Over the last 10 years, digital subthreshold logic circuits have been developed for applications in the ultra-low power design domain, where performance is not the priority. Recently, devices optimized for subthreshold operation have been introduced as potential construction blocks. However, for these devices, a strong sensitivity to process variations is expected due to the exponential relationship of the subthreshold drive current and the threshold voltage. In this thesis, a yield optimization technique is proposed to suppress the variability of a device optimized for subthreshold operation. The goal of this technique is to construct and inscribe a maximum yield cube in the 3-D feasible region composed of oxide thickness, gate length, and channel doping concentration. The center of this cube is chosen as the maximum yield design point with the highest immunity against variations. By using the technique, a transistor is optimized for subthreshold operation in terms of the desired total leakage current and intrinsic delay bounds. To develop the concept of the technique, sample devices are designed for 90nm and 65nm technologies. Monte Carlo simulations verify the accuracy of the technique for meeting power and delay constraints under technology-specific variances of the design parameters of the device.

Over the last 10 years, digital subthreshold logic circuits have been developed for applications in the ultra-low power design domain, where performance is not the priority. Recently, devices optimized for subthreshold operation have been introduced as potential construction blocks. However, for these devices, a strong sensitivity to process variations is expected due to the exponential relationship of the subthreshold drive current and the threshold voltage. In this thesis, a yield optimization technique is proposed to suppress the variability of a device optimized for subthreshold operation. The goal of this technique is to construct and inscribe a maximum yield cube in the 3-D feasible region composed of oxide thickness, gate length, and channel doping concentration. The center of this cube is chosen as the maximum yield design point with the highest immunity against variations. By using the technique, a transistor is optimized for subthreshold operation in terms of the desired total leakage current and intrinsic delay bounds. To develop the concept of the technique, sample devices are designed for 90nm and 65nm technologies. Monte Carlo simulations verify the accuracy of the technique for meeting power and delay constraints under technology-specific variances of the design parameters of the device.

Since the traditional CRT(Cathode Ray Tube) replaced by FPD(Flat Panel Display), e.g. LCD¡BOLED¡BPDP, FPD industry is regarded as the important one of global industry following Semi-conductor industry. The main stream of Large-Area Displays is TFT-LCD(Thin Film Transistor-Liquid Crystal Display) and it¡¦s applied a-Si:H TFT (the hydrogenated Amorphous Silicon Thin Film Transistor) as pixel-switch device on LCD. In a-Si:H TFT Cell process, the active region material(a-Si:H) with higher Photoconductivity results into higher off-state current under light illumination and that causes color performance discrepancy as incomplete On/Off operation of pixel-switch devices. As long as the introduction of F into a-Si:H modify the density of states in the gap of a-Si:H(:F), that may result the shift of the Fermi level toward the valence band edge and The density-of-states increasing. It¡¦s effective to decrease the photo leakage current. Due to electro-optical properties of liquid crystal(LC), to drive Pixel-switch device in TFT-LCD shall force On/Off voltage to change Twist Angle of LC is corresponding to have Stress on TFT device. According to DC Stress experiment results, it¡¦s found TFT device with SiF4 dopant can reach better reliability. This issue is aimed to research the photo leakage current variation of a-Si:H TFT at low temperature and ON/Off state effect by stress on TFT device.

Στην παρούσα διδακτορική διατριβή, διερευνήθηκε η συμπεριφορά μονωτήρων πορσελάνης και μονωτήρων πορσελάνης με επικάλυψη από RTV SIR, σε πραγματικές και εργαστηριακές συνθήκες, με μετρήσεις του ρεύματος διαρροής. Στην περίπτωση των μετρήσεων σε πραγματικές συνθήκες, με την χρήση κατάλληλου εξοπλισμού, κατέστη δυνατή η συνεχής καταγραφή της συμπεριφοράς δώδεκα μονωτήρων πορσελάνης 150kV, οι οποίοι αποτελούσαν ενεργές συνιστώσες δύο υποσταθμών του Συστήματος Μεταφοράς Κρήτης. Από αυτούς σε δέκα είχαν τοποθετηθεί επικαλύψεις από RTV SIR. Παράλληλα πραγματοποιήθηκαν και μετεωρολογικές μετρήσεις, σε συγχρονισμό με αυτές του ρεύματος διαρροής, δίνοντας την δυνατότητα συσχέτισης των μετεωρολογικών παραμέτρων με την αντίστοιχη συμπεριφορά των μονωτήρων. Οι μετρήσεις του ρεύματος διαρροής ανέδειξαν δύο περιόδους δραστηριότητας. Στην περίπτωση των μονωτήρων πορσελάνης η περίοδος αιχμής καταγράφεται στο τέλος της καλοκαιρινής περιόδου, από τον μήνα Αύγουστο μέχρι και τον Οκτώβριο. Τον υπόλοιπο χρόνο καταγράφεται δραστηριότητα, ιδιαίτερα την άνοιξη, σε σημαντικά χαμηλότερα επίπεδα όμως. Η συμπεριφορά αυτή είναι σε συμφωνία με την μηνιαία κατανομή των σφαλμάτων λόγω ρύπανσης στο Σύστημα Κρήτης, την περίοδο 1969 – 2005. Αντίθετα, στην περίπτωση των μονωτήρων με επικάλυψη από RTV SIR, την περίοδο από τον Αύγουστο μέχρι και τον Οκτώβριο, κατεγράφησαν εξαιρετικά χαμηλά ως και μηδενικά επίπεδα δραστηριότητας. Για τα υλικά αυτά, η αιχμή της επιφανειακής δραστηριότητας καταγράφεται κατά την χειμερινή περίοδο. Βέβαια πρέπει να σημειωθεί ότι ακόμη και τότε, η δραστηριότητα στην επιφάνεια των επικαλύψεων από RTV SIR είναι σαφώς ασθενέστερη σε σχέση με αυτήν των μονωτήρων πορσελάνης, το αντίστοιχο χρονικό διάστημα. Η ταυτόχρονη καταγραφή των μετεωρολογικών παραμέτρων ανέδειξε ως παράμετρο κλειδί τον παρατηρούμενο μηχανισμό ύγρανσης σε κάθε περίοδο. Η καλοκαιρινή αιχμή των μονωτήρων πορσελάνης αποδίδεται στην υγροσκοπική συμπεριφορά των ρύπων και στον μηχανισμό της συμπύκνωσης. Είναι σημαντικό ότι οι δύο αυτοί μηχανισμοί δεν μπορούν να μεταβάλουν την κατάσταση της επιφάνειας, ενώ προσβάλλουν το συνολικό μήκος ερπυσμού. Έτσι απουσία βροχοπτώσεων, ο φυσικός καθαρισμός των μονωτήρων το καλοκαίρι είναι δύσκολος, επιτρέποντας την προοδευτική συγκέντρωση της κρίσιμης ποσότητας ρύπανσης. Αντίθετα στους μονωτήρες με RTV SIR, παρά την παρουσία υγρασίας, διατηρείται η υδρόφοβη συμπεριφορά της επιφάνειας, η οποία επιβάλει τελικά την καταστολή της επιφανειακής δραστηριότητας. Αντίθετα τον χειμώνα, η εμφάνιση ασθενών βροχοπτώσεων μπορεί να οδηγήσει σε μεταβολή της κατάστασης της επιφάνειας των μονωτήρων. Στην περίπτωση της πορσελάνης προκύπτει ο καθαρισμός αυτής, ενώ στις επικαλύψεις από RTV SIR, όπου ο καθαρισμός είναι δυσκολότερος, παρατηρείται απώλεια της επιφανειακής υδροφοβίας, με αποτέλεσμα την καταγεγραμμένη δραστηριότητα. Είναι πάντως σημαντικό ότι σε κάθε περίπτωση, τα επίπεδα επιφανειακής δραστηριότητας στις επικαλύψεις από RTV SIR ήταν σαφώς χαμηλότερα από αυτά των μονωτήρων πορσελάνης. Στα πλαίσια των μετρήσεων σε πραγματικές συνθήκες, κατέστη δυνατή και η καταγραφή στιγμιότυπων του ρεύματος διαρροής. Στην περίπτωση των μονωτήρων πορσελάνης, προέκυψε ότι το απαιτούμενο ρεύμα για τον σχηματισμό ξηρών ζωνών, εξαρτάται από τον μηχανισμό ύγρανσης. Στην περίπτωση μηχανισμών όπως η συμπύκνωση, ένα ρεύμα της τάξης των 2mA αρκεί. Αντίθετα στην περίπτωση των βροχοπτώσεων έχουν καταγραφεί ρεύματα της τάξης των 15mA, χωρίς σημάδια ανάπτυξης ξηρών ζωνών. Η ανάπτυξη ξηρών ζωνών υποδηλώνεται στην κυματομορφή του ρεύματος από την εμφάνιση διαστημάτων μηδενικού ρεύματος σε κάθε ημιπερίοδο, τα οποία μάλιστα μεσολαβώντας μεταξύ των διαδοχικών εκκενώσεων υποδεικνύουν την ανεξαρτησία αυτών. Τα μη γραμμικά χαρακτηριστικά του ρεύματος στην περίπτωση αυτή αντικατοπτρίζονται στην εμφάνισης μιας συνιστώσας του ρεύματος στα 150Hz. Αυτά ισχύουν για το εύρος των τιμών ρεύματος που κατεγράφησαν στην περίπτωση αυτή (14mA<ILC<140mA). Αντίστοιχα χαρακτηριστικά προκύπτουν και στην περίπτωση των μονωτήρων με επικάλυψη από RTV SIR. Επιπλέον όμως στην περίπτωση αυτή, κατεγράφησαν εκκενώσεις που δεν σχετίζονται με ξηρές ζώνες, αλλά περισσότερο με το ενδεχόμενο διατήρησης υδρόφοβης συμπεριφοράς για τμήματα της επιφάνειας του μονωτήρα. Επιπλέον η αλλοίωση της κυματομορφής του ρεύματος είναι εντονότερη στην περίπτωση αυτή, κάτι που συνεπάγεται υψηλότερα επίπεδα στην αρμονική των 150Hz. Παράλληλα με τις μετρήσεις σε πραγματικές συνθήκες, οι συνθέσεις των υλικών RTV SIR που χρησιμοποιήθηκαν, αξιολογήθηκαν και σε εργαστηριακές συνθήκες. Πραγματοποιήθηκαν δύο δοκιμές σε θάλαμο υδατονέφωσης άλατος, όπου οι συνθήκες καταπόνησης μοιάζουν με το ενδεχόμενο της βροχόπτωσης σε πραγματικές συνθήκες. Από την συμπεριφορά των υλικών στις συγκεκριμένες συνθήκες και τις μετρήσεις που έγιναν, προκύπτει ότι οι συνθέσεις με ΑΤΗ είναι περισσότερο ανθεκτικές, τόσο όσον αφορά το φαινόμενο corona (υδρόφοβη επιφάνεια), όσο και την καταπόνηση από ξηρές ζώνες, στην περίπτωση απώλειας της υδροφοβίας, σε σχέση με αυτές όπου χρησιμοποιείται silica. Η διαφορά οφείλεται στον τρόπο προστασίας που προσφέρει ο κάθε τύπος πρόσμιξης. Πάντως πρέπει να σημειωθεί ότι είναι απαραίτητη η περαιτέρω εργαστηριακή διερεύνηση της επίδρασης του τύπου της πρόσμιξης, όσον αφορά την επίδραση τόσο των εκκενώσεων corona όσο και των εκκενώσεων ξηρών ζωνών. / In the present study the performance of porcelain and RTV SIR coated porcelain insulators has been investigated in field and laboratory conditions, by leakage current measurements. In field conditions, by the use of the appropriate equipment it was possible to continuously monitor a group of twelve 150kV porcelain insulators, installed in two high voltage substations, of the Transmission System in Crete. Ten of them were coated with RTV SIR. In addition simultaneous measurements of meteorological parameters were performed, allowing the correlation of the LC measurements to the environmental conditions. The leakage current measurements performed indicated two periods of intense surface activity. In the case of porcelain, the summer period and especially the months From August to October, represent the period of intense surface activity. During the rest of the year the recorded LC levels are remarkably lower. This monthly distribution comes in agreement with the observed pollution flashovers distribution, for the period 1969-2005. The opposite activity distribution is observed for the RTV SIR coated insulators. In this case the levels of surface activity in the summer period are remarkably low and the period of intense activity for the coatings is observed during the winter. It is worth mentioning however that even in this case the levels of activity are remarkably lower than the corresponding levels in the case of porcelain, for the same time period. The opposite behavior of porcelain and coated porcelain insulators can be correlated to the environmental conditions and especially the wetting mechanism present. During the summer, insulator wetting is possible as the result of two mechanisms, the hydroscopic behavior of the pollution layer and condensation. Both mechanisms are capable of wetting the total leakage creepage distance, without cleaning the insulators surface in the same time. As a result a critical amount of pollution can be formed on the insulator surface, considering also the low levels of precipitation. So in the case of porcelain the formation of surface conductivity is possible in contradiction to the RTV SIR coated insulator, where the formed surface hydrophobicity is maintained. On the other hand during the winter, light precipitation can support the development of surface activity, since it is possible to disturb the surface condition. In the case of porcelain this will result to the cleaning of the surface. However in the case of RTV SIR coatings a loss of hydrophobicity is observed which allows the development of surface activity, considering also that cleaning is more difficult in this case. It is worth noticing however that in all cases the observed activity on the RTV SIR coated insulators is remarkably lower than the corresponding activity in the case of uncoated porcelain insulators. The leakage current waveforms for finite time periods are also included in the information provided by the field measurements performed. In the case of porcelain insulators, the analysis of the corresponding waveforms indicated that the current required for the formation of dry bands depends on the wetting mechanism present. In the case of mechanisms such as condensation, a current in the range of 1 – 2mA is capable to support the formation of dry bands. However in the case of precipitation the necessary current is higher, reaching a level of 15mA. Further the formation of dry bands is reflected to the leakage current waveform by zero current periods which are observed between the current conduction periods. This behavior indicates that the observed activity can be considered as a sequence of independent current pulses. Additionally the FFT analysis correlates the non linear current behavior to an increased 150Hz component. These characteristics have been traced in all the waveforms recorded, in the range from 14mA to 150mA. In the case of RTV SIR coated insulators the recorded waveforms are in large extent similar to the waveforms on the porcelain insulators. However additional phenomena, correlated with the existence of areas which maintain the hydrophobic behavior. In addition the non linear behavior is enhanced in this case, something that results in higher levels of a current component at 150Hz. The performance of the employed RTV SIR coatings was also investigated in laboratory conditions. Two tests were performed in a salt fog chamber, were the stress conditions are similar to the conditions observed in the case of light rain. The material performance observed and the corresponding measurements performed in both tests, indicate that the formulations tested, the endurance of the ATH filled coatings is higher than the silica filled, both in the case of corona and dry band discharge stress. The difference observed can be correlated with the action of each filler type. However the influence of the filler action needs to be further investigated.

This thesis describes the design of an instrument for the protection of personnel carrying out live line bare hand maintenance of power transmission circuits. This instrument monitors the leakage current along equipment to ground and sounds an alarm when preset thresholds are exceeded. The leakage current monitor is microprocessor controlled, resulting in both flexible and user friendly operation. The prototype constructed, has been tested and found to perform very well.

In decades, the tremendous development of integrated circuits industry could be mostly attributed to SiO2, since its satisfactory properties as a gate dielectric candidate. The effectivity of SiO2 has been challenged since dielectric layer was scaled down below 3nm, when the gate leakage current of SiO2 became unacceptable. Institution to silicon-based CMOS techniques were proposed, but they have their own limitations. Nowadays, materials with high dielectric constants are mainstream gate dielectric materials in industry, but a SiO2 interfacial layer is still necessary to avoid gap between gate dielectric layer and Si substrate, and to minimize interface trap charges. In this thesis work, by applying lateral heating process on Si wafer with thermally grown ultrathin SiO2, the gate leakage current density could be reduced by 3-5 order of magnitude. MOS capacitors were fabricated, and electrical properties were tested with semiconductor parameter analyzer and LCR meter. The underlying mechanism of this appealing phenomenon was explored. Since unacceptable gate leakage current is one of the main reasons which prevent the scaling trend in semiconductor industry, this technology brings a possibility to post-pone the end of scaling trend, and pave a way for extensive application in industry. A new method for fabrication of MOS capacitors metal gate has been developed, and lift-off process has been replaced by wet etching process. This method provides better contact between dielectric layer and metal gate, meanwhile much easier operation.

The focus of this thesis is on radio frequency magnetron sputtered aluminum oxide thin films developed for use as the gate dielectric for organic thin film transistors. The effect of top metal electrodes on the electrical characteristics of aluminum oxide metal-insulator-metal capacitors has been studied to determine an optimum material combination for minimizing the leakage current, while maximizing the breakdown field. The leakage current and breakdown characteristics were observed to have a strong dependence on the top electrode material. Devices with Al top electrodes exhibited significantly higher breakdown voltages compared to devices with Au, Ni, Cu and Ag electrodes. Introducing an Al diffusion barrier dramatically increased the breakdown field and reduced the leakage current for capacitors with Ag, Au and Cu top electrodes. The electrical characteristics were found to relate well to material properties, of the contacting metals, such as ionization potential and diffusion coefficient.

The focus of this thesis is on radio frequency magnetron sputtered aluminum oxide thin films developed for use as the gate dielectric for organic thin film transistors. The effect of top metal electrodes on the electrical characteristics of aluminum oxide metal-insulator-metal capacitors has been studied to determine an optimum material combination for minimizing the leakage current, while maximizing the breakdown field. The leakage current and breakdown characteristics were observed to have a strong dependence on the top electrode material. Devices with Al top electrodes exhibited significantly higher breakdown voltages compared to devices with Au, Ni, Cu and Ag electrodes. Introducing an Al diffusion barrier dramatically increased the breakdown field and reduced the leakage current for capacitors with Ag, Au and Cu top electrodes. The electrical characteristics were found to relate well to material properties, of the contacting metals, such as ionization potential and diffusion coefficient.

- / In the present diplomatic project is realised a process of measurements of the leakage current by the system of measurement that was manufactured by gentleman Sideraki, in the frames of his doctoral thesis. The measurements were realised in the substation 150 kV of Crete‟s system of transport and in the substation 150 kV of A.I.S. Linoperamaton. In the first chapter, is presented the phenomenon of the insulators‟ pollution, while it constitutes the prime cause of the leakage current. Here are presented the main causes of pollution, its unfavourable results as well as her ways of confrontation. There is an extensive report, in the use of multilateral insulators and multilateral coatings for the confrontation of the phenomenon of pollution. Particularly is stressed the hydrophobic characteristic of these materials which leads to the reduction of the quantity of pollution on the surface of the insulators. The second chapter, is a presentation of the work of various researchers over the world, in the effort of studying the leakage current and its connection with the phenomena of ageing and flashover. Also there are mentioned the experimental provisions that they used in the frames of their studies and are reported the mainer conclusions to which they were led. Common point of their conclusions constitutes the make of ascertainment that the biggest price of the leakage current, constitutes an unreliable indicator of description of the distressed insulator‟s surface, since important information on the situation of the surface we draw also from the waveform of the leakage current. In the third chapter there is a concise presentation of Fourier„s theory and transforming, while an extensive report in the distinguishable Fourier transforming (DFT) is also mentioned. Finally, is presented the fast Fourier transforming (FFT), which constitutes an algorithm of fast calculation of distinguishable transformation Fourier and is used among others for the spectrum analysis of signals of distinguishable time. In the fourth chapter, is realised the treatment of measurements, with the use of Labview for the study of the leakage current in the field of frequency. Leakage current‟s waveforms are categorized, with criteria, the lack of linearity and the presence of discharges or not and conclusions are exported on these waveforms, from their spectrum analysis.

abstract: Two major challenges in the transformer-less, single-phase PV string inverters are common mode leakage currents and double-line-frequency power decoupling. In the proposed doubly-grounded inverter topology with innovative active-power-decoupling approach, both of these issues are simultaneously addressed. The topology allows the PV negative terminal to be directly connected to the neutral, thereby eliminating the common-mode ground-currents. The decoupling capacitance requirement is minimized by a dynamically-variable dc-link with large voltage swing, allowing an all-film-capacitor implementation. Furthermore, the use of wide-bandgap devices enables the converter operation at higher switching frequency, resulting in smaller magnetic components. The operating principles, design and optimization, and control methods are explained in detail, and compared with other transformer-less, active-decoupling topologies. A 3 kVA, 100 kHz single-phase hardware prototype at 400 V dc nominal input and 240 V ac output has been developed using SiC MOSFETs with only 45 μF/1100 V dc-link capacitance. The proposed doubly-grounded topology is then extended for split-phase PV inverter application which results in significant reduction in both the peak and RMS values of the boost stage inductor current and allows for easy design of zero voltage transition. A topological enhancement involving T-type dc-ac stage is also developed which takes advantage of the three-level switching states with reduced voltage stress on the main switches, lower switching loss and almost halved inductor current ripple. In addition, this thesis also proposed two new schemes to improve the efficiency of conventional H-bridge inverter topology. The first scheme is to add an auxiliary zero-voltage-transition (ZVT) circuit to realize zero-voltage-switching (ZVS) for all the main switches and inherent zero-current-switching (ZCS) for the auxiliary switches. The advantages include the provision to implement zero state modulation schemes to decrease the inductor current THD, naturally adaptive auxiliary inductor current and elimination of need for large balancing capacitors. The second proposed scheme improves the system efficiency while still meeting a given THD requirement by implementing variable instantaneous switching frequency within a line frequency cycle. This scheme aims at minimizing the combined switching loss and inductor core loss by including different characteristics of the losses relative to the instantaneous switching frequency in the optimization process. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017