Effect of current crowding and device structure on operation of gallium nitride-based light-emitting diodes

The photon recycling semiconductor (PRS) light emitting diode (LED) is a concept to produce both colorful and white LEDs. PRS LEDs have two or more active regions in one chip. Only the active region that emits in the shortest wavelength, which is called the primary active region, is injected with current. Other active regions are excited by the optical power emitted from the primary active region. The optimum wavelengths are analyzed and chosen for the dichromatic white PRS LED. The maximum luminous efficiency is calculated to be 336 lumens/Watt. The emission spectrum, color and color linearity of the device are characterized.; The current-crowding effect is analyzed both theoretically and experimentally for GaN/InGaN LEDs grown on insulating sapphire. The calculation yields an exponential decay of the current density under the p-type contact with a characteristic current spreading length. It is shown that a thick n-type cladding layer will alleviate the current crowding effect. The experimental results show good agreement with theory.; The optical output power of GaN/InGaN LEDs saturates when the injected current is beyond a certain magnitude. This effect is studied using pulsed current with different duty cycles. The current at which optical saturation occurs decreases as the duty cycle of the pulse current increases. This result suggests that heat generation is the major reason for the optical power saturation.; Based on the above study, different contact layouts are designed to reduce the effect of current crowding. The optical output of LEDs with a ring-shaped p-type contact is higher than LEDs with a square-shaped p-type contact. The more uniform current distribution and larger light extraction surface contribute to the optical output increase.; Additional experiments are performed with different mesa patterns. It is found that the optical power output of devices with interdigitated patterns is generally higher than devices with traditional square-shaped patterns. The optical power output increases as the contact finger width decreases. It is also found that the leakage current of the interdigitated pattern increases with the mesa perimeter. After passivation with a SiO₂ film, the leakage current decreases and the optical output and overall device resistance increase.