Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications

Resonant frequencies of the two-dimensional plasma in FETs increase with the reduction of the channel dimensions and can reach the THz range for sub-micron gate lengths. Nonlinear properties of the electron plasma in the transistor channel can be used for the detection and mixing of THz frequencies. At cryogenic temperatures resonant and gate voltage tunable detection related to plasma waves resonances, is observed. At room temperature, when plasma oscillations are overdamped, the FET can operate as an efficient broadband THz detector. We present the main theoretical and experimental results on THz detection by FETs in the context of their possible application for THz imaging.Comment: 22 pages, 12 figures, review pape

Sistema de polarização pulsado para caracterização de transístores de potência de RF

With the exponential evolution of mobile networks, the systems are demanded, not only to work at high power levels and to be more efficient, but also to operate at higher frequencies and with more bandwidth. For that, the scientific community is always trying to obtain the best performance possible for the mobile networks base stations. The efficiency limiting component of these base stations is the RF power amplifier, that is composed by a transistor capable of amplifying the signal that is wanted to transmit. Nowadays, the most used devices are the GaN HEMT (Gallium Nitride High Electron Mobility Transistor), that allow operations with high power density and high bandwidth. However, these devices suffer from several dynamic phenomena that cause the current to collapse and, consequently, the reduction of the power delivery capability. To predict and try to compensate these dynamic behaviours, it is necessary to characterize the transistors in a state where these phenomena are known, which means that isodynamic measurements should be obtained. This work focuses on designing a system that allows to perform pulsed measurements on a transistor and that avoids temperature effects and other dispersive phenomena characteristic of the mentioned technology (known as trapping). For that, two circuits were developed: one for the transistor gate, with a voltage range from −10V to 2V, a maximum current of 2A and a settling time of 500 ns, and another for the transistor drain, with a maximum voltage of 160V, a current of, at least, 15A and a settling time of 4 μs. With this system, it was possible to analyse the impact of the temperature on the measurements, through the study of the pulse width and duty-cycle variation, and it was also possible to study the impact of the trapping, by applying the double pulse technique. As the trapping is a dynamic phenomenon, there are associated time constants that should be known, so it is possible to study the dynamic nature of the devices. Finally, to prove that the system could be used to characterize several devices, GaN HEMT transistors rated for different power values were characterized. In the end, it was possible to obtain their pulsed isodynamic I/V curves, as well as the time constants associated with the trapping effect. With all the obtained results, it was possible to conclude that the system is capable of performing the desired measurements needed to characterize a device, and that the dynamic of the mentioned effects is characteristic of each device, so it should always be carefully analysed.Com a evolução exponencial das redes móveis, é exigido aos sistemas que, não só, operem com mais potência e que sejam mais eficientes, mas também que operem a frequências mais altas e com maior largura de banda. Para isto, a comunidade científica está dedicada a tentar obter o melhor desempenho possível para as estações base das redes móveis. O componente que domina o rendimento destas estações base é o amplificador de potência RF, composto por um transístor capaz de amplificar o sinal que se deseja transmitir. Hoje em dia, os dispositivos mais usados são os GaN HEMT (do inglês, Gallium Nitride High Electron Mobility Transistor ), que permitem atingir grandes densidades de potência e grande largura de banda. Mas, estes dispositivos sofrem de diversos fenómenos dinâmicos que causam o colapso da corrente e, consequentemente, a diminuição da capacidade de entrega de potência. De modo a que se consiga prever e tentar compensar estes comportamentos dinâmicos, é necessário caracterizar os transístores num estado onde estes fenómenos sejam conhecidos, o que significa obter medidas isodinâmicas. Este trabalho foca-se em construir um sistema que permita efetuar medidas pulsadas num transístor e evitar que efeitos de temperatura e de outros fenómenos dispersivos inerentes da tecnologia mencionada (conhecidos como trapping, do inglês) apareçam. Para tal, foram desenvolvidos dois circuitos: um para porta do transístor, cuja gama de tensões é de −10V a 2V, a corrente máxima é de 2A e o tempo de estabelecimento é de 500 ns, e outro para o dreno do transístor, cuja tensão máxima é de 160V, a corrente ´e de, pelo menos, 15A e o tempo de estabelecimento é de 4 μs. Com este sistema, foi possível analisar o impacto da temperatura nas medidas, através do estudo da variação da largura de pulso e do duty-cycle, e estudar o efeito do trapping, aplicando a técnica do duplo pulso. Como o trapping é um fenómeno dinâmico, existem constantes de tempo associadas que devem ser conhecidas, para que se possa estudar a natureza da dinâmica presente nos dispositivos. Finalmente, para comprovar que o sistema pode ser usado para testar diversos dispositivos, foram caracterizados transístores GaN HEMT de diferentes potências. No fim, foi possível obter as suas curvas I/V isodinâmicas pulsadas, assim como os valores das constantes de tempo associadas ao efeito de trapping. Com todos os resultados obtidos, foi possível comprovar que o sistema é capaz de efetuar as medidas desejadas para a caracterização dos dispositivos, e que a dinâmica dos efeitos mencionados ´e dependente de cada transístor e deve ser sempre cuidadosamente analisada.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

A Parallel Programmer for Non-Volatile Analog Memory Arrays

Since their introduction in 1967, floating-gate transistors have enjoyed widespread success as non-volatile digital memory elements in EEPROM and flash memory. In recent decades, however, a renewed interest in floating-gate transistors has focused on their viability as non-volatile analog memory, as well as programmable voltage and current sources. They have been used extensively in this capacity to solve traditional problems associated with analog circuit design, such as to correct for fabrication mismatch, to reduce comparator offset, and for amplifier auto-zeroing. They have also been used to implement adaptive circuits, learning systems, and reconfigurable systems. Despite these applications, their proliferation has been limited by complex programming procedures, which typically require high-precision test equipment and intimate knowledge of the programmer circuit to perform.;This work strives to alleviate this limitation by presenting an improved method for fast and accurate programming of floating-gate transistors. This novel programming circuit uses a digital-to-analog converter and an array of sample-and-hold circuits to facilitate fast parallel programming of floating-gate memory arrays and eliminate the need for high accuracy voltage sources. Additionally, this circuit employs a serial peripheral interface which digitizes control of the programmer, simplifying the programming procedure and enabling the implementation of software applications that obscure programming complexity from the end user. The efficient and simple parallel programming system was fabricated in a 0.5?m standard CMOS process and will be used to demonstrate the effectiveness of this new method

Pulsed plasma thruster ignition system: investigation, test design and results

Fra i sistemi di propulsione elettrica per satelliti, il Pulsed Plasma Thruster, PPT, è quello dal design più semplice. È anche il primo sistema di propulsione elettrica utilizzato in un satellite artificiale, ossia ZOND-2 lanciato nel 1964 dall’Unione Sovietica. Tuttavia, dopo circa 50 anni di ricerca, la comprensione teorica e sperimentale di questo dispositivo rimane limitata. Questo elaborato di tesi magistrale indaga sul sottosistema di accensione del PPT, cercando di mettere in luce alcuni aspetti legati al lifetime della spark plug, SP. Tale SP, o candela, è l’attuatore del sottosistema di accensione. Questa produce una scintilla sulla sua superficie, la quale permette la realizzazione della scarica elettrica principale fra i due elettrodi del motore. Questa scarica crea una sottile parete di plasma che, per mezzo della forza elettromagnetica di Lorentz, produce la spinta del PPT. Poiché la SP si trova all’interno del catodo del motore e si affaccia nella camera di scarica, questa soffre di fenomeni di corrosione e di deposizione carbonacea proveniente dal propellente. Questi fenomeni possono limitare notevolmente il lifetime della SP. I parametri connessi alla vita operativa della SP sono numerosi. In questo elaborato si è analizzata la possibilità di utilizzare una elettronica di accensione della candela alternativa alla classica soluzione che utilizza un trasformatore. Il sottosistema di accensione classico e quello nuovo sono stati realizzati e testati, per metterne in luce le differenze ed i possibili vantaggi/svantaggi

Stacking of IGBT devices for fast high-voltage high-current applications

The development of solid-state switches for pulsed power applications has been of considerable interest since high-power semiconductor devices became available. However, the use of solid-state devices in the pulsed power environment has usually been restricted by device limitations in either their voltage/current ratings or their switching speed. The stacking of fast medium-voltage devices, such as IGBTs, to improve the voltage rating, makes solid-state switches a potential substitute for conventional switches such as hard glass tubes, thyratrons and spark gaps. Previous studies into stacking IGBTs have been concerned with specific devices, designed or modified particularly for a specific application. The present study is concerned with stacking fast and commercially available IGBTs and their application to the generation of pulsed electric field and the switching of a high intensity Xenon flashlamp. The aim of the first section of the present study was to investigate different solid-state switching devices with a stacking capability and this led to the choice of the Insulated Gate Bipolar Transistor (IGBT). It was found that the collector-emitter voltage decreases in two stages in most of the available IGBTs. Experiments and simulation showed that a reason for this behaviour could be fast variations in device parasitic parameters particularly gate-collector capacitance. Choosing the proper IGBT, as well as dealing with problems such as unbalanced voltage and current sharing, are important aspects of stacking and these were reported in this study. Dynamic and steady state voltage imbalances caused by gate driver delay was controlled using an array of synchronised pulses, isolated with magnetic and optical coupling. The design procedure for pulse transformers, optical modules, the drive circuits required to minimise possible jitter and time delays, and over-voltage protection of IGBT modules are also important aspects of stacking, and were reported in this study. The second purpose of this study was to investigate the switching performance of both magnetically coupled and optically coupled stacks, in pulse power applications such as Pulse Electric Field (PEF) inactivation of microorganisms and UV light inactivation of food-related pathogenic bacteria. The stack, consisting of 50 1.2 kV IGBTs with the voltage and current capabilities of 10 kV, 400 A, was incorporated into a coaxial cable Blumlein type pulse - generator and its performance was successfully tested with both magnetic and optical coupling. As a second application of the switch, a fully integrated solid-state Marx generator was designed and assembled to drive a UV flashlamp for the purpose of microbiological inactivation. The generator has an output voltage rating of 3 kV and a peak current rating of 2 kA, although the modular approach taken allows for a number of voltage and current ratings to be achieved. The performance of the switch was successfully tested over a period of more than 10⁶ pulses when it was applied to pulse a xenon flashlamp.The development of solid-state switches for pulsed power applications has been of considerable interest since high-power semiconductor devices became available. However, the use of solid-state devices in the pulsed power environment has usually been restricted by device limitations in either their voltage/current ratings or their switching speed. The stacking of fast medium-voltage devices, such as IGBTs, to improve the voltage rating, makes solid-state switches a potential substitute for conventional switches such as hard glass tubes, thyratrons and spark gaps. Previous studies into stacking IGBTs have been concerned with specific devices, designed or modified particularly for a specific application. The present study is concerned with stacking fast and commercially available IGBTs and their application to the generation of pulsed electric field and the switching of a high intensity Xenon flashlamp. The aim of the first section of the present study was to investigate different solid-state switching devices with a stacking capability and this led to the choice of the Insulated Gate Bipolar Transistor (IGBT). It was found that the collector-emitter voltage decreases in two stages in most of the available IGBTs. Experiments and simulation showed that a reason for this behaviour could be fast variations in device parasitic parameters particularly gate-collector capacitance. Choosing the proper IGBT, as well as dealing with problems such as unbalanced voltage and current sharing, are important aspects of stacking and these were reported in this study. Dynamic and steady state voltage imbalances caused by gate driver delay was controlled using an array of synchronised pulses, isolated with magnetic and optical coupling. The design procedure for pulse transformers, optical modules, the drive circuits required to minimise possible jitter and time delays, and over-voltage protection of IGBT modules are also important aspects of stacking, and were reported in this study. The second purpose of this study was to investigate the switching performance of both magnetically coupled and optically coupled stacks, in pulse power applications such as Pulse Electric Field (PEF) inactivation of microorganisms and UV light inactivation of food-related pathogenic bacteria. The stack, consisting of 50 1.2 kV IGBTs with the voltage and current capabilities of 10 kV, 400 A, was incorporated into a coaxial cable Blumlein type pulse - generator and its performance was successfully tested with both magnetic and optical coupling. As a second application of the switch, a fully integrated solid-state Marx generator was designed and assembled to drive a UV flashlamp for the purpose of microbiological inactivation. The generator has an output voltage rating of 3 kV and a peak current rating of 2 kA, although the modular approach taken allows for a number of voltage and current ratings to be achieved. The performance of the switch was successfully tested over a period of more than 10⁶ pulses when it was applied to pulse a xenon flashlamp

Charge detection in semiconductor nanostructures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 137-145).In this thesis nanometer scale charge sensors are used to study charge transport in two solid state systems: Lateral GaAs quantum dots and hydrogenated amorphous silicon (a-Si:H). In both of these experiments we use time-resolved charge sensing to study electron transport in regimes that are not accessible to traditional transport measurements. For the lateral GaAs quantum dot experiments, we use a GaAs quantum point contact integrated with the dot as a charge sensor. We use this sensor to observe single electrons hopping on and off the dot in real time. By measuring the time intervals for which the dot contains one and zero electrons, we probe the rate F at which electrons tunnel on and off the dot from the leads. We measure F as a function of the drain source bias V, and gate voltages V applied to the dot. At zero magnetic field, we show that the dependencies of F on Vda and V can be understood in terms of a simple quantum mechanical model which takes into account variations in the electron energy relative to the top of the tunnel barriers separating the dot from the leads. We also show that the tunneling is dominated by elastic processes. At high magnetic fields, we show that tunneling into the excited spin state of the dot can be completely suppressed relative to tunneling into the ground spin state. The extent of the suppression depends on the shape of the electrostatic potential defining the quantum dot. For the a-Si:H experiments, we pattern a nanometer scale strip of a-Si:H adjacent to a narrow silicon MOSFET (metal-oxide-semiconductor field-effect transistor), which serves as an integrated charge sensor. We show that the MOSFET can be used to detect charging of the a-Si:H strip. By performing time-resolved measurements of this charging, we are able to measure extremely high resistances (~ 1017 Q) for the a-Si:H strip at T ~ 100 K. At higher temperatures, where the resistance of the a-Si:H strip is not too large, we show that the resistances obtained from our charge detection method agree with those obtained by measuring current. Our device geometry allows us to probe a variety of electron transport phenomena for the a-Si:H, including the field effect and dispersive transport, using charge detection. We extract the density of localized states at the Fermi level for the a-Si:H and obtain consistent results. We discuss the effect of screening by the substrate on the sensitivity of the MOSFET to charge in the a-Si:H, and show that the MOSFET can detect switching noise in the a-Si:H.by Kenneth MacLean.Ph.D