LED Technology
GaN-on-Si offers designers a bright future
Matthias Diephaus explains how recent advances in GaN-on-silicon process technology are enabling new families of white LEDs that support more competitively priced indoor and outdoor applications
D
esires for energy savings, reduced maintenance costs and enhanced performance – not to mention
international eco legislation – are key drivers behind a growing demand for LED lighting that, according to Lux Research, will reach $80 billion in 2020. General lighting applications such as
LED replacement lamps as well as indoor and outdoor luminaires typically call for white LEDs in tones ranging from warm white for residential environments to cool white for utilitarian applications such as high-bay lamps and streetlights. White LEDs are also in demand for automotive lighting such as headlamps and daytime running lamps, and also in display backlighting applications for
present a persuasive sales proposition offering a faster return on investment. Cost reductions can be sought across various aspects, from supply-chain and packaging issues to technical factors such as control electronics and the LED itself. In the case of the latter, cost reductions may be achieved through a combination of lower cost materials and processes, while also increasing the lumens generated per LED and driving up lumens-per-Watt efficacy.
LED construction White LEDs are typically manufactured using gallium-nitride (GaN) epitaxy on a substrate. Substrate characteristics such as high hardness, a high melting point
used in industrial or streetlights, or in the 2700K to 3500K range for warm white LEDs typically used in residential room- lighting.
Depending on package technology, the phosphor coated die may be attached to a thermally efficient substrate and a leadframe providing anode and cathode connections, and a reflector and optically clear
polycarbonate lens applied, as shown in Figure 1. Alternatively the die may be attached within the reflector cavity of a PLCC-type package and encapsulated using an epoxy or silicone material. Historically, sapphire wafer has been the substrate of choice for LED die fabrication. Production of GaN-on- sapphire LEDs has typically involved 2- inch or 4-inch wafers. More recently, technical barriers have been overcome allowing production of 6-inch sapphire wafers of sufficient quality to allow an adequate LED yield. Larger wafer sizes allow more die per wafer, so delivering economies of scale, and also enable higher area efficiency by reducing the effects of edge losses and unused space between individual die. However, other factors such as the cost of new equipment for processes such as etching and testing of 6-inch wafers can prevent manufacturers achieving rapid cost reductions.
Cost-down substrate technology A number of alternative substrate technologies have been developed, offering various improved characteristics. GaN-on-GaN and GaN on silicon carbide (GaN-on-SiC) have zero or very low mismatch between the substrate
material and the active LED structure, which allows increased efficiency. The use of pure silicon wafers, on the other hand, can deliver substantial savings, not only because silicon is just one-eighth the cost of a sapphire substrate (according to figures from Lux Research), but also because manufacturers can use proven semiconductor fabrication equipment and processes for wafers up to 8-inch (200mm). Eight-inch silicon wafers are readily available, while proven, cost-effective package technologies are also established. Figure 2 summarises the current status of GaN LED manufacturing on sapphire and silicon wafers at various sizes. Leading research agencies have published various forecasts predicting the likely effects of GaN-on-Si LED technology. In December 2013, for example, IMS Research found the market for GaN-on-Si wafers would grow at 69% CAGR to achieve more than 40% penetration by 2020. IMS compared the difficulty in producing large sapphire ingots with the abundant availability of low-cost silicon wafers in 200mm and larger sizes, and also cited the ready availability of equipment and processes supporting silicon wafers of 200mm and larger, as important factors that will drive GaN-on-Si market share.
GaN-on-Si roadmaps Toshiba has worked with lighting technology specialist Bridgelux to implement a process for fabricating GaN LEDs on 200mm silicon wafers that combines advanced Toshiba silicon process and manufacturing capabilities with Bridgelux crystal growth and LED
Figure 1: Phosphor coating determines emitted white-light characteristics. A clear lens or epoxy encapsulation is applied at packaging stage
devices ranging from mobiles to large- screen televisions.
Cost is a critical factor governing the adoption of LED lighting, particularly in residential and other consumer-oriented sectors. Replacement lamps and luminaires that are competitively priced, relative to alternatives such as incandescent or fluorescent lighting, can
38 February 2014
and high resistance to cracking and erosion are needed to withstand LED manufacturing processes. The raw LED chip emits light at predominantly blue wavelengths, which are converted by applying a phosphor coating engineered to emit white light at the desired colour temperature. This may be between 4000K and 6500K for cool white LEDs as
Components in Electronics
Figure 2: Larger wafer sizes enable lower cost per die, but are more difficult to produce in sapphire
www.cieonline.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48