AMS Acta
(4.910 recursos)
Repository of the University of Bologna.
Mostrando recursos 1 - 20 de 1.392
1.
La soluzione dell'enigma di Piero della Francesca - Molari, Pier Gabriele
L’interpretazione dell’iconografia della Flagellazione di Piero della Francesca è dibattuta da lungo tempo e non ha trovato fino ad ora una risposta definitiva malgrado vari tentativi.
In questo saggio, il dipinto viene associato alla tavola di Brera detta di San Bernardino ed alle tavolette della Galleria degli Uffizi di Firenze con i ritratti dei signori di Montefeltro pensando ad un'unica opera realizzata per la meditazione di Federico da Montefeltro.
Questo inedito e forse ardito accostamento pone l'insieme dei quattro dipinti sotto la stessa ottica e ne permette una definitiva interpretazione iconografica come pure una datazione certa.
Collocando poi l’insieme delle tavole nel...
2.
A Simple Parallel-Plate Resonator Technique for Microwave. Characterization of Thin Resistive Films - Vorobiev, A.; Deleniv, A.; Talanov, V.; Gevorgian, S.
A parallel-plate resonator method is proposed for non-destructive characterisation of resistive films used in microwave integrated circuits. A slot made in one of the plates is used to measure surface impedance of a reference film and film under test. The surface impedance of the film under test is extract from these two measurements using a simple procedure. X-band experimental verification is given for a number of resistive films.
3.
Evidence of surface trap effects on pseudomorphic HEMT submitted to impact ionisation stresses - Malbert, N.; Labat, N.; Lambert, B.; Touboul, A.; Pataut, G.
In this paper, AlGaAs/InGaAs pseudomorphic HEMTs have been submitted to DC life-tests in impact ionisation regime with and without thermal stress. Control devices present an identical and steep shape of the on-state breakdown locus while aged devices present a large dispersion of this characteristic. It seems that some slight modification of the surface properties and/or micro-defects located in the gate-drain region affect the on-state breakdown voltage loci measured with a gate current of 0.13mA/mm. After the two life-tests, the temperature evolution of the surface leakage contribution to the reverse gate current has increased while impact ionisation current remains unchanged. This...
4.
Etching behaviour of GaAs with chlorine chemically assisted ion beam etching depending on the surface temperature - Dienelt, J.; Zimmer, K.; Rauschenbach, B.
The chemically assisted ion beam etching (CAIBE) of GaAs as well as the sputtering and the pure chemical etching (CE) at different surface temperatures and chlorine fluxes have been studied. At elevated surface temperature a noticeable increase of the etch rate compared to room temperature etching for CE and CAIBE was achieved whereas sputtering is not depending on the temperature. An etch rate of higher than 1 µm/min at an ion beam energy of 400 eV, a chlorine flux of 10 sccm Cl2, and a temperature of 420 K could be obtained. An electronic beam chopper has been utilised in...
5.
Electrical and photoluminescence properties of bulk GaAs after surface gettering - Gorelenok, A.T.; Andrievskii, V.F.; Kamanin, A.V.; Kohanovskii, S.I.; Shmidt, N.M
The successful results on surface gettering of background impurities and defects in 1.6 mm thick (111) GaAs wafers have been obtained. For the gettering, the wafers were coated by a 1000 Å thick yttrium film either on one side or on both sides followed by a heat treatment. It has allowed the electron concentration to decrease from (1–2)´10 15 cm -3 down to 10 8 –10 10 cm -3 and the mobility to increase from 1500–2000 cm 2 V -1 s -1 up to 7000 cm 2 V -1 s -1 at 300 K. The distribution profiles of the electron...
6.
Enhanced Performance GaAs-Based HBTs using a GaInNAs Base Layer - Stevens, K.S.; Welser, R. E.; Deluca, P.M.; Landini, B. E.; Lutz, C. R.; Wolfsdorf-Brenner, T. L.
GaInNAs base layers are enabling performance enhancements over standard GaAs-based HBTs, while preserving the cost effective GaAs platform. We have demonstrated InGaP/GaInNAs HBTs which offer 115 mV lower operating voltage, improved diode symmetry for reduced VCE,offset and Vknee, and superior gain temperature stability as compared with standard InGaP/GaAs HBTs. These enhancements have been realized while simultaneously preserving DC current gain and base doping ( ββ β80, Rsb 530 Ω/†, base thickness -500 Å, base doping ~ 4×10 19 cm 2 ) at the high levels necessary for commercial applications. GaInNAs enables these performance enhancements due to a lower energy gap...
7.
Correlation between Chemical and Electrical Properties of n-InGaP Surfaces Grown by MOVPE - Hashizume, Tamotsu; Saitoh, Toshiya
Correlation between chemical and electrical properties of n-InGaP surfaces grown by metal-organic chemical vapor epitaxy (MOVPE) was investigated by x-ray photoelectron spectroscopy (XPS), current-voltage (I-V) and UHV contactless capacitance-voltage (C-V) methods. An air-exposed surface exhibited highly In-rich phase where the InPO 4 -like natural oxide was dominant. Poor I-V characteristics appeared in Schottky contacts fabricated on the air-exposed surfaces. Chemical treatments in HCl and HF solutions were found to be effective in reducing natural oxide and in recovering the surface stoichiometry. The UHV contactless C-V results showed no pronounced Fermi level pinning at the chemically treated InGaP surfaces. Furthermore, such...
8.
AlGaN/GaN HFET’S - Eastman, Lester F.
The strong spontaneous and piezoelectric polarizations in pseudomorphic AlGaN/GaN on SiC substrates are used to induce 2DEG sheet density of > 1 x 10 13 /cm 2 with mobility up to 1,700 cm 2 /vs. The processing sequence and HEMT electrode layout are presented. The output power and power-added efficiency in the 8-10 GHz frequency range are shown for CW bias conditions and optimum tuning in class B operation. Single-gate, center-fed 100 µm periphery devices gave 11.2 W/mm at 32% power-added efficiency; two-gate, 200 µm periphery devices with 50 µm pitch gave 8.7 W/mm at 48% power-added efficiency, and 12-gate,...
9.
Low frequency noise conversion in fets under nonlinear operation - Danneville, F.; Tamen, B.; Cappy, A.; Juraver, J-B; Llopis, O.; Graffeuil, J.
Based upon the active line concept, the conversion mechanisms of microscopic low frequency noise (e.g. generation-recombination noise) located in the channel of a Field Effect Transistor (FET) which is driven by a large RF signal is demonstrated. The first consequence is that the based band (low frequency) input gate noise voltage spectral density is dependent on the magnitude of the input RF power applied to the FET. Moreover, the microscopic generation-recombination noise sources located in the channel are responsible of up-converted input gate noise voltage spectral density around the RF frequency.
10.
High performance test fixture for 10-Port MMIC's characterisation - Kamenopolsky, Stanimir; Dankov, Plamen
High-performance measuring multi-port 3-level fixture for active device characterization in the Ku-band is described in this paper. It is a low-cost alternative of the expensive microwave "on-wafer" measurements with CPW probes. The method of modeling of the separate components of the each embedding RF-path of the fixture is used for de-embedding purposes. Testing of several phantom-type structures on the chip-carrier level confirms the ability of the considered fixture to give repeatable results for the extracted own MMIC's parameters: mainly for the insertion phase and the gain.
11.
A wideband automated measurement system for on-wafer noise parameter measurements at 50-75 GHZ - Kantanen, M.; Lahdes, M.; Tuovinen, J.; Vähä-Heikkilä, T.; Kangaslahti, P.; Jukkala, P.; Hughes, N.
A wideband automated on-wafer noise parameter measurement system has been built. Using measurement system developed here, noise parameters of a chip device can be determined over entire 50-75 GHz range in an automated manner. As an example, measured noise parameters of an InP HEMT are shown over 50-75 GHz.
12.
Theoretical and experimental assessment of the non-linear scattering functions for the cad of non-linear microwave circuits - Schreurs, D.; Verspecht, J.; Acciari, G.; Colantonio, P.; Giannini, F.; Limiti, E.; Leuzzi, G.
The Non-Linear Scattering Functions have been theoretically defined and experimentally measured for the linear-equivalent design of non-linear circuits in arbitrary large signal conditions. Non-linear measures and simulations have been compared, with good agreement. Linear CAD concepts can therefore be extended to non-linear circuits in a rigorous way.
13.
New trends in characterization and modeling of High Power devices - Bouysse, Ph.; Barataud, D.; Sommet, R.; Teyssier, J.P.; Nébus, J.M.; Quéré, R.
In this paper, we present some non exhaustive measurement techniques used for characterization and modeling of high power devices. Capabilities offered by pulsed measurements can be an efficient approach to deal with self-heating effects and sometimes trapping effects encountered in microwave high power devices. Pulsed techniques allow to separate self-heating and trapping effects, making like this the modeling process easier. A pulsed RF and pulsed bias load-pull system is then described as well as a time waveform measurement system. Capabilities of these two characterization tools are illustrated with results obtained on a high power silicon bipolar transistor and on a...
14.
A Novel method for the Higher Order components extraction of the Channel current in GaAs MESFET - Kim, Youngsik; Kim, JiYoun; Kim, Sungwoo; Kim, Bumman
We propose a new simple and accurate method for the higher order Taylor coefficient extraction of the channel current in GaAs MESFET. In this method, the nonlinear channel currents are directly measured through a hole current sensor and spectrum analyzer. Taylor coefficients up to 3rd order have been successfully extracted from the measured currents of the low frequency(4MHz, 25MHz) two-tone test, and a resonator is added to the load to remove the gate frequency component from the drain voltage, which makes the problem simple. The proposed extraction procedure is simple and straightforward.
15.
A novel approach for highly linear automatic gain control of a hemt small-signal amplifier - Malaver, Emigdio; Garcia, José Angel; Tazon, Antonio; Mediavilla, Angel
A novel approach for highly linear automatic gain control (AGC) in small-signal ampli¯ers is presented in this paper. A HEMT based topology was implemented, biasing the transistor in the transition between the saturated and linear operation regions. Gain control with low distortion is achieved by simultaneous adjustments of the gate to source (Vgs) and drain to source (Vds) voltages, along the line where the second derivative of the transconductance (Gm3) has a null. Comparatively to the traditional approach, with the transistor biased in the saturated region, this ampli¯er has better intermodulation behavior and e±ciency, without important reduction in the gain...
16.
Parameter extraction and evaluation of the Bias dependence of Tf, for the VBIC Model used on a GaAs HBT - Olavsbråten, Morten
This paper presents a practical method of extracting the bias dependent parameters of the forward transit time Tf, implemented in the VBIC model used on a GaAs HBT. The work should be of great value for the circuit designers, who want to have an easy practical way of including good model parameters from a few simple measurements. This paper includes an evaluation of the implemented bias dependence of Tf in the VBIC model. The VBIC model is extracted from measurements on a GaAs HBT made by Caswell Technology. The model shows good agreement with these measurements.
17.
Pulsed power operation of commercially available silicon carbide mesfets - Walden, Mark G.
Sample devices of commercially available SiC MESFETs were measured under pulsed conditions. The results show significant improvements over traditional III-V devices measured under the same conditions. The measurements indicate that SiC devices may have several advantages for use in pulsed applications such as Phased Array Radar.
18.
Efficient CM-FEM modeling of coplanar waveguides for high-speed e/o modulators - Carbonera, F.; Bertazzi, F.; Goano, M.; Ghione, G.
A general approach is proposed for the accurate and efficient computation of the characteristic parameters of coplanar waveguides having electrodes of complex cross-section on multi-layered, planar or non-planar substrates, as presently considered for high-speed (semiconductor or LiNbO3) electrooptic modulators. The approach is based on the coupling of quasi-TEM finite element method with numerical conformal mapping, and provides the frequency-dependent parameters of an equivalent transmission line model. The present technique, implemented in MATLAB as a practical design tool, is compared with semi-analytical methods (when applicable) and full-wave finite element method: excellent agreement is demonstrated with the full-wave solution, at a fraction...
19.
High tuning speed optical receiver front-end for packet-switched WDM Networks - Manfrin, S.K.; Orengo, G.; Giannini, F.; Romero, M. A.
In this work two distinct optical receiver conjurations are investigated for use in the downstream direction of ATM PON networks based on wavelength division multiplexing (WDM). The goal is to overcome the slow tuning speed of optical filters, achieving channel selection in the nanosecond time frame and allowing the implementation of an efficient ATM/WDM protocol.
20.
40 Gbit/s GaAs MMIC Signal Processor for Optical Communication Systems - Monteiro, Paulo P.; Violas, Manuel; Sousa Ribeiro, Rui; Ferreira da Rocha, José
We will present the design and characterisation of a transversal microwave filter as a signal processor for 40 Gbit/s optical receivers. The work reports, for the first time, a versatile transversal filter, implemented as a monolithic microwave integrated circuit (MMIC) with capacity to adjust the frequency response for signal processing at these very high bit rates. The main features are the electrical response tuning facility and signal gain.
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