GaN targets LEDs and power transistors
Gallium nitride is a semiconductor compound that is being targeted for power transistors and LEDs in the commercial, defense, and aerospace sectors. It is making inroads into the areas traditionally held by GaAs in RF and microwave applications, particularly in terms of the switching, power supplies, and amplifiers. (It is currently not being used at millimeter-wave frequencies, which are still being served primarily by GaAs.)
It is also being used in the manufacture of light-emitting diodes (LEDs), and is particularly useful for violet (405nm) ones (which are used in Blu-ray systems). It can also be combined with aluminum and indium to create other color LEDs. The growth of single-crystal semiconductor hollow nanotubes would be advantageous in potential nanoscale electronics, optoelectronics and biochemical-sensing applications. Here we report an ‘epitaxial casting’ approach for the synthesis of single-crystal GaN nanotubes with inner diameters of 30–200 nm and wall thicknesses of 5–50 nm.
As compared to other semiconductor technologies, GaN is known for its high output power and high efficiency, which is particularly important for power amplifiers at high frequencies (like in mobile phones).
Who, Where, When?
Although the first GaN HEMT transistor was invented more than 15 years ago by Asif Khan, significant development efforts on practical power devices employing GaN-on silicon technology are fairly recent. Tremendous progress in this technology is expected over the next 10- 20 years, and in just five years figures of merit could improve by an order of magnitude.
TriQuint Semiconductor has been working with GaN since 2008, and its current market is mostly in the defense/aerospace business, but GaN is gaining traction in the commercial wireless infrastructure market, such as in base stations, repeaters, femtocells, and possibly WiMAX. TriQuint’s available products are GaN on silicon carbide. Enabling technology for GaN in the RF and microwave space includes 3D and thermal modeling.
Nitronex was spun out of NC State and produced its first GaN products in 2006 and recently announced that it had shipped more than 500,000 production GaN devices. Its main areas of applications are cable TV and military/aerospace. Their process is GaN on silicon. Other manufacturers working with GaN include NXP Semiconductors, Microsemi, and Toshiba.
Other universities working with GaN include the University of Florida, Carnegie Mellon University, and University of Notre Dame.
GaN is typically deposited on either silicon, silicon carbide, or sapphire. Combining superior electron mobility, high breakdown voltage and good thermal conductivity, it is particularly suitable for optoelectronics and advanced power semiconductors.
It is hard to grow gallium nitride on silicon, mainly because the materials expand and contract at very different rates, explains Colin Humphreys, a materials science researcher at Cambridge University. The process is carried out at temperatures around 1,000 °C, and, upon cooling, the gallium nitride cracks because it is under tension, Humphreys says. One way to solve the problem is to insert additional thin films around the gallium nitride to compress the material and balance out the tension produced during cooling. In fact, Humphreys and his colleagues have used this trick to make gallium-nitride LEDs on silicon; their devices produce 70 lumens per watt.
Critics of GaN point out the problems that occur when the material is exposed to high voltages. These so called ‘thermal issues’ are particularly challenging because they reduce the energy efficiency of GaN devices: the very property that makes them desirable for high-power applications. However, industry and university researchers are working on this. For instance, a team at NC State developed an argon implant technique that it claims improves performance enough to handle 10X as much power.
For LED applications to be successful, overcoming the effects of defects (slight dislocations in the crystalline structure) in GaN will be necessary. One process, invented by EE professor Salah Bedair and materials-science professor Nadia El-Masry, intentionally introduces voids into the GaN film near its interface with a sapphire substrate. As a result, the thousands of defects typically present are sucked into the voids thereby boosting the output of the devices using that purified film.
GaN technology still needs further refinement to also be economically competitive. To achieve this, inexpensive and efficient production methods for epitaxial deposition of GaN/(Al)GaN structures on larger-diameter silicon wafers are very promising.
M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, “Structural characterization of nonpolar (110) a-plane GaN thin films grown on (102) r-plane sapphire,” Appl. Phys. Lett. 81, 469 (2002); doi:10.1063/1.1493220.
GaN Strengthens Grip On Power, RF Design, June 10, 2011.
High-Power/High-Performance GaN resource page.
Turning 6-inch GaN LED manufacturing into reality. Jan 31, 2011.
- J. G. R. He, Y. Zhang, S. Lee, H. Yan, H. J. Choi, and P. Yang, Single-crystal gallium nitride nanotubes, Nature 422, p. 599–602, 2003.
- IR slashes GaN manufacturing costs, 21 September 2011.
- GaN Strengthens Grip On Power, 10 June 2011.
- Siltronic AG Joins imec’s GaN-on-Si Research Program to Develop Technology for Next-Gen Power Semiconductors and LEDs, 09 June 2011.
- A New Way to Churn Out Cheap LED Lighting, 21 March, 2011.
- New Technique Boosts High-Power Potential for Gallium Nitride Electronics, 2 February 2011.
- R. C. Johnson, GaN depo process said to make brighter LEDs, 25 January 2011.