News Article

Epitaxial Cascading Of Nitride LEDs Overcomes Efficiency Droop

As the number of the cascaded LEDs increases, efficiency droop is greatly reduced
Researchers at the Ohio State University have experimentally demonstrated epitaxial cascading of multiple p-n junction diodes with low series resistance.

They have shown that cascading multiple LEDs circumvents nitride LED efficiency droop and reduces overall joule heating.

Efficiency droop in GaN LEDs is one of the major roadblocks to widespread adoption of solid state lighting. In the last decade there has been extensive work on identifying and overcoming the nitride LED efficiency droop. But the underlying reason is still under debate and no designs has been completely successful in solving the problem. It is possible that material limitations of III-nitride material system may preclude complete elimination in traditional single active region LED structures.

Cascading multiple LEDs pushes the input power of the peak efficiency to higher values by exploiting each 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 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 LED can be boosted at elevated powers, as depicted in Figure 2 below.

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).


AngelTech Live III: Join us on 12 April 2021!

AngelTech Live III will be broadcast on 12 April 2021, 10am BST, rebroadcast on 14 April (10am CTT) and 16 April (10am PST) and will feature online versions of the market-leading physical events: CS International and PIC International PLUS a brand new Silicon Semiconductor International Track!

Thanks to the great diversity of the semiconductor industry, we are always chasing new markets and developing a range of exciting technologies.

2021 is no different. Over the last few months interest in deep-UV LEDs has rocketed, due to its capability to disinfect and sanitise areas and combat Covid-19. We shall consider a roadmap for this device, along with technologies for boosting its output.

We shall also look at microLEDs, a display with many wonderful attributes, identifying processes for handling the mass transfer of tiny emitters that hold the key to commercialisation of this technology.

We shall also discuss electrification of transportation, underpinned by wide bandgap power electronics and supported by blue lasers that are ideal for processing copper.

Additional areas we will cover include the development of GaN ICs, to improve the reach of power electronics; the great strides that have been made with gallium oxide; and a look at new materials, such as cubic GaN and AlScN.

Having attracted 1500 delegates over the last 2 online summits, the 3rd event promises to be even bigger and better – with 3 interactive sessions over 1 day and will once again prove to be a key event across the semiconductor and photonic integrated circuits calendar.

So make sure you sign up today and discover the latest cutting edge developments across the compound semiconductor and integrated photonics value chain.



Search the news archive

To close this popup you can press escape or click the close icon.
Register - Step 1

You may choose to subscribe to the Compound Semiconductor Magazine, the Compound Semiconductor Newsletter, or both. You may also request additional information if required, before submitting your application.

Please subscribe me to:


You chose the industry type of "Other"

Please enter the industry that you work in:
Please enter the industry that you work in:
Live Event