industry  LEDs


Plessey slashes LED costs


High-quality growth of LEDs on silicon is notoriously difficult, with stresses and strains causing wafers to bow. But these problems are not insurmountable: Plessey Semiconductors is now churning out GaN-on-silicon LED chips from flat epiwafers using a recipe developed at the University of Cambridge. Richard Stevenson reports.


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ou’ll need deep pockets to buy an LED bulb that can replace a 60 W incandescent. If you insist on the best that money can buy, you’ll go in search of the Philips bulb that won the Department of Energy’s Lighting Prize, and have to fork out $30. If you are willing to sacrifice efficiency and colour quality, you’ll be able to trim a few dollars off your outlay, but you must still expect to pay over $20.


Retailing for these prices makes the 60 W-equivalent LED bulb the preserve of the early adopter. Claims of 15 year lifetimes and


efficiencies exceeding those of the compact fluorescent are undeniably attractive, but most people are not going to get their wallet out until prices fall substantially – maybe to $10 or less.


Bulb prices will tumble when LED chips are made more cheaply, because these devices account for a very high proportion of the cost of the bulb. According to the most recent Solid-State Lighting Manufacturing Roadmap that was published in August 2012, in 2011 the LED package accounted for more than half of the total manufacturing cost of the bulb.


One of the most promising options for trimming the costs of LED manufacture is to switch substrates, replacing sapphire with silicon. This platform is not just cheaper; it allows wafer processing on lines that were installed many years ago for making silicon-based products, so there is the opportunity to either create LED production lines with minimal capital expenditure, or to outsource LED manufacture at very competitive rates.


Savings associated with making LEDs on silicon are well known within the industry, with several companies trying to develop technologies for producing this type of device. Lattice Power from China has arguably led the way, bringing to market the first white- emitting, GaN-on-silicon LEDs, which were made on 2-inch GaN substrates. It is developing a process for 6-inch silicon wafers, which should enter production this year.


An X-ray diffraction tool scrutinizes the crystal quality of the LED epiwafers


28 www.compoundsemiconductor.net January / February 2013


Meanwhile, the Californian outfit Bridgelux has developed warm- white LEDs in it labs that deliver 125 lm/W at a drive current of 350 mA, and its technology is now being used by Toshiba to make devices on 200 mm silicon. In addition, the likes of Samsung, Osram, Azzurro and Plessey Semiconductors are developing technologies for GaN-on-silicon LEDs on 6-inch or 200 mm silicon substrates.


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