varactor for microwave application was developed for the MMIC VCO circuit design. The experiment MMIC VCO exhibits 1-GHz tunable bandwidth at the central frequency of 7 GHz when the BST varactor bias changed from 0 to 24 V. An output power of 17 dBm and phase noise of -81 dBc/Hz (at offset of 100 kHz) are obtained at drain bias of 8 V and BST varactor of 0 V. The results indicate that the monolithic BST varactor is suitable for microwave application, and the integration of the BST varactor can be successfully applied to develop various tunable GaN MMICs.

Tunable microwave filters based on discrete ferroelectric and semiconductor varactors

This study presents a direct comparison between semiconductor and barium strontium titanate (BST) tuning technologies. Tapped line combline two-pole and four-pole bandpass filters were designed in a coplanar waveguide configuration. Discrete BST interdigital capacitors were fabricated and integrated into the filter circuits in a hybrid manner. The BST varactors performed as tuning elements in the filters. The same filter was also made using gallium arsenide (GaAs) varactors and the tuning and loss performance were compared with the BST filters. The centre frequency of the two-pole BST filter tuned 30.3% from 1.55 to 2.02 GHz with insertion loss ranging from 3.7 to 1.1 dB, at an electric field of 6.5 V/mu m. This filter has a figure of merit of 0.87 dB(-1). The two-pole GaAs filter showed a wide tuning range from 0.94 to 2.44 GHz at 14 V with insertion loss ranging from 4.2 to 0.8 dB. The results are discussed in detail, as it is important to clarify the comparison with relation to the quality of the materials, as well as the particular filter designs and centre frequencies involved.

Tunable slotted ground structured bandstop filter with BST varactors

This paper presents an investigation of a tunable bandstop filter (BSF) using slotted ground structure. A kind of open loop slot resonator is used for the bandstop response and its characteristics according to various dimensions are considered to decide a proper shape for the final design. A barium-strontium-titanate (BST) varactor chip is deployed to tune each resonator and its location is determined by electromagnetic simulation for an optimum frequency tuning. A three-pole tunable BSF of this type is measured at the centre frequency from 4.5 to 5.5 GHz and its rejection is more than 20 dB at the stop band while the insertion loss at the pass band is less than 1.5 dB.

Structural and Electrical Properties of Nanocrystalline Diamond based Barium Strontium Titanate Varactors

Barium strontium titanate (BST) varactors fabricated on silicon substrates experience reliability and yield issues due to the defects induced in the interdiffusion of the layers in the bottom electrode stack. The structural and electrical properties of BST varactors fabricated on nanocrystalline diamond (NCD) films as the diffusion barrier layer are reported here. The structural properties and the quality of the interfaces were studied along with the electrical properties of the varactors. The varactors fabricated on NCD films displayed improved symmetry of capacitance-voltage (C-V) characteristics and 70% higher quality factor due to better interface stability with reduced bottom electrode losses.

A TRL-Like Calibration for Tunable Interdigitated BST Varactors

We report on a de-embedding technique for measuring the reflection coefficients of comblike capacitors patterned on barium-strontium-titanate (BST) ferroelectric films. The technique is derived from the thru-reflect-line calibration technique and is combined with conformal mapping to retrieve the complex permittivity of BST thin films. The accuracy of the method is demonstrated over a broad frequency for 0.3-mum-thick BaxSr1-xTiO3, with x = 0.5, in a paraelectric state, grown by a sol-gel technique. The samples are characterized up to 35 GHz and exhibit a constant permittivity of 245 plusmn 12%, which is in good agreement with the values deduced from propagation wave constants of BST transmission lines.

BST-Varactor Tunable Dual-Mode Filter Using Variable ${Z}_{C}$ Transmission Line

Ferroelectric materials like barium-strontium-titanate (BST) have become attractive for microwave tunable circuit applications. This letter presents an investigation of BST-varactor tunable dual-mode filter using variable characteristic impedance or Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> transmission line. The interdigital BST varactors are fabricated and modeled, which are then deployed to realize a bandpass filter with tunable bandwidth capability. The use of tunable impedance line provides additional flexibilities in the design of microwave tunable circuits. A tunable bandpass filter of this type with a center frequency of 1.8 GHz and a bandwidth tuning ratio of 1.15:1 is demonstrated experimentally.

RF MEMS, BST, and GaAs varactor system-level response in complex modulation systems

This article presents the response of RF microelectromechanical systems (RF MEMS), barium strontium titanate (BST), and gallium arsenide (GaAs)-based tunable filters and reconfigurable matching networks to a wideband code-division-multiple-access signal cen- tered at 1.95 GHz. The RF MEMS tunable filter and impedance tuner result in very low inter- modulation distortion and spectral regrowth compared to their BST and GaAs counterparts. The linearity of the BST and GaAs tunable networks improves considerably by using a series combination of BST and GaAs varactors, but the RF MEMS-based networks still show the best linearity of all three technologies. Also, it is shown that the reconfigurable networks, tuned with capacitive RF MEMS can handle up to 1 W of RF power with no self-actuation. V C 2007

Modeling the capacitive nonlinearity in thin-film BST varactors

A simple closed-form expression for the dielectric nonlinearity in thin-film high-permittivity barium strontium titanate (BST) devices is obtained from a third-order power-series expansion for the field-polarization relation. The expression is parameterized in terms of easily measurable quantities of zero-field capacitance and tuning ratio, and compares favorably with data on several representative BST compositions and device sizes. The temperature dependence of the capacitors is treated using a simple linear temperature coefficient in the zero-field capacitance that also compares favorably with experimental data on BST capacitors. The influence of interfacial (dead layer), fringing, and parasitic shunt capacitance on the experimental C-V curves is discussed. The results are potentially useful for circuit and electromagnetic simulation.