In this paper, we present complete operating bench experiments used to characterize a type of tunable devices in the microwave domain. This type of tunable devices requires unusual high AC/DC bias voltage (up to 400V). The experimental characterization system is based on the reflection and transmission parameters of a quadrupole placed between the two ports of a network analyzer, in the 8-12 GHz range. For the dielectric characterizations of materials, it is possible to determine the real part (’) and the imaginary part (’’) of the dielectric constant ( of the material. For tunable devices using polymer dispersed liquid crystals, it is necessary to apply a high electric driving field (up to 4V/µm @1 kHz). In the test bench, we used a microstrip phase shifter which presents a 100µm-thick active part. Therefore a 400V external voltage bias device was developed. We first present an applicative circuit, that is, a phase shifter, which is machined to study the electronic behaviors of anisotropic materials. We then present the design and characteristics of the bias circuit. Secondly, we present experimental results for two Polymer Dispersed Liquid Crystals made with two different monomers (NOA65-Norland, and PolyPropylene Glycol Di Acrylate540), with a commercial liquid crystal (5CB-Merck).

A tunable, liquid-crystal (LC)-based, subwavelength Fabry-Perot (FP) resonator has been designed and fabricated in the 27-GHz band. The grid mirrors are identical and consist of metallic capacitive meshes characterized by a high filling ratio. This ensures simultaneously a high reflectivity and a strong negative phase of the reflection coefficient on the mirror so as to reduce the resonator thickness as much as possible. The resonator has been filled with a 110-mum thick E3 nematic LC. A frequency shift of 6 % has been obtained numerically (finite-difference time- domain (FDTD) simulations) using permittivity values measured under 1-T driving magnetic field (isin/ / = 2.91, isinperp = 2.52). An experimental tunability of 2% has been obtained by applying a 10-kHz ac bias voltage of 10 V. This lower value is due to the low driving field (~ 0.1 V/mum), the nonperfect polyvinylalcohol (PVA) layer, and the relatively high thickness of the E3 mixture.

We present an experimental procedure for dielectric characterization of liquid crystals in the frequency range 10(3)-10(10) Hz. This procedure is based an open-end strip line measuring cell and a 2D S.D.A. electromagnetic simulation software. Effective permittivities and tensor permittivity elements are determined for a commercial nematic liquid crystal. An application to an electrically tunable microwave phase-shifter is also presented. Transmission coefficient phase variations of about 0.5 degrees /cm length are obtained per GHz showing the liquid crystals potentialities for such applications.

A novel FET concept, using low temperature grown GaAs as surface passivation and buffer layer material, has been developed to tolerate high levels of input overdrive and to improve electrostatic discharge (ESD) resistance. It is shown that high input levels 17 dB beyond the 1 dB compression point, do not lead to the burnout of the device and that, at the same time, the ESD failure voltage can be increased compared to conventional GaAs based MESFETs.

We describe a novel type of GaAs FET using LT GaAs as a lossy dielectric. In this device, between gate and drain, an LT GaAs surface layer is used as a low conductive bypass with linear IV characteristics providing a controlled leakage path. This leads to a field redistribution in the high field space charge and drift region which lowers the maximum field and improves the breakdown characteristics. Experimentally a high power density is observed at 1 MHz of up to 2.7 W/mm. At higher frequencies, the output power decreases down to 0.6 W/mm. We propose to explain this reduction by a new mechanism based on the charge density distribution in the LT GaAs which leads to the formation of a parasitic second gate. This parasitic gate compresses the maximum available open channel current and RF power density. We propose an experimental optimisation of the structure, where the current compression has not been observed while high current levels and breakdown voltages are still maintained. From large signal measurements at 0.85 GHz a maximum output power density of 1.75 W/mm is extracted. We discuss this experimental window and compare to theoretical expectations.

We study the behavior of low temperature (LT) GaAs FET's at 300 and 77 K. The dependence with frequency of their transconductance is measured and a decrease is observed in the MHz range at room temperature and in the kHz range at liquid nitrogen temperature. This phenomenon is related to the dielectric relaxation of the LT GaAs which is due to the residual conductivity of this material. The behavior of the transconductance is also correlated to the large signal characterization. From the results, we propose improvements of the device structure by recessing the gate into the LT GaAs. The transconductance measurements show a strong improvement in the device characteristics.

Low-temperature (LT) GaAs FETs have been realized for high I × V products. They have been extensively characterized under dc, rf, pulsed, and large-signal conditions. The results are analyzed and related to the device structure. The electrical passivant role of the LT GaAs has been demonstrated. Under dc conditions I × V products of 2.5 W/mm are obtained. Under microwave frequencies, only 0.4 W/mm were measured on this device. We discuss the relationship between the dielectric relaxation of LT GaAs and the low-power performance of the device. New device structures are proposed. © 1996 John Wiley & Sons, Inc.

Low temperature grown GaAs has been fabricated containing a limited amount of excess arsenic. The material has a low conductivity in the order of 100KΩ cm, due to hopping in a deep donor band. This σ-LT-GaAs was grown reproducibly by using the lattice mismatch as the primary parameter for substrate temperature calibration. Breakdown fields, in the order of 100kV/cm, are observed for planar structures and increased at low measurement emperatures. Low hopping conductivity and high breakdown field are also observed in the lossy dielectric metal-insulator-semiconductor field-effect transistor device using σ-LT-GaAs as a surface layer. The record radio frequency power density of 4.0W/mm at 77K is extracted from the dc output characteristics.

A lossy dielectric FET has been realized and evaluated for the first time by employing /spl sigma/-LT-GaAs. 3.5 W/mm RF power capability extracted from the DC output characteristics give evidence for a field redistribution which overcomes the power limitation of Schottky gate FET devices. 60 GHz f/sub max/ values have been obtained for 1 /spl mu/m devices. Gate-drain breakdown voltages above 30 V have been identified at 2 GHz in conjunction with a channel sheet charge of 5/spl middot/10/sup 12/ cm/sup -2/. Parasitics, specific to the lossy dielectric have been widely eliminated. However limitations specific to the /spl sigma/-LT-GaAs material need still to be overcome, which is discussed.

LT-GaAs MISFETs have been realized indicating a record 2.1W/mm RF power handling capability. To understand the properties of such LT-GaAs power MISFET structures, the MIS-system containing a LT-GaAs insulator and an AlAs interfacial diffusion barrier to the channel has been analysed. A noticeable parallel conductance was found in the insulator leading to a high gate to drain breakdown voltage, however also to a gm-dispersion in the MHz regime. An electronic equivalent circuit for use in the FET model has been established. At the insulator-semiconductor interface, a low interface potential in the range of 0.3eV below the conduction band is seen, indicating uncommon interface and passivation properties.

As-grown (σ-conductive) and annealed (semi-insulating (SI)) low temperature (LT) double layers have been analysed by conductivity profiling. The σ-LTGaAs/GaAs junction represents an NT+/GaAs junction with no noticeable depletion effects in the σ-LTGaAs. The SILTGaAs/GaAs junction is an SI(n−)GaAs/ GaAs junction with depletion effects in LTGaAs. The Fermi-level of the LTGaAs lies in all cases above the nGaAs free surface potential and is estimated to satisfy 0.18 eV < | Ec − EF | < Φs.

LT-GaAs MISFETs had been realized indicating a record 2.7 W/mm RF power handling capability. To optimize such LT-GaAs power MISFET structures, the MIS system containing a LT-GaAs insulator and an AlAs interfacial diffusion barrier to the channel has been analysed. A noticeable parallel conductance was found in the insulator which is thought to be one of the key parameters to realize high gate to drain breakdown voltages. This conductivity however leads also to a gm -dispersion in the MHz range. Locus-curves of this system demonstrate a higher resistivity in the AlAs layer than in the LT-GaAs layer, indicating that the simple model of a single lossy capacitance does not describe the MIS diode completely. An extended electronic equivalent circuit for use in the FET model has been established

Low temperature (LT) GaAs MISFETs with 680 mA/mm drain current and 28 V drain voltage have been fabricated. This represents the highest I-V product of a GaAs FET to date, indicating an RF-power handling capability of 2.1 W/mm. The weak dependence of the breakdown voltage on the doping-thickness product indicates that further improvements are possible.<>

In this communication, a microstrip line cell method for dielectric spectroscopy is described (Fig. 2).The cell and an electromagnetic simulation software allow us to study the dielectric dispersion and absorption spectra for both components of the dielectric permittivity tensor of uniaxial systems like nematic liquid crystals.The sample is introduced by capillarity and orientated with a surface treatment (polymer coating and rubbing) or an external magnetic field.Results are compared for different liquid crystals presenting a nematic phase at ambient temperature. The developed simulation software, based on electrical integral equation solving with moments method in Fourier space (Spectral Domain Approach), is also presented.Applied to microstrip- lines, this method considers surface waves, radiation, substrate anisotropy and dielectric losses.