Electrical Characterization of Blue Light Emitting Diodes as a Function of Temperature
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Electrical Characterization of Blue Light Emitting Diodes as a Function of Temperature Madhu Murthy1*, Alphonse Kamto1, Michael A. Awaah1**, Dake Wang2, Minseo Park2, Frederick J. Walker3, and Kalyankumar Das1 1 Department of Electrical Engineering Tuskegee University, Tuskegee, AL 36088. * Present address: Department of Electrical and Computer Engineering, George Mason University, Fairfax, VA 22030 ** Present address: Intel Corp., CTM, Santa Clara, CA 95052 2 Department of Physics, Auburn University, Auburn, AL 36849 3 ORNL/University of Tennessee, Oak Ridge, TN 37831 ABSTRACT Blue light emitting diodes (LEDs) based on an AlGaN/GaN/AlGaN double heterojunction structure were electrically characterized as a function of temperature. Currentvoltage (I-V), capacitance-voltage (C-V) and reverse recovery storage time measurements were conducted at temperatures in the range between -8o and 75o C. Capacitance-voltage measurements were also carried out as a function of frequency (20 Hz-1 MHz) at room temperature. It was observed that the diode turn-on voltage decreased with increasing temperature, however, reverse leakage currents monitored at -1, -5 and -10 V showed only a slight increase with increasing temperature. The concentration of deep states and their position in the band gap, were extracted from logarithmic plots of the forward characteristics. Three closely spaced levels were obtained, most likely located in the range between EV and EV + 0.6 eV with approximate concentrations of the order of 1017/cm3. The effect of temperature on these states and their densities are not clearly understood at this point. Recombination lifetimes, as obtained from experimentally determined reverse recovery storage times, remained constant over the range of temperature considered. A higher value of diode capacitance was observed at low measurement frequencies (20 Hz-1kHz), gradually dropping to a lower value over a frequency range between (1 kHz-100 kHz) and remained constant from 100 kHz to 1 MHz. A loss peak centered about 10 kHz was observed in the corresponding plot of g m as a function of frequency, f. The position of the peak in the g m - frequency (f) plot and dC / d (for f in the range 1 kHz-100 kHz ), yielded a concentration of deep-states of approximately 2.2 x 1015/cm3, located at 0.39 eV above the valence band edge. INTRODUCTION Gallium nitride and more recently InGaN based high intensity blue light emitting diodes (LEDs) are of great interest considering their potential as devices that may find application in general purpose illumination. These devices are normally fabricated in AlGaN/(GaN/InGaN)/AlGaN heteroepitaxial films grown either on sapphire or 6H SiC substrates [1, 2]. A high density of crystallographic defects of the order of 108-1010/cm2 have been observed in these heteroepitaxial films [3]. Operation of these LEDs is apparently insensitive to the high level of defects present in the material used for their fabrication [4]. However, it has been demonstrated recently that pure edge dislocati
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