Emerging wide bandgap (WBG) semiconductors hold the potential to revolutionize the electronics world, promising to advance the global industry in much the same way as the invention of the silicon (Si) chip over 50 years ago enabled the modern computer era. The electronic bandgap is what allows semiconductor devices to switch currents on and off to achieve a desired electrical function, and WBG materials, the category of electronic materials in which the bandgap energy exceeds approximately 2 electronvolts (eV), exhibit characteristics and processes that make them superior to Si for many applications. The most mature and developed WBG materials to date are silicon carbide (SiC) and gallium nitride (GaN), which possess bandgaps of 3.3 eV and 3.4 eV respectively, whereas Si has a bandgap of 1.1eV. SiC and GaN devices are starting to become more commercially available. Smaller, faster, and more efficient than counterpart Si-based components, these WBG devices also offer greater expected reliability in tougher operating conditions.
Advantages of WBG semiconductors over Si in power electronics include lower losses for higher efficiency, higher switching frequencies for more compact designs, higher operating temperature (far beyond 150° C, the approximate maximum of Si), robustness in harsh environments, and high breakdown voltages. Diverse applications range from industrial functions, such as motor drives and power supplies, to automotive and transportation systems including hybrid and electric vehicles, aircraft, ships, and traction, to wireless communications, military systems, space programs, and clean energy generation from solar inverters and wind turbines.
A characteristic of WBG materials is the ability to emit light in the visible spectrum. This has allowed innovation in the solid-state lighting industry, where developments in WBG-based light emitting diodes (LEDs) have resulted in devices with greater lighting efficiency and much longer lifetimes than incandescent bulbs. In fact, LEDs provide light output on the order 160 lumens per watt, with a service life between 35,000 to 50,000 hours, while incandescent lights provide less than 20 lumens per watt and last between 1,000 to 2,000 hours. WBG technology is also used in laser diodes, with next generation DVD players, including Blu-ray and HD DVD formats, employing GaN-based blue lasers.
SiC is by far the most mature WBG technology, attributable in part to its excellent thermal conductivity. Due to SiC’s ability to effectively transfer generated heat away from itself, SiC is especially well suited for the highest power applications such as photovoltaic systems and wind turbines, as well as high temperature operating environments like down-hole drilling where temperatures can exceed 200° C. GaN is popular as a more cost-effective alternative to SiC, but given that today’s GaN is bonded over a Si or SiC substrate as opposed to grown on a bulk-GaN substrate for cost reasons, GaN is not as thermally conductive as SiC or even standard Si. The WBG benefits of GaN, such as high voltage operation, high switching frequencies, and outstanding reliability - coupled with expectations that GaN will reach price parity with Si equivalents by 2015, keep it as a front-running choice for power electronics up to 900 V, as well as a superb choice for next generation consumer electronics, where size, efficiency, and price greatly matter.
The power electronics industry is ushering in a new era marked by the emerging availability of wide bandgap (WBG) semiconductors. With power device innovations in conventional silicon (Si) nearly reaching their theoretical limits and the new WBG materials offering important advantages over Si, the power electronics industry is heralding opportunities previously not thought possible, as well as anticipating significant improvement in existing applications.
The advantages of SiC over Si for power devices include lower losses for higher efficiency, higher switching frequencies for more compact designs, robustness in harsh environments, and high breakdown voltages. SiC also exhibits significantly higher thermal conductivity than Si, with temperature having little influence on its switching and thermal characteristics. This allows operation of SiC devices in temperatures far beyond 150° C, the maximum operating temperature of Si, as well as a reduction in thermal management requirements for lower cost and smaller form factors.
SiC is the most maturely developed of the WBG technologies, culminating in the recent commercial availability of SiC power electronics. SiC-based power discretes, including diodes, rectifiers, super junction transistors (SiC BJTs), JFETs, MOSFETs and thyristors are now in production by a number of manufacturers, including Cree, GeneSic, Infineon, ROHM, STMicroelectronics, Semelab/TT Electronics, and Central Semiconductor.
The SiC Schottky diode is currently the most prevalent type of SiC power device, with variants available today for operation up to 1700 V, in temperatures up to 250° C. Schottky diodes are known for their lower forward voltage drops than standard diodes, making them beneficial in high efficiency applications such as photovoltaic (PV) systems. SiC Schottky diodes offer a much lower reverse leakage current, and higher reverse voltage than Si Schottky diodes, improving the efficiency and reliability of new PV systems. In standalone (off-grid) systems, the diodes prevent batteries from discharging through the solar panels at night. In grid-connected systems, the diodes prevent reverse current from flowing between adjacent strings. Not surprisingly, the first confirmed end products using SiC power electronics are PV systems.
Schottky diodes are also known for very fast switching, making them useful in applications such as switch mode power supplies (SMPS). High speed switching allows the use of small inductors and capacitors for smaller form factors, without trading off efficiency. Hence, SiC Schottky diodes, offering the industry’s highest switching speeds, will enable the next generation of smaller, lighter switch mode power converters.
Power modules incorporating SiC diodes are available to simplify design efforts. SiC diodes are often coupled with IGBTs, so modules combining the two exist, such as the GB100XCP IGBT/SiC diode co-pack from GeneSic. There are also modules which combine SiC diodes and SiC MOSFETs, such as Cree’s CAS100H12AM1 1.2 kV, 100 A SiC half-bridge modules.
A more recent entrant (in the last couple of years) in the SiC world of power devices is the “super” junction transistor (SJT), or super-high current gain SiC-based BJT. These power switches target 1.2kV (now) to 10 kV (future), high temperature (>300° C), and high-efficiency medium to high-frequency power conversion applications, such as SMPS, Uninterruptible Power Supply (UPS), aerospace, defense, down-hole oil drilling, geothermal, Hybrid Electric Vehicle (HEV) and inverter applications. The SiC SJTs offer significant benefits over Si IGBTs, SiC MOSFETs and JFETs including reduction in power losses for improved system efficiency, and reduction in thermal management requirements to lower cost and size. SiC SJTs are also a direct replacement for Si IGBTs, so they can be driven using the standard IGBT/MOSFET gate drivers, whereas SiC MOSFETs and JFETs require specialized gate drivers.
As a nascent technology, SiC presents a higher purchasing cost than Si, spawning investigation into less expensive WBG materials and leading to developments in gallium nitride (GaN). Power devices using GaN material bonded over a Si or SiC substrate (generally still referred to as simply GaN) are more cost effective than SiC, and anticipated to become more widely available in the near future. While otherwise preserving the same performance benefits over Si as SiC, the mismatch in substrate (bulk-GaN as a substrate is currently prohibitively expensive) actually reduces GaN’s high theoretical thermal conductivity to slightly lower than Si. Therefore GaN-based devices are targeted at less temperature stringent applications. The price of GaN –based devices is expected to be comparable to Si equivalent counterparts by 2015, making GaN an excellent choice for next generation power applications and end products.
Target applications for WBG power devices are diverse - ranging from industrial functions, such as motor drives and power supplies, to automotive and transportation systems including hybrid and electric vehicles, aircraft, ships, and traction, to wireless communications, military systems, space programs, and clean energy generation from solar inverters and wind turbines.
SiC is expected to grow the most in renewable energy applications such as solar power systems and grid storage. Both SiC and GaN are anticipated to be adopted equally well in automotive and transportation systems. GaN is forecasted to eclipse SiC in IT and electronics, as well as more general applications.
Regardless of how the actual applications play out in the future, WBG power devices will make an impact in power electronics. Overall sales of power discretes (MOSFETs, IGBTs, BJTs, rectifiers, etc.), power modules, and power ICs are projected to launch power electronics from the $18 to $20 billion market that it is today to approximately $65 billion in 2020 – with the share of SiC devices in 2022 expected to near $1.8 billion, up from around $200M today, while the GaN market is projected to grow from almost nothing today to over $1 billion in 2022.
Silicon-based RF power transistors are reaching limits of power density, breakdown voltage, and operating frequency, thus opening up the opportunity for adoption of wide bandgap (WBG) semiconductors such as gallium nitride (GaN) in RF signal processing applications. GaN offers key advantages over silicon. The high power density of GaN leads to smaller devices as well as smaller designs due to reduced input and output capacitance requirements, an increase in operational bandwidth, and easier impedance matching. GaN’s high breakdown field allows higher voltage operation and also eases impedance matching. The broadband capability of GaN devices provides coverage for a broad frequency range to support both the application’s center frequency as well as the signal modulation bandwidth. Additional advantages of GaN include lower losses for higher efficiency, and high-temperature operation (in the case of GaN on bulk-GaN substrate).
Available WBG-based RF devices include GaN high-electron-mobility transistors (HEMTs) and GaN monolithic microwave integrated circuits (MMICs). The HEMT is a field effect transistor incorporating a junction between two materials with different bandgaps, enabling it to operate at higher frequencies than ordinary transistors. Applications where high gain and low noise at high frequencies are desired are suitable for HEMTs. GaN HEMTs are attractive in RF applications over devices of other materials due to their high-power performance. The MMIC is an integrated circuit that operates at microwave frequencies (300 MHz to 300 GHz). These devices typically perform functions such as high-frequency switching, microwave mixing, power amplification, and low-noise amplification. GaN enables advanced performance MMICs for high performance RF applications. Applications for GaN RF devices include broadband amplifiers, radar, telecom base stations, military communications, and satellite communications.
Though light emitting diodes (LEDs) have been available since the 1960’s, high-brightness blue LED products only arrived relatively recently - in the early 1990’s, arising from critical developments with gallium nitride (GaN), a wide bandgap (WBG) semiconductor material. The color of an LED is determined by the energy bandgap of the semiconductor, and current blue LEDs are based on GaN and InGaN (indium gallium nitride). When blue LEDs are mixed with red and green LEDs or coated in yellow phosphor, the more popular method, the result is high-intensity white light. The availability of LED-based illumination revolutionized the solid-state (semiconductor based) lighting industry by providing a much higher efficiency and longer lifetime alternative to filament-based incandescent lighting, and a mercury-free alternative to compact fluorescent light bulbs. Energy-saving WBG-based LEDs produce more than 10 times more light per watt, and last 30 times longer than comparable incandescent bulbs. LED makers today offer products with lighting efficiency greater than 150 lumens per watt, with lifetimes of around 40,000 hours. Compare this to the 20 lumens per watt lighting output, and 1000-2000 hours rating of incandescent bulbs and it is easy to envision how LED lighting will gain widespread adoption despite a higher initial purchase cost. Indeed, LED lighting sales is projected to grow massively over the next few years, overtaking sales of incandescent bulbs by year 2018.
In addition to efficiency and lifetime improvements in general lighting, LEDs offer advantages that enable a myriad of applications. LEDs possess directionality, permitting products to be optimized for directional indoor lighting applications such as track lighting and spot lights. The robustness and durability of LEDs allows innovation in areas where incandescent lighting is too fragile. The compact size makes ever-smaller implementations possible. The high switching speed of LEDs enable improvements in applications such as television displays, where fast turn-on/off produces exceptional visual quality. Backlighting for mobile phones, automobile lighting, aviation lighting, advertising displays, traffic signals, and even flashlights are just some of the popular uses for LEDs.
Progress in GaN technology has also led to developments in blue, violet, and ultra-violet (UV) laser diodes. Blue and violet lasers are being used in Blu-ray players to read the high capacity optical storage discs, for xenon lamp replacement in projection systems, and in laser printing and medical imaging technologies. UV lasers are finding applications in watermark inspection for anti-counterfeiting, medical instrument disinfection and sterilization, and water or air purification.