Compact modeling technology for the simulation of integrated circuits based on graphene field-effect transistors

transformatiu CRUE-CSICUTP en procés de revisióAltres ajuts: GraphCAT project reference 001-P-001702The progress made toward the definition of a modular compact modeling technology for graphene field-effect transistors (GFETs) that enables the electrical analysis of arbitrary GFET-based integrated circuits is reported. A set of primary models embracing the main physical principles defines the ideal GFET response under DC, transient (time domain), AC (frequency domain), and noise (frequency domain) analysis. Another set of secondary models accounts for the GFET non-idealities, such as extrinsic-, short-channel-, trapping/detrapping-, self-heating-, and non-quasi static-effects, which can have a significant impact under static and/or dynamic operation. At both device and circuit levels, significant consistency is demonstrated between the simulation output and experimental data for relevant operating conditions. Additionally, a perspective of the challenges during the scale up of the GFET modeling technology toward higher technology readiness levels while drawing a collaborative scenario among fabrication technology groups, modeling groups, and circuit designers, is provided

Compact Modeling Technology for the Simulation of Integrated Circuits Based on Graphene Field-Effect Transistors

The progress made toward the definition of a modular compact modeling technology for graphene field-effect transistors (GFETs) that enables the electrical analysis of arbitrary GFET-based integrated circuits is reported. A set of primary models embracing the main physical principles defines the ideal GFET response under DC, transient (time domain), AC (frequency domain), and noise (frequency domain) analysis. Another set of secondary models accounts for the GFET non-idealities, such as extrinsic-, short-channel-, trapping/detrapping-, self-heating-, and non-quasi static-effects, which can have a significant impact under static and/or dynamic operation. At both device and circuit levels, significant consistency is demonstrated between the simulation output and experimental data for relevant operating conditions. Additionally, a perspective of the challenges during the scale up of the GFET modeling technology toward higher technology readiness levels while drawing a collaborative scenario among fabrication technology groups, modeling groups, and circuit designers, is provided.European Commission 881603Spanish Government European Commission RTI2018-097876-B-C21 European CommissionDepartament de Recerca i Universitat 001-P-00170

Characterization and simulation of a CdTe detector for use in PET

The Voxel Imaging PET (VIP) Path nder project got the 4 year European Research Council FP7 grant in 2010 to prove the feasibility of using CdTe detectors in a novel conceptual design of PET scanner. The work presented in this thesis is a part of the VIP project and consists of, on the one hand, the characterization of a CdTe detector in terms of energy resolution and coincidence time resolution and, on the other hand, the simulation of the setup with the single detector in order to extend the results to the full PET scanner. An energy resolution of 0.98% at 511 keV with a bias voltage of 1000 V/mm has been measured at low temperature T=-8 ºC. The coincidence time distribution of two twin detectors has been found to be as low as 6 ns FWHM for events with energies above 500 keV under the same temperature and bias conditions. The measured energy and time resolution values are compatible with similar ndings available in the literature and prove the excellent potential of CdTe for PET applications. This results have been presented in form of a poster contribution at the IEEE NSS/MIC & RTSD 2011 conference in October 2011 in Valencia and at the iWoRID 2012 conference in July 2012 in Coimbra, Portugal. They have been also submitted for publication to "Journal of Instrumentation (JINST)" in September 2012

A physics-based model of SiC-based MESFETs

Silicon Carbide (SiC) has been investigated as an alternative material to Silicon (Si) for enhancing the power-handling capability of semiconductor devices for simultaneous high-temperature and high frequency applications. Its high thermal conductivity, high bandgap, low permittivity, high saturation velocity, moderate mobility, material hardness and chemical inertness make it a prime candidate for power electronics, heat and light sensors, and MEMS applications. The MESFET is the most viable power transistor based on SiC. The performance of SiC MESFETs is limited by trapping and thermal effects. A physics-based analytical model of the SiC MESFET incorporating trapping and thermal effects is reported. The model takes into account the field and temperature dependencies of carrier transport parameters and carrier trapping effects. Both surface and substrate traps have been incorporated in the model to calculate the observed current slump in the I-V characteristics. The trapping and detrapping from surface traps control the channel opening at the drain end of the channel that requires the drain resistance to be gate and drain voltage dependent. The substrate traps capture channel electrons at high drain bias when the buffer layer is fully depleted resulting in current collapse at low drain bias in the following I-V trace. The detrapping of the captured electrons is initiated with the increasing drain bias and the channel electron concentration increases which is accelerated by increased thermal effects. As a result, restoration of collapsed drain current is obtained before the trapping effect is reinitiated at high drain bias. The calculated results using the current model are in good agreement with experimental data. A small-signal model for the MESFET has also been proposed. Calculations for the output conductance, the transconductance, the gate-source and gate-drain capacitance has also been presented

Caracterização, modelação e compensação de efeitos de memória lenta em amplificadores de potência baseados em GAN HEMTS

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have emerged as the most compelling technology for the transmission of highpower radio-frequency (RF) signals for cellular mobile communications and radar applications. However, despite their remarkable power capabilities, the deployment of GaN HEMT-based RF power amplifiers (PAs) in the mobile communications infrastructure is often ruled out in favor of alternative siliconbased technologies. One of the main reasons for this is the pervasiveness of nonlinear long-term memory effects in GaN HEMT technology caused by thermal and charge-trapping phenomena. While these effects can be compensated for using sophisticated digital predistortion algorithms, their implementation and model-extraction complexity—as well as the power necessary for their real-time execution—make them unsuitable for modern small cells and large-scale multiple-input multiple-output transceivers, where the power necessary for the linearization of each amplification element is of great concern. In order to address these issues and further the deployment of high-powerdensity high-efficiency GaN HEMT-based RF PAs in next-generation communications and radar applications, in this thesis we propose novel methods for the characterization, modeling, and compensation of long-term memory effects in GaN HEMT-based RF PAs. More specifically, we propose a method for the characterization of the dynamic self-biasing behavior of GaN HEMTbased RF PAs; multiple behavioral models of charge trapping and their implementation as analog electronic circuits for the accurate real-time prediction of the dynamic variation of the threshold voltage of GaN HEMTs; a method for the compensation of the pulse-to-pulse instability of GaN HEMT-based RF PAs for radar applications; and a hybrid analog/digital scheme for the linearization of GaN HEMT-based RF PAs for next-generation communications applications.Os transístores de alta mobilidade eletrónica de nitreto de gálio (GaN HEMTs) são considerados a tecnologia mais atrativa para a transmissão de sinais de radiofrequência de alta potência para comunicações móveis celulares e aplicações de radar. No entanto, apesar das suas notáveis capacidades de transmissão de potência, a utilização de amplificadores de potência (PAs) baseados em GaN HEMTs é frequentemente desconsiderada em favor de tecnologias alternativas baseadas em transístores de silício. Uma das principais razões disto acontecer é a existência pervasiva na tecnologia GaN HEMT de efeitos de memória lenta causados por fenómenos térmicos e de captura eletrónica. Apesar destes efeitos poderem ser compensados através de algoritmos sofisticados de predistorção digital, estes algoritmos não são adequados para transmissores modernos de células pequenas e interfaces massivas de múltipla entrada e múltipla saída devido à sua complexidade de implementação e extração de modelo, assim como a elevada potência necessária para a sua execução em tempo real. De forma a promover a utilização de PAs de alta densidade de potência e elevada eficiência baseados em GaN HEMTs em aplicações de comunicação e radar de nova geração, nesta tese propomos novos métodos de caracterização, modelação, e compensação de efeitos de memória lenta em PAs baseados em GaN HEMTs. Mais especificamente, nesta tese propomos um método de caracterização do comportamento dinâmico de autopolarização de PAs baseados em GaN HEMTs; vários modelos comportamentais de fenómenos de captura eletrónica e a sua implementação como circuitos eletrónicos analógicos para a previsão em tempo real da variação dinâmica da tensão de limiar de condução de GaN HEMTs; um método de compensação da instabilidade entre pulsos de PAs baseados em GaN HEMTs para aplicações de radar; e um esquema híbrido analógico/digital de linearização de PAs baseados em GaN HEMTs para comunicações de nova geração.Programa Doutoral em Telecomunicaçõe

X-RAY INDUCED EFFECT ON CHARGE CARRIER TRAPPING LIFETIME OF a-Se PHOTOCONDUCTORS AND THE RECOVERY PROCESS

Amorphous selenium (a-Se) alloy x-ray detectors are currently used in commercial mammographic detectors for breast cancer detection and diagnosis. They have been only recently commercialized and there are now at least five companies manufacturing these detectors. This work focuses on the study of the X-ray induce effects on the carrier trapping lifetime in a-Se, and the recovery process of the X-ray induced damage in the bulk of a-Se samples. The x-ray dose effect on the carrier trapping lifetime was studied alongside the temperature effect on the induced x-ray damage and recovery process. The carrier trapping lifetime reduces as the accumulated dose deposited in the a-Se samples increases. Upon the cessation of x-ray exposure, carrier lifetime recovered slowly (over many hours) back to its original state. The damage was not permanent. Several a-Se detectors samples have been exposed to high doses of x-ray and the recovery process has been observed under different temperature, 23.5 oC and 35.5 oC. The time of flight (TOF) measurement technique was employed to measure the carrier drift mobility and the interrupted filed time of flight (IFTOF) technique was used to measure the carrier trapping lifetime . All samples used in this project are pure a-Se for hole transport measurements, a-Se: 0.3%: 2.5ppm Cl and a-Se: 0.5%: 10ppm Cl for electron transport measurements. Sample thickness ranges from 50 μm to 200 μm with a variance of ±5 μm at different positions on the sample. The applied dose rate during the x-ray irradiation ranges from 1.9 Gy/s to 2.5 Gy/s. The difference in dose rate does not affect the change in the hole trapping lifetime but has a non-significant effect on the electron trapping lifetime. The rate of decrease in the hole normalized lifetime is more rapid at 35.5oC than at room temperature (23.5oC). The recovery processes were also observed to be more rapid at the higher temperature

An experimental and theoretical study of the dark current and x-ray sensitivity of amorphous selenium x-ray photoconductors

Recently, the world of diagnostic radiography has seen the integration of digital flat panel x-ray image detectors into x-ray imaging systems, replacing analog film screens. These flat panel x-ray imagers (FPXIs) have been shown to produce high quality x-ray images and provide many advantages that are inherent to a fully digital technology. Direct conversion FPXIs based on a photoconductive layer of stabilized amorphous selenium (a-Se) have been commercialized and have proven particularly effective in the field of mammography. In the operation of these detectors, incident x-ray photons are converted directly to charge carriers in the a-Se layer and drifted to electrodes on either side of the layer by a large applied field (10 V/μm). The applied field causes a dark current to flow which is not due to the incident radiation and this becomes a source of noise which can reduce the dynamic range of the detector. The level of dark current in commercialized detectors has been reduced by the deposition of thin n- and p- type blocking layers between the electrodes and the bulk of the a-Se. Despite recent research into the dark current in metal/a-Se/metal sandwich structures, much is still unknown about the true cause and nature of this phenomenon. The work in this Ph.D. thesis describes an experimental and theoretical study of the dark current in these structures. Experiments have been performed on five separate sets of a-Se samples which approximate the photoconductive layer in an FPXI. The dark current has been measured as a function of time, sample structure, applied field, sample thickness and contact metal used. This work has conclusively shown that the dark current is almost entirely due to the injection of charge carriers from the contacts and the contribution of Poole-Frenkel enhanced bulk thermal generation is negligible. There is also evidence that while the dark current is initially controlled by the injection of holes from the positive contact, several minutes after the application of the bias, the dark current due to hole injection may decay to the point where the electron current becomes significant and even dominant. These conclusions are supported by numerical calculations of the dark current transients which have been calibrated to match experimental results. Work detailed in this Ph.D. thesis also focuses on Monte Carlo modeling of the x-ray sensitivity of a-Se FPXIs. The higher the x-ray sensitivity of a detector, the lower the radiation dose required to acquire an acceptable image. FPXIs can experience a decrease in the x-ray sensitivity of the photoconductive layer with accumulating exposure, leading to a phenomenon known as “ghosting”. Modeling this decrease in sensitivity can uncover the reasons behind it. The Monte Carlo model described in this thesis is a continuation of a previous model which now considers the effects of the n- and p-like blocking layers and the flow of dark current between x-ray exposures. The simulation results explain how deep trapping of photogenerated charge carriers, and the resulting effect on the electric field distribution, contribute to sensitivity loss. The model has shown excellent agreement with experimental data and has accurately predicted a sensitivity recovery once exposure has ceased which is due to primarily to the relaxation of metastable x-ray-induced carrier trap states