LEDs ♦ news digest
injected electron for multiple emissions rather than a single emission process as in conventional LEDs.
Therefore, higher radiative output power can be obtained at lower current levels and efficiency loss due to droop can be minimised. The design can be applied to all existing nitride emitters regardless of energy of the emission.
To show the feasibility of cascading GaN emitters, the authors demonstrate devices using multiple (1, 2 and 4) epitaxially cascaded p-n junctions with gadolinium nitride (GdN) visible wavelength transparent tunnel junctions by plasma assisted MBE.
All of the devices have n-type GaN on top and bottom layers device since tunnel junctions eliminates the need for a p-type contact, as shown in Figure 1.
Figure 2: The change in wall plug efficiency of the modeled commercial LED (N=1) and cascaded LEDs with N=5, N=20, and N=50
The enhancement is not only due to superior external quantum efficiency, but also suppression of joule heating. Since the LED is operated at higher voltage and lower current, resistive losses are lower.
This work is described in detail in the paper, “Tunneling- based carrier regeneration in cascaded GaN light emitting diodes to overcome efficiency droop,” by Fatih Akyol in Applied Physics Letters, 103, 081107 (2013).
http://dx.doi.org/10.1063/1.4819737
Figure 1: Epitaxial design of the cascaded p-n junctions
As the p-n junctions forward biased, tunnel junctions get reverse biased. Electrons from valance band of the p-type layer tunnel into conduction band of n-type layer, leaving a hole behind, in p-type layer. The carriers generated in this process get injected into p-n junction diode regions, thus tunnel junctions work as carrier regeneration centres, supplying majority carriers to device active regions.
The cascaded diode structures showed rectifying behaviour. Diode turn-on voltage increased with N-repeats of the device sections, as expected. Analysis of series resistances of the 100µm2 devices leads to a very low resistance ~ 5x10-4 Ω-cm2 per tunnel junction.
Using the performance parameters of these tunnel junctions, the authors calculated the characteristics of LEDs designed with multiple cascaded junctions, with each junction simulating the characteristics of a commercial LED.
The calculation shows that as the number of the cascaded LEDs increases, efficiency droop is greatly reduced, and the wall plug efficiency of a conventional
Intematix awarded GAL phosphor patents for LEDs
When combined with red nitride or other red phosphors, arrangements also covered by these patents, CRIs up to 98 (out of 100) have been demonstrated. The firm’s latest technology is also instrumental to meeting new standards such as California’s Quality LED Lamp Specification
Intematix Corporation has had patents covering its green and yellow GAL phosphor products and certain LuAG phosphors granted by the United States Patent and Trademark Office.
Patent numbers 8,529,791 and 8,475,683 regarding green and yellow emitting, garnet-based phosphors used in lighting and display applications were issued on September 10th, 2013 and July 2nd, 2013, respectively.
Intematix GAL phosphors enable exceptional performance for LED lighting compared to other options
October 2013
www.compoundsemiconductor.net 71
LED can be boosted at elevated powers, as depicted in Figure 2 below.
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