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A linear galvanic isolator was developed, using a compact and high sensitivity Quantum Well Hall Effect (QWHE) sensor. This sensor is based on a GaAs-InGaAs-AlGaAs heterostructure, with a maximum capacitance of 5.5 pF and a 3 dB bandwidth of 40.2 MHz. As part of this work, a printed transmitter coil was also designed as part of this QWHE galvanic isolator. A linear relationship between input current and output voltage, at each frequency of the isolation device, were observed (R2 ≈ 1.0). The relative errors of frequency response between 0 and 100 kHz were ≤ 5.4 %. Two gain temperature coefficients were also obtained at low and high temperatures, α1 and α2, with a temperature of 280 K being the boundary between the two regions. The mean values of α1 and α2 were (7.09±0.27) x 10-4 K-1 and (3.22±0.17) x 10-4 K-1 respectively. The controlling mechanisms for the amplitudes of α1 and α2 are suggested to be due to the temperature variation of the two-dimensional electron gas (2DEG) electron mobility, arising from the heterostructure nature of the QWHE sensor used. The QWHE isolator has high accuracy, large bandwidth, high frequency-gain linearity and thermal stability. Compared with commercial optical isolators, Silicon Hall sensor-based isolators and coil isolators, this QWHE isolator does not require any external light sensor transistor or ferrite toroid to enhance its sensitivity. As such, it is not affected by any nonlinear transistor behaviour, B-H curve or magnetic retention. The results indicate that this high sensitivity, highly linear QWHE isolator is suitable for use as a low cost, high efficiency, linear galvanic isolation device.

In this study the results of optical, electrical and photovoltaic experiments which were carried out to investigate the memory effect of TlGaSe2 ferroelectric - semiconductor with incommensurate (INC) - phase are presented. The aim is to show that electrical and open circuit photovoltage measurements can be effectively used for the study of memory effect of INC - structure in TlGaSe2.

Electrical switching phenomenon was observed in TlGaSe2 layered ferroelectric-semiconductor applying different types of electrodes on different TlGaSe2 samples in both directions parallel and perpendicular to the pane of the layers. The non-linear current-voltage (CV) characteristics were measured by sweeping the current while measuring the voltage drop and could be classified as current-controlled S-type negative resistance phenomenon. The effects of temperature, illumination and as well as long time annealing within the incommensurate phase on the switching characteristics were also been studied. The switching phenomenon is discussed on the basis of the models widely used for disordered semiconductors. It was shown that TlGaSe2 crystal demonstrates the peculiar behavior that is typical to chalcogenide glassy semiconductors (CGS). (C) 2014 Elsevier Ltd. All rights reserved.

The strong enhancement, by several orders of magnitude, of the excitonic peak within the photoconductivity spectrum of TlGaSe2 semiconductor was observed. The samples were polarized in external dc electric field, which was applied prior to the measurements. Due to the accumulation of charges near the surface, an internal electric field was formed. Electron-hole pairs that were created after the absorption of light are fallen in and then separated by the built-in electric field, which prevents radiative recombination process. (C) 2014 AIP Publishing LLC.

We report the internal electric field induced rectification in TlGaSe2 layered semiconductor. This built-in internal electric field was imprinted into the TlGaSe2 sample prior to the measurements while cooling the samples under the external electric field. We employ various pairs of metallic electrodes (In-In, Au-In, Au-Cu), which are deposited on a smooth mirror-like surfaces of TlGaSe2 crystal, respectively, and used to measure the current flow in two directions, parallel and perpendicular to the plane of layers. The current-voltage (I-V) characteristics were measured on the samples with imprinted internal electric field. The diode-like I-V characteristics was observed with the rectification polarity coinciding with the direction of the imprinted electric field. This effect was observed in both directions parallel and perpendicular to the plane of layers. Our results support that near the metal-TlGaSe2 interface the barrier is formed after the pretreatment of the sample in the electric field. So, the diode with a controlled rectification direction, which is achieved by changing the polarization direction using an external bias, is fabricated. The field induced motion of the charge, which piles up under the respective contact that forms a barrier is assumed. The memristive behavior with characteristic pinched hysteresis loop is also observed. (C) 2014 AIP Publishing LLC.

The effect of the preillumination on the dark and the photo-conductivity of the TlGaSe2 layered semiconductor is investigated within the temperature range of 80-300 K After the illumination predominantly at high temperatures, a substantial decrease in both dark and photo-conductivities was observed. Observed phenomena resemble the Staebler-Wronski effect, which is typical for the amorphous semiconductors. The main contribution of this work is to show that the TlGaSe2 single crystals with well apparent crystalline structure can demonstrate certain characteristics peculiar to amorphous semiconductors. (C) 2014 Elsevier Masson SAS. All rights reserved.

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2005, Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2005, Bu çalışmada pasif sistemlerin kontrolü için geliştirilen teknikler tanıtılmaya çalışılmıştır. İlgilenilen sistemler Euler-Lagrange sistemleri ile sınırlı tutulmuştur. Önce Klasik Mekaniğe dair temel kavramlar açıklanmaya çalışılmış daha sonra Euler-Lagrange denklemlerinin hem Newton’un hareket yasalarından hem de Hamilton İlkesinden çıkartılması ve verilen bir sistemin Euler-Lagrange modelinin çıkartılması gösterilmeye çalışılmıştır. Daha sonra Euler-Lagrange modeli verilen bir sistemin kontrolünde kullanılan Pasifliğe Dayalı Kontrol tekniğinin teorik alt yapısının açıklanması için gerekli temel kavramlar ve teoremler açıklanmaya çalışılmıştır. Son olarak anlatılan tekniklerin birkaç uygulaması verilmiş ve simülasyonları yapılmıştır., In this study the techniques which have been developed for control of passive systems are presented. The systems, which are in interest, are Euler-Lagrange systems. First of all some fundamental properties of Classical Mechanics are presented. Then the Euler-Lagrange equations are derived from both Newton s Laws and Hamilton s principle. And derivation of the Euler-Lagrange model of a given system is also presented. Then, following this, the theoretical concepts of the passivity based control are presented. At the end the techniques are applied to some mechanical systems. The simulation results are also included., Yüksek Lisans, M.Sc.