Tarık Asar’s research while affiliated with Gazi University and other places

In this paper, we present the results of a comparative analysis of two alternative SiNx passivation techniques of AlGaN/GaN high electron mobility transistor (HEMT) manufactured using identical epitaxial structure and fabrication processes. AlGaN/GaN HEMT has demonstrated excellent device characteristics, making them excellent candidates for high power, high frequency, and low noise applications. However, the full potential of GaN HEMTs in large signal operation at high frequency is limited by trapping effects and leakage currents at the interface between the epitaxial structure and passivation layer. A SiNx passivation layer has commonly been used to prevent electron trapping at the surface by providing extra positive charges to neutralize trapped negative electrons on the surface. This comparative study investigates the effects of a 75 nm SiNx passivation layer fabricated using both plasma-enhanced chemical vapor deposition (PECVD) and inductively coupled plasma chemical vapor deposition (ICPCVD) techniques on the DC and RF performance of the transistor.

The effects of Ar sputtering deposition pressure on the optical, structural, morphological, and electrical properties of amorphous InGaZnO thin films were investigated. The InGaZnO thin films, which have amorphous structures determined by grazing incidence X-Ray diffraction, contained In, Ga, Zn, and O confirmed by secondary ion mass spectrometry method. Additionally, when the thicknesses of the deposited thin films are examined, it was seen that the profilometer measurement results of the crater formed by secondary ion mass spectrometer and scanning electron microscope measurement results are nearly similar, and it is approximately 100 nm. The surface roughness, obtained from Atomic Force Microscopy results, show that decreased up to a particular value with the increase of the working pressure, and then the surface roughness increased. The optical band gaps of the films were obtained in the range of 3.50 eV-3.58 eV via Tauc relation by using the Ultraviolet-Visible measurements carried out in the wavelength range of 200 nm-1100 nm. It was seen that the optical band gap was decreased with the increase in Ar pressure. The electrical properties of InGaZnO thin film-based Schottky diodes, such as the barrier height, ideality factor, saturation current, series resistance, and shunt resistance, have also been studied comprehensively. The electrical results showed that diode properties change with increasing deposition pressure.

The Ag/MgPc/GaAs/Au-Ge organic heterojunction diode is fabricated by low cost spin coating technique. Dielectric and impedance measurements within 500 kHz–1 MHz range of organic heterojunction diode are investigated at room temperature for renewable energy applications. A broaden peak and decay of capacitance to negative values are recorded for 500, 700 kHz and 1 MHz applied frequencies within −2, +5 V biasing voltage range. The negative capacitance (NC) of our organic device is observed inside the forward bias voltage range. Increase of conductance with decline in frequency within forward bias voltage is recorded inside the same voltage range. Ac conductivity is frequency dependent in particular within forward voltage range and exceeds 3.5 µS/m at lower frequency. Dielectric constant of organic diode follows similar variation as that of C-V for measured frequencies. Dimensionless parameter M″ versus M′ for applied frequencies is represented in Cole-Cole diagram. It is recorded a highest value of 0.6 for 1 MHz.

The Ag/MgPc/GaAs/Au-Ge organic heterojunction diode is fabricated by low cost spin coating technique. Dielectric and impedance measurements within 500 kHz-1MHz range of organic heterojunction diode are investigated at room temperature for renewable energy applications. A broaden peak and decay of capacitance to negative values are recorded for 500, 700 kHz and 1 MHz applied frequencies within-2V, +5V biasing voltage range. The negative capaci-tance of our organic device is observed inside the forward bias voltage range. Increase of conductance with decline in frequency within forward bias voltage is recorded inside the same voltage range. Ac conductivity is frequency dependent in particular within forward voltage range and exceeds 3.5 µS/m at lower frequency. Dielectric constant of organic diode follows similar variation as that of C-V for measured frequencies. Dimensionless parameter M" versus M' for applied frequencies is represented in Cole-Cole diagram. It is recorded a highest value of 0.6 for 1 MHz.

The morphological, optical and electrical properties of undoped Zinc Oxide (ZnO) and 0.5% and 2% Ag-doped ZnO based Photodiodes were investigated. ZnO samples, which were deposited on p-type Si semiconductor substrates using spin a coating method, were produced by sol–gel method. The morphological properties of the ZnO films were investigated by atomic force microscopy. The results revealed that thin films are in nanofiber like structure that nanofiber spread homogeneously on the substrate. Additionally, the permittivity, absorbance, reflectivity and bandgap energy values were calculated by using the UV measurement results. The optic bandgaps of the films were also calculated by the optical absorption method. Each sample shows permittivity properties in the visible region; samples show absorption properties in the region where the applied wavelength is smaller than 400 nm. Reflecting boundaries were shift to higher boundaries with increased dopant effect. The transmittance of samples was found low in the UV region, but it shows higher characteristics which are between 78 and 88% in the visible region. To investigate the electrical properties of produced thin films, Current–Voltage, Capacitance–Voltage, Conductance–Voltage characteristics and Current–Time measurements were obtained in different illumination intensities. It was seen that the samples show photoconductive performance.