Gallium Nitride on Silicon (GaN on Si) LEDs are anticipated to be the key to reducing LED pricing sufficiently to turn the tide in cost-sensitive residential markets. Ongoing research efforts are beginning to deliver results, while manufacturers using traditional GaN on sapphire materials are also succeeding in bringing costs down.
This article will provide an overview of the state-of-the-art in GaN on Si developments, and its potential, by taking advantage of high volume, larger, wafer-size, semiconductor-processing technology, to deliver lower production costs. In the UK, Plessey Semiconductors is one of the first producers of LEDs using GaN on Si wafers. Cree, meanwhile, is a pioneer in GaN on SiC materials for the LED market. Their latest TW (true white) series of LED bulbs is setting standards in the US.
We will review a select number of LED-based replacement bulbs and tubes from companies including Cree, ROHM, and LEDdynamics, targeted at the retrofit market in the residential sector.
Figure 1: Left: Cree’s latest TW 60 W bulb replacement uses GaN on SiC LEDs. Right: LEDdynamics’ EverLED TR range of LED replacement tubes incorporating Philips Lumileds Luxeon LEDs.
Commercial advantage vs. technical challenge
Gallium Nitride (GaN) has been commonly used for LEDs since the 1990s. Its hardness, heat capacity, and thermal conductivity properties were critical for the development of the high brightness, daylight visible, color, and white devices in widespread use today. Yet despite the compelling features of LEDs, unit costs remain a barrier in some consumer sectors. The huge attraction of developing Gallium Nitride on Silicon substrates for LED chips is to reduce production costs. According to market research company, Lux Research, in a recent report,¹ material costs of GaN on Si represent just one-eighth of those of traditional sapphire substrates.
The cost benefit arises through the ability to use larger wafer sizes, thereby increasing the yield per wafer. This is typically 6 inch, 8 inch, or even 12 inch wafers, compared to 4 inch for sapphire-based substrates, although 6 inch sapphire substrate wafers are now feasible. Further, by using fully-equipped and already depreciated silicon foundries left behind by the fast moving integrated circuit market, production costs can theoretically be reduced.
Many major LED manufacturers have recognized the potential of GaN on Si technology to reduce production costs. Companies including Osram, ROHM, Toshiba (through their acquisition of Bridgelux assets), and Plessey Semiconductors are now beginning to make GaN on Si LED chips commercially available. However, according to Lux, GaN on sapphire remains the material of choice for 90% of LEDs manufactured today. For the future, Lux predicts that the market for epitaxial wafers will grow to reach $4 billion by 2020, of which GaN on Si wafers will achieve 10%.
The Lux report states that there are technical challenges to overcome for GaN on Si, predominantly due to the physical mismatch between silicon and GaN crystal lattices. This mismatch, as well as the dynamics of wafer size, yield and cost, are why Lux believes the market for GaN on Si will be limited, and that the technology may even miss out on the market opportunity.
The GaN on Si mismatch is recognized as more problematic than that experienced between GaN and sapphire substrates, which in turn is more significant than that found with GaN on Silicon Carbide (SiC) technology. SiC, as promoted by Cree, is expected to take an 18% market share by 2020.
Newcomer Soraa is pioneering GaN on GaN technology that eliminates the mismatch problem altogether, though GaN on GaN does not offer the cost advantage potential that GaN on Si promises. More on Soraa’s approach is covered in a separate article,² while more on the so-called ‘substrate wars’ can be found in another TechZone article.³
Material lead
In the UK, Plessey Semiconductors is taking a lead in developing high-brightness LEDs based on proprietary GaN on Si technology, acquired in 2011 from University of Cambridge spin-out, CamGaN. Using 6 inch silicon wafers at its foundry in Plymouth, UK (with an R&D path to 8 inch wafers), Plessey is perfecting the technique of using much thinner GaN layers, just 2 µm thick on the buffer and epilayer only, compared to 6 to 8 µm other GaN on Si companies are using. The company claims to be able to match industry-standard performance at a dramatically-reduced cost of manufacture, in order to meet its goal of driving an affordable switch from standard 60 W bulbs to LED lamps.
Cost advantages are gained by using the larger 6 inch silicon wafers, which are significantly less expensive than alternatives, plus there are a greater number of die per wafer and reduced scrap. Thinner layers allow less deposition time and more batches per day.
Details of Plessey’s first GaN on Si LEDs, the Magic PL111010 and PLB010050 series, are now finalized and available, and the PL111010 is available in production quantities. Advanced product information on another eight or so devices is also available. The company has reported that it will soon be offering LEDs in the 60 to 70 lm/W range, as well as devices for incandescent lamp replacements.
Toshiba claims to be migrating from GaN on sapphire to GaN on silicon, with the aim of providing a cost-competitive alternative to current LEDs for general-purpose applications. The key to reduced costs, according to Toshiba, is the ability to manufacture the LEDs on lower cost, 200 mm (8 in) silicon wafers using existing fabrication facilities. Following their partnership with Bridgelux for their GaN on Si technology, Toshiba rapidly began announcing new devices. The first parts in their new Leteras range of devices moved into mass production in November last year (2013).
This included the 1 W TL1F2 LEDs, at the heart of the range, which feature high luminous efficacy and a full, correlated color temperature range from 2700 to 6500K, with minimum CRI values of 70 or 80. Comparative luminous efficiency together with low and uniform forward voltage are claimed to be the key to demonstrating that the substrate mismatch challenge has been largely overcome. Typical luminous flux of the 1 W LEDs ranges from 104 to 235 lumen depending on color temperature and CRI. Additional families, specifically targeting indoor, retrofit applications include the 0.6 to 0.9 W TL3GA, initially available in 4000 and 5000K CCT and 80 CRI versions, and the 0.2 to 0.5 W TL2FK series offered across the 2700 to 6500K range.
Another European development from German company Azzurro, uses a buffer layer between the silicon substrate and GaN to overcome the mismatch problem. Its template wafer can then be made available to LED manufacturers to use to produce their own LED designs. Collaboration with Osram Semiconductor as well as companies in the Far East is likely to yield GaN on Si LEDs in the near future.
Figure 2: Comparison of the GaN on sapphire and Azzurro’s GaN on Si approaches.
Replacing incandescents
Meanwhile, the market for LED-based replacements for standard domestic incandescent bulbs is highly competitive and prices are falling, irrespective of the material technology used. Quality, however, is an issue, as some of the cheapest models available in some markets do not match the performance and quality standards that consumers expect.
In recent years, LED technology has improved and manufacturers have worked hard to demonstrate that LED replacements not only match (or exceed) the quality and performance of incumbent technologies, but offer additional, desirable qualities. As a result, a number of factors are likely to contribute towards convincing the public to switch to LED lamps. These include: improved CRI, now beyond 90 in some models; dimmability and improved omnidirectional light output; the availability of high brightness (100 W equivalents) lamps; and the ability to select color temperature from warm (2700K), similar to an incandescent, to a cooler 5000K ‘daylight’ appearance.
However, manufacturers are aware that the higher unit cost generally remains the key limiting factor to mass adoption. Prices vary considerably, from around $13 to well in excess of $120 for an LED bulb, compared to less than $2 for a conventional bulb. To justify the premium price, most vendors are quoting overall cost savings over the lifetime of the bulb. This is often in the region from $100 to more than $300, dependent on the local cost of electricity. An increasing number of consumers are now beginning to understand and accept the ‘lifetime cost savings’ arguments, but not enough. In response, the LED industry is setting itself a target to reduce prices to $10 for a standard 60 W replacement, in order to stimulate mass adoption.
3D for omnidirectional light
Cree, having developed an extensive LED chip, module, and lamp business based on GaN on SiC technology, has recently announced a range of TW (true white) LED bulbs, designed as direct replacements for 40 W and 60 W incandescents. These soft-white bulbs are rated at 8.5 W (450 lumens) and 13.5 W (800 lumens) respectively, with a color temperature of 2700K, a CRI of 93, and a claimed lifetime saving of $128.
Figure 3: The inside of Cree’s TW series 60 W LED replacement bulb showing the filament tower.
At the heart of Cree’s TW series 60 W replacement bulb is a filament tower containing 20 LED modules (10 in the 40 W replacement version) to provide an omnidirectional light, comparable to traditional bulbs (see Figure 3 above). The other key differentiator with the TW series is the incorporation of neodymium in the glass (giving it the blue appearance seen in Figure 1), which acts as a notch (spectral) filter, filtering out excess yellow and making the light warmer.
Interestingly, Cree also sells LEDs for other OEMs to use in their fixtures. One of the ranges of devices they use in their own bulb replacements, such as the one shown above, is the XT-E series. The XT-E HVW high-voltage LED features a CCT of 2700K for a soft white light, 12 V forward voltage, and an output of 92 lm/W. Other devices in the family cover CCTs up to 6200K, with light output ranging from 67 to 105 lm/W, and forward voltage of 12, 24 or 48 V, creating a range of applications including daylight bulbs and spotlight replacements.
Last year, Cree introduced new members of their CXA family of chip-on-board (COB) LED arrays, also destined for retrofit lamp applications. The CXA1304 range delivers up to 1034 lumens, with a typical efficacy of 102 lm/W. CCT range is from 2700 to 5000K. With a smaller, 6 mm light-emitting surface, it is ideal for tighter-beamed downlights and small reflector bulbs. A typical device in the range, the CXA1304-0000-000C00A427F for a warm white light (2700K), is specified at 368 lumens with a forward voltage of 9 V.
The 12 mm CXA1816 is destined for replacements for 70 W ceramic metal halide spotlights, as well as smaller luminaires and retrofit lamps. This range features devices offering up to 3000 lumens, with versions covering 2700 to 5000K color temperature. The CXA1816-0000-000N00Q2250F is a typical example of a cool white, 5000K device with a light output of 2180 lumens and a forward voltage of 37 V.
ROHM Semiconductor has also adopted the 3D approach to assembling the LEDs inside the light bulb housing, in order to generate a diffused light similar to that emanated by standard bulbs (see Figure 4 below). The company offers models to provide either a daylight white or a warmer light to mimic incandescent bulbs. The 6 W R-B10N1 for daylight white applications features a luminous flux of 390 lumens and a forward voltage of 100 VAC. The 6 W R-B10L1 incandescent version features a light output of 330 lm.
Figure 4: Polar graph of luminous lighting intensity for the 3D LED module within ROHM’s R-B10N1 light bulb.
Fluorescent going obsolete
The replacement market for fluorescent tubes is potentially even larger than for incandescent bulbs as they are so widely used in industrial and commercial spaces, as well as in residential applications, such as garages and workshops. The economic case for LED replacements is comparable, once consumers start to look at the lifetime costs rather than just the unit costs. Again, there is a huge variability in pricing. Standard T8 fluorescent tubes can cost less than $5. LED replacements cost from $35, predominantly for cheap imports from China, to some units priced at more than $100, a considerable premium to pay.
Some companies quote a payback period of less than 2 years in terms of energy savings, longer lifetime, and reduced maintenance costs, especially for larger, 24/7 applications. Clearly here, there is a significant opportunity for lower-cost GaN on Si LEDs to make a price difference, and stimulate a faster switch to energy-saving LED versions.
Other operational benefits of LED lamps could potentially make a strong impact in the fluorescent tube replacement market. That LED tubes turn on and off instantly with no flicker and no buzz are key attractions. Other factors such as color consistency and choice of color temperature also appeal. For the ecology-conscious consumers, the choice of a lamp that contains no hazardous waste (lead, mercury, or gas) is important.
A number of manufacturers have developed replacement tubes that can be simply fitted into the popular T8/T12 fixtures, while others have developed new fixtures for customers requiring new or upgraded lighting solutions. Companies including Cree and Osram not only produce the retrofit tubes, but also the LEDs for other OEMs to produce units for local markets.
Figure 5: LEDdynamics EverLED TR plug-in replacements for fluorescent tubes.
One of the first available LED plug-in fluorescent tube replacements came some years ago from LEDdynamics with their EverLED-TR and EverLED-VE ranges. Claimed to be compatible with virtually all standard fluorescent ballasts, including emergency back-up ballasts, it can be fitted to easily replace the existing tube. Rated at 20 W, the TR-range 48 inch T8 tubes are available in 3000K (ET8S48830), 4000K (ET8S48840), and 5000K (ET8S48750) color temperatures and feature a typical light output of 1600 to 2000 lumens, with light efficacy of up to 100 lm/W. CRI is quoted at 85 for the 3000 and 4000K units and 75 for the 5000K. The US-manufactured TR-range units cost around $113 each, and are guaranteed for 10 years of operation.
Lower-cost versions, at around $69 each, are claimed to deliver the same quality and rugged construction, expected to last 10 years, but are guaranteed for 5 years. The ET8V48830, ET8V48840, and ET8V48750 are the equivalent warm, neutral, and cool white versions.
A wide choice of LED fluorescent tube replacement lamps is available from ROHM, with models ranging from 20 W to 110 W, as well as waterproof versions, to suit a broad scope of applications. The 40 W R-FAC40BN1 replaces a standard 48 inch tube, offering a CCT of 5000K to provide a cool white light. Luminous flux is quoted at 1980 lm and input voltage is 100 to 242 VAC. With just 11 W power consumption, the lower-cost R-FAC20BN1 offers comparable features, though with a lower luminous flux of 890 lm. Lifespan of these tubes is quoted at 40,000 hours, equivalent to more than 9 years at 12 hours per day.
Summary
Lower-cost LEDs are required to bring down the prices of LED-based standard replacements for traditional incandescent light bulbs used widely in residential markets, as well as fluorescent tubes used even more extensively throughout the industrial, commercial, and residential sectors. GaN on Si is believed by many to be the way forward, although there are technical challenges to overcome.
Several of the major LED manufacturers are continuing to develop and perfect their GaN on Si processes, and production volumes of lower-cost, mass-produced devices are now becoming readily available.
The race is on to see whether GaN on Si will deliver lower-cost LEDs while maintaining performance, before the dominating GaN on sapphire technology succeeds in reducing its costs sufficiently to capture these huge potential retrofit markets first.
References:
- Lux Research state of the market report
- Digi-Key TechZone article: New Material Technologies Deliver Improved LED Replacements for the Popular MR16 Light Bulb
- Digi-Key TechZone article: Will Silicon Substrates Push LED Lighting Into the Mainstream?
