Technical Insight
LED tunnel junctions spread the charge (Research Review)
One of the limitations of conventional GaN-based LEDs is the low conductivity of p-type GaN:Mg used as the top contact layer. A large area p-type ohmic contact is required to spread current from the p-bonding pad across the junction area to prevent current crowding in the high-resistivity GaN:Mg. This electrode must be semi transparent for maximum light emission. A group at Chonbuk National University, Korea, have published a method to facilitate lateral current spreading by reducing lateral hole conduction (Appl. Phys. Lett. 2001 78(21) 3265). This involves using buried tunnel contact junctions on the p-side. A reverse biased tunnel junction (TJ) supplies holes to the p-type crystal above the active region by lateral electron currents. This allows the use of a low resistivity n-type GaN contact layer that further facilitates current spreading, and also the use of an n-type ohmic contact. Device fabrication is simplified as only n-type ohmic contacts are needed, resulting in a single metalization process. The TJ concept has been demonstrated for GaAs-based edge-emitters and VCSELs, but this is the first reported use for GaN-based LEDs. At 4.9 V (20 mA), the operating voltage of a TJ-LED is higher than that of a conventional GaN-based LED (4.5 V at 20 mA). This is as a result of a series resistance across the tunnel junction. However, the light output of the TJ-LED is twice that of a conventional LED. This is because there is no need for a large metal contact to spread current, and light emission occurs across the whole junction area because of the low resistivity n-type contact layer. shows the blue light emission at 5, 20, and 100 mA for (a) a conventional LED without a semi-transparent electrode and (b) a TJ-LED. At 100 mA operation, the emission is slightly more intense around the electrode. This is the result of current crowding in the n-type GaN contact layer. The uniformity of the radiance was further increased in the TJ-LED by increasing the doping level of the n-type GaN to improve its conductivity.
