Estimation of Junction Temperature in Operating Light Emitting Diodes
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0955-I15-13
ESTIMATION OF JUNCTION TEMPERATURE IN OPERATING LIGHT EMITTING DIODES Md. Shahrukh Sakhawat1, Arindra N Guha1, Okechukwu Akpa1, Ping Z Hagler2, Dake Wang2, Minseo Park2, and Kalyankumar Das1 1 Electrical Engineering Department, Tuskegee University, Tuskegee, AL, 36088 2 Physics Department, Auburn University, Auburn, AL, 36830 ABSTRACT Light emitting diodes (LEDs) are normally operated at high current levels resulting in substantial junction heating. However, the junction temperature cannot be measured directly. In the study reported here, it was estimated junction temperature in LEDs with peak emission 400 nm. Temperature was estimated from current-voltage (I-V) measurements as a function temperature and peak-shift of optical emission spectra with increasing temperature. For a diode operated at a current-level of 100 mA, a temperature of 100 oC was estimate from current voltage measurement and 160 oC from the peak-shift in the optical emission spectra. The difference in temperature estimated using the two different technique is not understood at this point. INTRODUCTION High intensity LEDs are normally operated at a high current-level resulting in a substantial rise in junction temperature [1]. However, junction temperature cannot be measured directly, although knowledge of the operating temperature is important for appropriate selection of packaging materials and suitable heat-sinking. It has been attempted to estimate junction temperature in LEDs, emitting at 400 nm, at high operating currents from current-voltage (I-V) measurements as a function of temperature and peak-shift in optical emission spectra with increasing temperature. The forward voltage, Vf, of a diode for a constant current decreases with increasing temperature. Thus from I-V measurements, dVf/dT can be obtained by measuring Vf, as a function of temperature while maintaining current constant. With an experimentally determined value for dVf/dT, an unknown junction temperature can be determined from the initial and final values of forward voltage (when a steady state junction temperature has been reached) Vf1 and Vf2, respectively. The steady state forward voltage, Vf2, for given current can be given by, Vf2 = (dVf/dT)∆T + Vf1,
……(1)
The rise in temperature, ∆T, can be given as,
∆T = (Vf1 - Vf2 )/ (dVf/dT),
……(2)
(dVf/dT) being a negative quantity, the junction temperature, Tj, will be given by, Tj = ∆T + room temperature.
……(3)
Optical emission studies indicate the peak of the LED emission spectra from the LED at a high operating currents occurs at longer wavelengths, indicating that radiative transitions occur across narrower bandgaps. The narrowing of the bandgap takes place due to junction heating at a high current. Dependence of bandgap with temperature is given by, E g (T ) = E g (0) +
αT 2 β +T
……(4)
where, Eg (T) and Eg(0) are the bandaps at temperatures T and 0 K, respectively, α and β are Varshni parameters, for GaN α is 0.77 meV/K2 and β is 600 K [2]. In the present study, Eg (T) and Eg(0) were taken as at the highe
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