The forward current-voltage (I-V) characteristics of a blue GaInN, a green GaInN, and a red AlGaInP LED are shown in Fig. 12.19. The forward turn-on voltage scales with the emission energy, indicating a well-behaved characteristic. Closer inspection of the forward voltage (at 1 mA) of the green LED (Vf, green = 2.65 V) indicates that it is very similar to the blue LED (Vf, blue = 2.75 V) even though the emission energies of the blue and green LED are quite different (Xblue = 470 nm, hvblue = 2.64 eV; Xgreen = 525 nm, hvgreen = 2.36 eV). The small difference in forward voltage indicates that carriers probably lose energy by phonon emission when injected from the GaN barrier into the GaInN active region. They lose more energy when being injected from the GaN barrier into the In-richer active region of the green LED. The energy, dissipated by emission of phonons, is supplied by the external voltage applied to the LED.

The diode series resistance can be inferred from the slope of the I-V characteristics. The blue and green diodes have a higher series resistance than the red AlGaInP diode. The larger resistance in GaInN LEDs can be attributed to several factors including the “lateral” resistance in the n-type buffer layer for devices grown on sapphire substrates, strong polarization effects occurring in the nitride material family, lower p-type conductivity in the cladding layer, and higher p-type contact resistance. The lower p-type conductivity is due to the high acceptor activation energy (approximately 200 meV) in GaN and GaInN so that only a small fraction of acceptors is activated.

The temperature dependence of the forward voltage at a diode current of 30 mA is shown in Fig. 12.20. For all diodes shown, the forward voltage decreases as the temperature is increased. The decrease in forward voltage is due to the decrease of the bandgap energy. In the blue and green GaInN diodes, the lower forward voltage is also due to the decrease in series resistance occurring at high temperatures. This resistance decrease is due to the higher acceptor activation occurring at elevated temperatures and the resulting higher conductivity of the p-type GaN and GaInN layers.