news digest ♦ LEDs
Cao’s team is now trying to find ways to create similar thin films in which each atomic layer is made of a different material. Cao is also working to create field- effect transistors and LEDs using the technique. Cao has filed a patent on the new technique.
Verticle reveals copper-based hexagonal UV-LED chip
The firm has extended its product line in gallium nitride (GaN) based UV-LEDs
Verticle’s new hexagonal shaped UV chip incorporates a copper substrate to enable a higher current.
Despite many advantages, the main drawback of UV-LEDs is lower optical power due to lower internal quantum efficiency (IQE).
Since improving the IQE of UV epitaxial wafers is a long-term prospect, an alternative way to boost UV-LED power is to drive more current in.
However, heat generation is a big issue with high current drive. What’s more, aluminium containing UV epitaxial wafers are more resistive than blue, meaning thermal management of UV-LEDs is more critical. This makes one of the main challenges lowering the junction temperature in UV-LEDs.
UV light emitting from the chip
In order to operate at higher current injection conditions and dissipating heat more efficiently, Verticle’s UV-LED chip uses a copper substrate. According to the firm, as shown in the figure below, its copper based vertical chip’s thermal resistance (Rth) is 2K/W lower than for a GaN- on-silicon vertical chip.
Thermal resistance and junction temperature of GaN-on- silicon and GaN-on-copper vertical LEDs as a function of injection current (at 250C).
As a result, Verticle maintains that the junction temperature (Tj) of GaN-on-copper is lower than for GaN-on-silicon. Also, the junction temperature difference is 20C at 350 mA current injection between two chips. And the difference becomes larger (60C) at a 1A current injection.
What’s more, Verticle claims its UV-LED chip can be driven at a higher current than other vertical chips constructed with different substrate materials.
Verticle also says its GaN-on-copper UV-LED chips do not saturate over 1A, while GaN-on-tungsten and GaN- on-sapphire UV-LED chips start to saturate after 500mA. This means that the GaN-on-copper chips have a higher heat dissipation capability compared to GaN on other substrates.
An SEM image of the hexagonal UV-LED chip
This could mean that copper based UV-LED chips have distinct advantages for applications where high current injection and good thermal properties are required.
A graph illustrating the firm’s hexagonal UV-LED chip performsnce is shown below. Data was taken for a 45 mm size chip with radiant fluxes measured at 416mW at 350mA current injection, 787mW at 700mA, and 1.025mW at 1A for the 392 nm wavelength, respectively.
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www.compoundsemiconductor.net June 2013
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