Browsing by Author "Çakmak, H."
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Item Open Access Current transport mechanisms and trap state investigations in (Ni/Au)-AlN/GaN Schottky barrier diodes(Elsevier, 2010-10-13) Arslan, E.; Bütün, S.; Şafak, Y.; Çakmak, H.; Yu, H.; Özbay, EkmelThe current transport mechanisms in (Ni/Au)-AlN/GaN Schottky barrier diodes (SBDs) were investigated by the use of current-voltage characteristics in the temperature range of 80-380 K. In order to determine the true current transport mechanisms for (Ni/Au)-AlN/GaN SBDs, by taking the Js(tunnel), E 0, and Rs as adjustable fit parameters, the experimental J-V data were fitted to the analytical expressions given for the current transport mechanisms in a wide range of applied biases and at different temperatures. Fitting results show the weak temperature dependent behavior in the saturation current and the temperature independent behavior of the tunneling parameters in this temperature range. Therefore, it has been concluded that the mechanism of charge transport in (Ni/Au)-AlN/GaN SBDs, along the dislocations intersecting the space charge region, is performed by tunneling. In addition, in order to analyze the trapping effects in (Ni/Au)-AlN/GaN SBDs, the capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics were measured in the frequency range 0.7-50 kHz. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming the models in which traps are located at the heterojunction interface. The density (Dt) and time constants (τt) of the trap states have been determined as a function of energy separation from the conduction-band edge (Ec - Et) as Dt≅ (5-8)×10 12eV-1 cm-2andτt≅(43-102) μs, respectively.Item Open Access Forward tunneling current in Pt/p-InGaN and Pt/n-InGaN Schottky barriers in a wide temperature range(Elsevier, 2012-07-27) Arslan, E.; Çakmak, H.; Özbay, EkmelThe current-transport mechanisms of the Pt contacts on p-InGaN and n-InGaN were investigated in a wide temperature range (80-360 K) and in the forward bias regime. It was found that the ideality factor (n) values and Schottky barrier heights (SBHs), as determined by thermionic emission (TE), were a strong function of temperature and Φb0 show the unusual behavior of increasing linearly with an increase in temperature from 80 to 360 K for both Schottky contacts. The tunneling saturation ( JTU(0)) and tunneling parameters (E 0) were calculated for both Schottky contacts. We observed a weak temperature dependence of the saturation current and a fairly small dependence on the temperature of the tunneling parameters in this temperature range. The results indicate that the dominant mechanism of the charge transport across the Pt/p-InGaN and Pt/n-InGaN Schottky contacts are electron tunneling to deep levels in the vicinity of mixed/screw dislocations in the temperature range of 80-360 K.Item Open Access Indium rich InGaN solar cells grown by MOCVD(Springer New York LLC, 2014) Çakmak, H.; Arslan, E.; Rudziński, M.; Demirel, P.; Unalan, H. E.; Strupiński, W.; Turan, R.; Öztürk, M.; Özbay, EkmelThis study focuses on both epitaxial growths of InxGa 1-xN epilayers with graded In content, and the performance of solar cells structures grown on sapphire substrate by using metal organic chemical vapor deposition. The high resolution X-ray and Hall Effect characterization were carried out after epitaxial InGaN solar cell structures growth. The In content of the graded InGaN layer was calculated from the X-ray reciprocal space mapping measurements. Indium contents of the graded InGaN epilayers change from 8.8 to 7.1 % in Sample A, 15.7-7.1 % in Sample B, and 26.6-15.1 % in Sample C. The current voltage measurements of the solar cell devices were carried out after a standard micro fabrication procedure. Sample B exhibits better performance with a short-circuit current density of 6 mA/cm2, open-circuit voltage of 0.25 V, fill factor of 39.13 %, and the best efficiency measured under a standard solar simulator with one-sun air mass 1.5 global light sources (100 mW/cm2) at room temperature for finished devices was 0.66 %.Item Open Access Nonalloyed ohmic contacts in AlGaN/GaN HEMTs with MOCVD regrowth of InGaN for Ka-band applications(IEEE, 2021-01-22) Çakmak, H.; Öztürk, Mustafa; Özbay, EkmelLow-resistance ohmic contacts in AlGaN/GaN high-electron-mobility transistor (HEMT) devices require high-temperature (HT) annealing (>800 °C) which can deteriorate material quality, surface morphology, and edge acuity of the metal stacks. This article demonstrates the highfrequency and high-power performance of the AlGaN/GaN HEMT devices with low-temperature metal-organic chemical vapor deposition (MOCVD) regrown degenerately doped InGaN ohmic contacts compared with GaN-based regrown contacts. Ohmic contacts fabricated by regrowth methods could be a valuable alternative for both metal-based alloyed ohmic contacts and implantation-based ohmic contacts. Using a T-gate and MOCVD regrown InGaN ohmic contacts, the AlGaN/GaN HEMT with an Lg of 150 nm and S-D spacing of 2.5 μm demonstrated a maximum drain current of 0.94 A/mm and a peak transconductance of 337 mS/mm. The same device exhibited a forward current gain frequency ft of 36.8 GHz and a maximum frequency of oscillation fmax of 75.0 GHz. A power density of 3.07 W/mm with a 60% drain efficiency was measured at 35 GHz with a Vds of 20 V and a quiescent current of 100 mA/mm.