Electric properties of GaN light-emitting diodes

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Abstract
The mechanisms of electrical conduction processes in MIS GaN : Zn, yellow and green light-emitting diodes (LED’s), are analyzed by considering the I-V characteristics at room temperature and nitrogen temperatures and the temperature dependences of currents through the diodes with a constant forward bias. The diode current transport mechanisms are different between the two temperature regions, lower or higher than 150 K. The forward conduction current I is the sum of two contributions: I=I1+I2, where I1∝V1/2 exp(λV) and I2∝V exp(λ′V). At temperatures lower than 150 K, I1 dominates the current conduction controlled by tunnel-induced impact ionization. The electrons tunnel through a triangular potential barrier at the i-n junction. At temperatures higher than 150 K, I2 dominates as a result of electron thermal excitation, tunneling, and impact ionization. The electron thermal excitation occurs from the conduction band of the n layer to shallow donors of the i layer at the i-n junction. The thermal activation energy for the excitation depends on the Zn concentration in the i layer. The activation energies observed are 100 meV for the yellow LED and 14 meV for the green LED. From these activation energies and the donor binding energy, the Fermi levels of the i layers for the yellow and green diodes are estimated to locate 0.13–0.14 and 0.04–0.05 eV below the conduction band, respectively.

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