Chapter 6: Junction and carrier temperature (click on figure for full-size image)

Fig. 6.1. Carrier temperatures in (a) GaInN blue and (b) AlGaInP red LEDs inferred from the high-energy slope of emission spectrum. Due to the alloy-broadening effect, the measured carrier temperatures overestimate the true carrier temperature (after Chhajed et al., 2004; Gessmann et al., 2003). Fig. 6.2. (a) Peak emssion wavelength versus oven temperatureof an AlGaN UV LED for pulsed current injection with 0.1 % duty cycle . (b) Emssion spectra and junction temperatures for different DC currents (after Xi et al., 2004).
Fig. 6.3. Junction temperature inferred from emission peak energy as a function of DC injection current for a 1 mm 1 mm deep UV LED emitting at 295 nm. The error bar stems from an uncertainty in the peak energy (after Xi et al., 2004). Fig. 6.4. Fundamental bandgap energy of GaN, GaP, GaAs, InP, Si, and Ge as a function of temperature. The bandgap energy is approximated by the Varshni formula which uses the fitting parameters alpha and beta (from data compiled by Ioffe, 2004).
Fig. 6.5. Current-voltage characteristic of GaAsP/GaAs LED emitting in the red part of the visible spectrum, measured at 77 and 295K. The threshold voltages are 2.0 and 1.6 V, at 77 and 300 K, respectively. Fig. 6.6. (a) Pulsed calibration procedure establishing the forward voltage versus junction temperature (Vf vs. Tj) relation and (b) determination of junction tempera- ture for different DC forward currents.
Fig. 6.7. (a) Pulsed calibration measurement (duty cycle 0.1 %) and (b) junction temperature (Tj) versus DC current of an AlGaN UV LED (after Xi et al., 2004). Fig. 6.8. Junction and carrier temperature of devices packaged in conventional 5mm packages as a function of DC injection current. The measured carrier temperature over-estimates the true carrier temperature due to alloy broadening (after Chhajed et al., 2004).
Fig. 6.9. LED drive circuit with series resistance Rs. The intersection between the diode I-V characteristics and the load lines are the points of operation. Small series resistances result in an increased diode current at high temperatures, thus allowing for compensation of a lower LED radiative efficiency.