Varactor Devices Research Articles

Giant Critical Thickness in Highly Conducting Epitaxial SrMoO3 Electrodes Investigated by Lift‐Off Membranes

AbstractWithin the huge perovskite materials family, thin films of highly conducting materials such as SrMoO3, SrNbO3, and SrVO3 are candidates for low‐loss bottom electrodes in epitaxial all‐oxide devices, in particular for high‐frequency applications. Recently, the fully coherent growth of more than 5 µm thick SrMoO3 electrodes in a varactor device prototype epitaxial heterostructure has been reported. This result raises the question of the strain mechanism in such anomalously thick coherent epitaxial layers. Here, this question is addressed by comparing the lattice constants of coherently strained layers and their free‐standing membranes. SrMoO3 is mainly elastically strained within the heterostructure and fully relaxed after removal of the substrate. These results indicate a giant critical thickness, making highly conducting perovskites even more outstanding materials for high‐frequency applications that require electrode thicknesses beyond the skin depth. The described technology of lifting off thick SrMoO3 membranes joins the emerging field of freestanding oxide layer technology, opening unexplored avenues for single crystal investigations, novel perovskite nanostructures, and wafer transfer of functional oxides, walking in the footsteps of recent developments in van der Waals epitaxial heterostructures.

Freestanding High-Power GaN Multi-Fin Camel Diode Varactors for Wideband Telecom Tunable Filters

The quality, resolution, and quantity of information transmitted in RF front end modules favor compact tunable filters with varactor elements that can handle high power and that are highly linear. Gallium nitride (GaN) varactors can theoretically reach the highest quality factor figure of merit (QFOM), owing to the GaN outstanding breakdown electric field and good electron mobility. Popularly pursued GaN Schottky diode varactors suffer from leakage currents at high reverse voltage biases at their junctions and because of GaN inherent threading dislocations in thin GaN layers on foreign substrates. We devised a novel device architecture to overcome these limitations. First, we employ an interdigitated anti-series multi-Fin camel diode structure composed of a thin <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{p}^{+}$ </tex-math></inline-formula> GaN top layer situated between the Schottky metal and an n-type GaN drift layer. The anti-series symmetric Fin structure leads to excellent linearity and minimizes device parasitics to achieve measured record high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factors. The p-GaN layer raises the barrier height to suppress reverse leakage current to maintain a high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factor at higher reverse biases than Schottky diodes. Second, we utilize GaN on Qromis Substrate Technology (QST) wafers that permit the growth of thick GaN layers with lower dislocation densities and lower leakage currents than GaN-on-Si and that are comparable to GaN-on-GaN. We report in this work the fabrication and dc, RF <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}$ </tex-math></inline-formula> -parameters of these diodes, and assess their potential with empirical models. Breakdown voltage of 146 V and extrapolated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factor of 84, 33, and 9 at 2, 5, and 18 GHz, respectively, are obtained in multi-Fin back-to-back varactor device configuration.

Characterization and Performance Analysis of BST-Based Ferroelectric Varactors in the Millimeter-Wave Domain

We present the realization and analysis of the microwave performances of interdigited varactors integrating thin ferroelectric layers of barium and strontium titanate (BaxSr1−xTiO3). Devices based on ferroelectric films of different compositions (x = 0.8 and x = 0.5) have been characterized in the millimeter-wave domain, from 200 MHz to 110 GHz. By applying different bias voltages, the tunability of the capacitance can reach up to 40% for the Ba0.8Sr0.2TiO3 composition, under relatively low applied electric fields of about 167 kV/cm. These promising characteristics allow the integration of the varactor devices in tunable antennas for a large frequency domain, from the microwaves to the millimeter waves range.

Tunable Microwave Device Fabrication on Low‐Temperature Crystallized Ba0.5Sr0.5TiO3 Thin Films by an Alternating Deposition and Laser Annealing Process

AbstractThis article demonstrates a method to fabricate crystalline Ba0.5Sr0.5TiO3 (BST) thin films at a lower temperature of ≈300 °C using an excimer laser by an alternating depositing and annealing process. Firstly, a BST thin film with a thickness of ≈120 nm is deposited at 300 °C (laser energy of ≈2 J cm−2) and subsequently laser annealed (66 mJ cm−2) at 300 °C. This process is repeated five times to obtain thicker device quality films. XRD patterns, TEM, Raman, and UV–Vis–NIR spectroscopy confirm that phase formation of BST thin films has taken place. The band edge value for BST thin films is observed to decrease systematically from 4.65 eV (amorphous, 300 °C) to 3.56 eV (5‐layers, crystalline, 300 °C). A varactor device with a Circular Patch Capacitor (CPC) structure is patterned on the five‐stage laser annealed BST thin films processed at 300 °C, which shows a microwave tunability of 34% at 1 GHz compared to conventionally deposited BST films (prepared at 700 °C). This study provides a way for fabricating ferroelectric thin film based tunable devices at low temperatures, making the process compatible with low melting substrates in flexible electronics and other situations where there is a temperature constraint.

Capacitive Sensing of Glucose in Electrolytes Using Graphene Quantum Capacitance Varactors.

A novel graphene-based variable capacitor (varactor) that senses glucose based on the quantum capacitance effect was successfully developed. The sensor utilizes a metal-oxide-graphene varactor device structure that is inherently compatible with passive wireless sensing, a key advantage for in vivo glucose sensing. The graphene varactors were functionalized with pyrene-1-boronic acid (PBA) by self-assembly driven by π-π interactions. Successful surface functionalization was confirmed by both Raman spectroscopy and capacitance-voltage characterization of the devices. Through glucose binding to the PBA, the glucose concentration in the buffer solutions modulates the level of electrostatic doping of the graphene surface to different degrees, which leads to capacitance changes and Dirac voltage shifts. These responses to the glucose concentration were shown to be reproducible and reversible over multiple measurement cycles, suggesting promise for eventual use in wireless glucose monitoring.

Geometry-Based Tunability Enhancement of Flexible Thin-Film Varactors

In this letter, flexible voltage-controlled capacitors (varactors) based on an amorphous Indium–Gallium–Zinc–Oxide (a-IGZO) semiconductor are presented. Two different varactor designs and their influence on the capacitance tuning characteristics are investigated. The first design consists of a top electrode finger structure which yields a maximum capacitance tunability of 6.9 at 10 kHz. Second, a novel interdigitated varactor structure results in a maximum tunability of 93.7 at 100 kHz. The design- and frequency-dependencies of the devices are evaluated through C–V measurements. Furthermore, we show bending stability of the devices down to a tensile radius of 6 mm without altering the performance. Finally, a varactor is combined with a thin-film resistor to demonstrate a tunable RC -circuit for impedance matching and low-pass filtering applications. The device fabrication flow and material stack are compatible with standard flexible thin-film transistor fabrication which enables parallel implementation of analog or logic circuitry and varactor devices.

Closed-form analysis of high-resistivity thin-film electrode in the hybrid varactor structure

Hybrid ferroelectric thin-film varactor device architecture is presented that allows for integrating the features of both structures of coplanar and parallel-plate varactors. In this structure, a high-resistivity oxide thin film is used as dc bias bottom electrode, while the existence of the high-resistivity oxide thin-film electrode is considered as disturbance to the original electric field in the coplanar varactor. Using analytic solution of electromagnetic field, the disturbance caused by high-resistivity thin film in microwave TEM wave is derived. The result shows that this disturbance could be modeled as serial or shunt resistance in the transmission line equivalent circuit and the magnitude of it is proportional to the conductivity and thickness of the conductor thin film. With conductivity and thickness low enough, the conductor film will be “transparent” to microwave field. Using high-resistance dc bias electrode “transparent” to microwave is expected to be able to bring much more flexibility in the developing of microwave ferroelectric tunable devices.

Design and performance of Yb/ZnS/C Schottky barriers

In this work, ZnS thin films are deposited onto glass and transparent ytterbium substrates under vacuum pressure of 10−5 mbar. The effects of the Yb substrate on the structural, mechanical, optical, dielectric and electrical performance of the ZnS are explored by means of the energy dispersion X-ray analyzer, X-ray diffraction, UV–VIS spectroscopy, current-voltage characteristics and impedance spectroscopy techniques. The techniques allowed determining the lattice parameters, the grain size, the degree of orientation, the microstrain, the dislocation density, the optical and the excitonic gaps, the energy band offsets and the dielectric resonance and dispersion. The (111) oriented planes of glass/ZnS and Yb/ZnS exhibited 2.06% lattice mismatch between Yb and ZnS and degree of orientation values of 63% and 51.6%, respectively. The interfacing of the ZnS with Yb shrunk the energy band gap of ZnS by 0.50 eV. On the other hand, the electrical analysis on the Yb/ZnS/C Schottky device has revealed a rectification ratio of 3.48 × 104 at a biasing voltage of 0.30 V. The barrier height and ideality factor was also determined. Moreover, the impedance spectroscopy analysis have shown that the Yb/ZnS/C device is very attractive for use as varactor devices of wide tunability. The device could also be employed as microwave resonator above 1337 MHz.

Effect of Gamma Ray Irradiation on Epitaxial Pb(Zr,Ti)O3 /SrRuO3 Tunable Varactor Devices

Epitaxial morphotropic PbZr0.52Ti0.48O3 (PZT) thin films were employed to enhance the dielectric tunability of microwave filter devices without compromising the device impedance matching and low bias voltage (<10 V) requirements. Epitaxial heterostructure of ferroelectric PZT(001)/SrRuO3 (SRO) were grown on single crystal SrTiO3 (001) substrates by pulsed laser deposition, and a platinum (Pt) electrode was deposited on top of the PZT film. The tunability of the Pt/PZT/SRO varactor devices are strongly dependent on bias voltage and exhibited good dielectric tunability of 55% at 100 kHz and 10 V. The capacitance (CP) of the heteroepitaxial varactor devices was 105 pF at 10 V applied bias with a corresponding small leakage current of 1 nA. The influence of gamma (γ-ray) irradiation on the intrinsic electrical properties of the epitaxial PZT varactor devices was investigated as a function of the irradiation dose from 0 kGy to 400 kGy, in terms of the capacitance–voltage (C–V) characteristics and loss tangent response. With enhancing γ-ray irradiation doses the ferroelectric capacitance was found to decrease accompanying degradation in the loss tangent values. The results indicate that tunability of the epitaxial PZT ferroelectric thin-film capacitors decreased with increasing gamma irradiation dose and degraded ∼25% at 400 kGy dose than unexposed devices. Possible reasons for the degradation behavior of dielectric properties and tunability due to radiation-induced defects has been discussed.

An 800-MHz LC-VCO with a Linear Control Characteristics in a 130-nm CMOS Technology

A semilinear LC-voltage-controlled oscillator (VCO) topology is proposed. To exploit a linear frequency tuning, the LC-VCO adopts the variable capacitance block of the varactor devices and the PMOS transistors. This block has the capacitance characteristics which are approximately inversely proportional to the square of the VCO control voltage. The proposed LC-VCO is implemented in a 130-nm CMOS technology. The measurement results show a semilinear frequency range of 503–873 MHz and a phase noise of −128 dBc/Hz at 1 MHz offset frequency. Power consumption is about 2 mA from a 1.2 V supply. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1972–1975, 2013

A 3.8-GHz highly linear &lt;i&gt;LC&lt;/i&gt;-VCO without a varactor device

A LC-VCO topology without a varactor device has been proposed. The oscillator includes a variable capacitance block for a frequency tuning. The block consists of two transistors and two capacitors. The LC-VCO has been fabricated in a 130-nm CMOS technology. The measured results show operation frequencies of 3.73GHz to 3.94GHz and a phase noise of −119.1dBc/Hz at 1-MHz offset frequency. Power consumption is 5.9mA from a 1.2V supply.

Enhanced capacitance ratio and low minimum capacitance of varactor devices based on depletion-mode Ga-doped ZnO TFTs with a drain-offset structure

Depletion-mode Ga-doped ZnO thin-film transistors (TFTs) with a drain-offset configuration were fabricated employing a plasma-enhanced chemical vapour deposited Si3N4 gate insulator and a metal organic chemical vapour deposition grown Ga-doped ZnO channel layer. For comparison, conventional TFTs with drain-to-gate overlap were also fabricated. Capacitance–voltage characteristics of gate-to-drain capacitors and current–voltage characteristics of TFTs were measured. Although drain-offset TFTs exhibit poor device characteristics compared with conventional TFTs, these TFTs show better Cmax/Cmin ratio and Cmin values. The Cmax/Cmin ratio is as large as 71.83 and Cmin is as small as 0.3 fF µm−1 normalized for channel width, demonstrating the potential of these devices as varactors for circuit applications. Their better varactor performance is ascribed to the presence of a very small capacitance in the drain-offset region. The effect of drain-offset length variation on TFT and gate-to-drain capacitor performance is also reported.

Passive and Active Reconfigurable Scan-Beam Hollow Patch Reflectarray Antennas

The design concept of passive and active reconfigurable reflectarray antennas has been proposed and tested. The antenna elements in the array are identical hollowed patches. In the first phase of study the slots are loaded with a SMD capacitor to set the required phase shift needed for array implementation. Simulations show promising results. Mounting a SMD capacitor in such a configuration can be considered as the first step in using capacitive loading on a slotted patch for active microstrip reflectarrays. It is shown that by adjusting the capacitance values it is possible to scan the beam. In the second phase, the patch elements are loaded with active varactor-diode device which its reflected phase can be varied. This phase alteration is based on the variation of the diode capacitance which can be achieved by varying the biasing voltage of the active varactor device. In latter approach by activating these varactor devices, the phase of each antenna element in the array configuration can be adopted dynamically and consequently, its beam direction can be reconfigured. The reflectarrays incorporating passive and active elements have been built and tested at 7.0 GHz and 6.0 GHz, respectively. The performance of the proposed reconfigurable antennas is excellent, and there is good agreement between the theoretical and measurement results which pioneers design of arbitrarily reconfigurable antennas.

Design and characterization of TlInSe 2 varactor devices

TlInSe 2 single crystal has been successfully prepared by the Bridgman crystal growth technique. The crystal, which exhibits compositional atomic percentages of 25.4%, 25.2% and 49.4% for Tl, In and Se, respectively, is found to be of tetragonal structure with lattice parameters of a=0.8035 and c=0.6883 nm. The crystals were used to design radio frequency sensitive varactor device. The temperature dependence of the current–voltage characteristics of the device allowed the calculation of the room temperature barrier height and ideality factor as 0.87 eV and as 3.2, respectively. Rising the device temperature increased the barrier height and decreased the ideality factor. This behavior was attributed to the current transport across the metal–semiconductor interface. The capacitance of the device is observed to increase with increasing voltage and increasing temperature as well. The temperature activation of the capacitance starts above 82 °C with a temperature coefficient of capacitance being 1.08×10 −3 K −1. Furthermore, the capacitance of the device was observed to increase with increasing frequency up to a maximum critical frequency of 4.0 kHz, after which the capacitance decreased with increasing frequency. The behavior reflected the ability of maximum amount of charge holding being at a 4.0 kHz. The analysis of the capacitance–voltage characteristics at fixed frequencies reflected a frequency dependent barrier height and acceptors density. The decrease in the barrier height and acceptors density with increasing frequency is mainly due to the inability of the free charge to follow the ac signal.

Analysis and design of a new reflection‐type 360° phase shifter with combined switch and varactor

AbstractAnalysis and design of a new 360° reflection type phase shifter is proposed.The new phase shifter uses a switch and one varactor device per reflective load. Equations for the analysis and design of the new phase shifter are presented. A 2.5 GHz phase shifter is fabricated using a varactor diode and a PIN diode per load. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 530–535, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24975

Reconfigurable scan‐beam hollow patch reflectarray antenna loaded with tunable capacitor

AbstractIn this article, the design concept of an active reconfigurable reflectarray antenna has been proposed and tested. The elementary radiators are hollow patch element loaded with varactor‐diode device in which the reflected phase can be varied. This phase alteration is based on the variation of the diode capacitance which can be achieved by varying the biasing voltage of the active varactor device. By activating these varactor devices, the phase of each antenna element in the array configuration can be adopted dynamically, and consequently, its radiation beam direction can be reconfigured. The advantage of the MEMS switches is the complexity of the integration and continuing beam scanning capability. The reflectarray incorporating active elements has been built and tested at 6.0 GHz. Narrow band multifrequency shared aperture is demonstrated theoretically and experimentally. The performance of the proposed active antenna is excellent, which pioneers the design of arbitrarily reconfigurable antennas. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 367–374, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24050

Performance Consideration of MOS and Junction Diodes for Varactor Application

The engineering and tradeoffs for quality factor, capacitance tuning ratio, capacitance mismatch, and low- frequency noise for different varactor device structures, geometry sizes, and operations have been compared to provide a comprehensive guideline of an optimized structure for the advanced radio-frequency voltage-controlled oscillator (VCO). Compared with junction varactors, n+/n-well (NW) MOS varactors exhibit a higher capacitance tuning ratio (~3) and a superior capacitance mismatch (< 0.1% for a drawn size of 25 mum2). However, a noticeable low-frequency noise (~ 10-18 A2/Hz) was observed in the accumulation-mode n+/NW MOS varactors. A derived figure of merit in selecting a suitable varactor in terms of VCO's phase noise has been proposed for the first time. The measurement was performed by using the 0.18- mum 1P6M with AlCu backend wafer.

Metal–oxide–semiconductor devices using Ga2O3 dielectrics on n-type GaN

Using a photoelectrochemical method involving a He–Cd laser, Ga2O3 oxide layers were directly grown on n-type GaN. We demonstrated the performance of the resultant metal–oxide–semiconductor devices based on the grown Ga2O3 layer. An extremely low reverse leakage current of 200 pA was achieved when devices operated at −20 V. Furthermore, high forward and reverse breakdown electric fields of 2.80 MV/cm and 5.70 MV/cm, respectively, were obtained. Using a photoassisted current–voltage method, a low interface state density of 2.53×1011 cm−2 eV−1 was estimated. The varactor devices permit formation of inversion layers, so that they may be applied for the fabrication of metal–oxide–semiconductor field-effect transistors.

Substrate transfer process for InP-based heterostructure barrier varactor devices

We report on transferred-substrate InP-based heterostructure barrier varactor (HBV) devices. Individual HBV devices having active device diameters ranging from 10 to 40 μm have been fabricated on a SiO2 glass host substrate following a novel epitaxial layer transfer technique. This process utilizes the negative photoresist SU-8 as a photopatternable intermediate bonding layer and could be extended to other InP-based devices. The transferred HBV device characteristics have been measured and compared to devices which have been fabricated on InP. The transferred devices exhibited highly symmetrical electrical characteristics due to the preservation of the high quality molecular beam epitaxy layers during the transfer process. A leakage current of 4.5 A/cm2 at 10 V together with an associated conductance value of 40 nS/μm2 were observed. The rf measurements revealed a zero bias capacitance of 1 fF/μm2 and a capacitance ratio of 5:1.

Transferred-substrate InP-based heterostructure barrier varactor diodes on quartz

InP-based heterostructure barrier varactor (HBV) devices employing air-bridge technology have been fabricated on a quartz host substrate following a transfer-substrate technique. Electrical characterization demonstrates highly symmetrical I(V) and C(V) characteristics due to the preservation of the high quality MBE epitaxial layers during the transfer process. Small signal RF measurements have been performed up to 110 GHz and display a marked reduction in the values of parasitic resistance and capacitance, thus confirming the ability of the devices to operate in the upper-part of the MM-wave spectrum.