The invention of the silicon (Si) integrated circuit over 50 years ago inexorably paved the way for the modern computing and electronics era that we enjoy today. But all good things must come to an end, as the saying goes, and in this case the foreseeable end is the dominance of silicon in the semiconductor industry. Moore’s Law predicts that the number of transistors incorporated on a chip doubles approximately every two years. In traditional silicon-based computing, Moore’s Law cannot be indefinitely sustained due to heat issues from packing in so many transistors, as well as leakage issues due to shrinking technology. Similarly, in the power electronics arena, it has become an increasing challenge to achieve new devices with greater power density and energy efficiency, year upon year, to meet market demands using silicon. Essentially, innovation in silicon is nearing its fundamental physical limits.
By some expert accounts, we have less than a decade left to extract additional performance before silicon capability is at its theoretical maximum. On the computational front, numerous efforts such as nanotechnology and three dimensional chips are competing to extend Moore’s law for silicon, while molecular and quantum computing are considerations for the post-silicon era. In power electronics, silicon carbide (SiC) and gallium nitride (GaN), both wide bandgap (WBG) semiconductors, have emerged as the front-running solution to the slow-down in silicon in the high power, high temperature segments. With roughly ten times better conduction and switching properties than silicon, WBG materials are a natural fit for power electronics, producing devices that are smaller, faster, and more efficient, with ability to withstand higher voltages and higher temperatures than counterpart silicon-based components. These features, together with greater durability and higher reliability, position WBG power devices as key enablers for today’s important emerging applications such as hybrid electric and electric vehicles and renewable energy generation and storage. WBG power devices also improve existing applications, particularly in efficiency gain. Yole Developpment research estimates that replacing silicon with SiC or GaN can increase DC-to-DC conversion efficiency in from 85% to 95%; boost AC-to-DC conversion efficiency from 85% to 90%; and optimize the efficiency of DC-to-AC conversion from 96% to 99%.
RF applications also benefit from WBG semiconductors. Consider not only the explosion of mobile device usage such as smart phones and tablets, but also the widespread, global trend towards online streaming of video into homes, creating more users and more data. The increasing traffic results in performance demands on wireless and telecommunications systems. It is no wonder that silicon-based RF power transistors are reaching limits of power density, breakdown voltage, and operating frequency. GaN enables advanced performance high-electron-mobility transistors (HEMTs) and monolithic microwave integrated circuits (MMICs) for high performance RF applications, and lower gate capacitance which equates to higher speeds and greater bandwidth.
WBG materials also emit light, and this optical property has helped fuel the rapid development of WBG semiconductors in recent years. In fact, the solid-state lighting industry is using GaN-based light emitting diodes (LEDs) to provide an energy-saving, durable, long-life alternative to incandescent bulbs that is so effective that sales of LED lighting is projected to grow massively over the next few years and surpass incandescent sales by 2018. LED lighting also provides a mercury-free alternative to compact fluorescent lamp (CFL) bulbs. GaN is also used in laser diodes, with the most recognizable implementation today being Blu-ray players.
Figure 1
What is a wide bandgap?
WBG materials are so-called due to a relatively wide energy bandgap as compared to conventional silicon. The electronic bandgap is the energy gap between the top of the valence band and the bottom of the conduction band in solid materials. Electrons can jump the gap to the conduction band by means of thermal or optical excitation. Some materials have no bandgap, but the existence of a bandgap allows semiconductor devices to partially conduct as the word “semiconductor” implies. It is the bandgap that gives semiconductors the ability to switch currents on and off as desired in order to achieve a given electrical function; after all, a transistor is just a very tiny switch embedded in a silicon-based substrate. A higher energy bandgap imparts characteristics that make WBG materials superior to silicon as a semiconductor. WBG-based devices tolerate much higher operating temperatures in a smaller size than the equivalent silicon-based device, enabling previously impossible applications. The popular WBG materials in use today are silicon carbide (SiC) and gallium nitride (GaN.) Whereas silicon possesses a bandgap of 1.1 electronvolts (eV), SiC and GaN have a bandgap of 3.3 eV and 3.4 eV, respectively. Insulators are materials with very large bandgaps, typically greater than 4 electronvolts (eV), and high resistivity. In general, they are not useful as semiconductors except in the case of diamond (C). Though technically an insulator with a bandgap of 5.5 eV, diamond exhibits properties that actually make it the ultimate semiconductor.
WBG in High Power, High Temperature Electronics
Power electronics is a fundamental industry; absolutely everything that uses electricity employs power management devices of some kind. As such, advancements in power devices enable advancements in an unlimited number of applications. That is why it is so exciting to see the adoption of WBG materials in power electronics. Leveraging a history that began first in light emitting diodes (LEDs), and then expanded into RF devices with SAW filters, WBG semiconductors debuted in the power electronics world in 1992 with the demonstration of the first 400V SiC Schottky diode. Since then the WBG power electronics portfolio has expanded to include 1200V SiC Schottky diodes as well as rectifiers, JFETs, MOSFETs, BJTs, and thyristors from a number of manufacturers including Cree and STMicroelectronics. An industry leader, Cree has been in WBG for some time now, with a portfolio including MOSFETs, Schottky diodes and rectifiers, LEDs, and more. In 2011, Cree launched its Z-FET™ line of SiC MOSFETs, which can provide record efficiencies while improving reliability in power-switching applications. STMicroelectronics’ STPSC families of SiC Diodes are available at 600V, 650V, and 1200V. STPSC6H12 is a high-performance 1200V SiC Schottky rectifier that is specifically designed to be used in photovoltaic inverters. It helps increase the inverter yield by up to 2% thanks to its ability to work at high frequency with low switching losses and ultrafast switching whatever the temperature. The losses generated by SiC diodes are 70% lower compared to bipolar diodes.
SiC is the most maturely developed of the WBG materials as evidenced by the number of SiC power devices available today from various manufacturers including Cree, GeneSiC, Infineon, Panasonic, ROHM, STMicroelectronics, Semelab/TT Electronics, and Central Semiconductor. The advantages of SiC over silicon for power devices include lower losses for higher efficiency, higher switching frequencies to trim down passive components for more compact designs, and higher breakdown voltages in the tens of kilovolts. SiC enables higher operating speeds and smaller sized magnetics in power electronic designs. SiC also exhibits significantly higher (3X) thermal conductivity than silicon, with temperature having little influence on its switching performance and thermal characteristics such as on-resistance. This allows low loss, high efficiency operation of SiC devices in temperatures beyond 150° C, the maximum operating temperature of silicon, as well as reduction in cooling/thermal management requirements (elimination of fans and heat sinks, for example) for lower system cost and smaller form factors. The high thermal conductivity also lends to the robustness of SiC devices. High thermal conductivity combined with the homogeneous substrate and epitaxy layers (SiC devices are built on a SiC substrate) allows for vertical power devices that can distribute heat effectively across the die as well as withstand high current surges and high transient voltages. These properties make SiC ideal for high power (>1200V, >100kW), high temperature (200° – 400° C) applications, but also suitable for less stringent usage as well. Renewable energy generation (solar inverters and wind turbines), geothermal (down-hole drilling), automotive (hybrid/electric vehicles), transportation (aircraft, ships, and rail traction), military systems, space programs, industrial motor drives, uninterruptable power supplies, and power factor correction (PFC) boost stages in offline power supplies are all suitable applications for SiC power devices.
As a new technology, SiC is presently more expensive to produce than silicon, leading to the development of GaN as a cost-effective alternative. GaN-based power devices are just now coming into the market. These devices are created with GaN bonded over a SiC or silicon substrate due to the prohibitive expense of using a homogeneous GaN substrate. While lowering cost, and otherwise preserving the same performance benefits over silicon as SiC, the mismatch in substrate actually lowers GaN’s high theoretical thermal conductivity to a bit less than the thermal conductivity of silicon. The WBG benefits of GaN-on-Si, such as high voltage operation, high switching frequencies, and outstanding reliability – coupled with expectations that GaN-on-Si will reach price parity with silicon equivalents as early as 2015, make GaN-based power devices attractive for sub-900 V applications. As costs go down, GaN power devices will also become viable for future generation consumer electronics, where size, efficiency, and price greatly matter.
SiC-based power semiconductors accounted for about $200 million USD in sales in 2013, but are expected to take off sharply in the next few years and by some projections, will near $1.8 billion USD by 2022. With GaN power devices just now entering the market, sales are projected to top $1 billion in that same time period. Despite these exponential growth expectations for SiC and GaN power devices, given that the global power semiconductor market is estimated to be worth around $65 billion USD in 2020 (per IHS/IMS Research), it’s clear that opportunities still continue for silicon for years to come. This is especially true in the low power, low voltage market where a low bandgap is desirable. Development in new materials for the low power, low voltage market is still in infancy. For example, graphene, a zero-bandgap material, has generated a lot of excitement due to its unique properties. Graphene has a tunable bandgap , excellent conductivity, durability, is light weight, and yet was recently isolated in 2004. Interestingly, heating SiC to high temperatures (>1,100 °C) under low pressures (~10-6 torr) reduces it to graphene.
WBG in RF Applications
GaN-based RF devices such as those available from TriQuint and Cree offer key physical and performance advantages over silicon, at a similar price-point. The high power density of GaN leads to smaller devices and systems due to reduced input and output capacitance requirements and an increase in operational bandwidth. 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. GaN also provides lower losses for higher efficiency operation. Vertical devices in GaN can also have better conductivity than SiC, but this is not a feasible realization today given the lack of homogeneous GaN substrate at reasonable cost.
The focus for GaN in RF is consequently on lateral HEMTs and MMICs. GaN enables advanced performance HEMTs and MMICs for high performance RF applications. 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 work best with HEMTs if high gain and low noise at high frequencies are desirable. 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. End applications for GaN RF devices include broadband amplifiers, radar, telecom base stations, military communications, and satellite communications.
WBG in the Optoelectronics and Lighting Industries
WBG materials have a long history in LEDs. The first LED action was demonstrated in 1907 using SiC, and the first generations of commercial LEDs that were available from the 1960’s through the 1980’s were based on SiC. In the early 1990’s, critical developments in GaN showed that it can produce 10-100 times brighter emissions than SiC. This brought about the arrival of the first high-brightness blue LEDs, from which came the dawn of the solid state lighting industry based on white lighting produced primarily by blue LEDs coated in phosphor. Fast forward to today, and we are looking at billions of LEDs already sold into the LED lighting market, with projections of massive growth in LED lighting sales over the next few years. LED lighting sales are expected to overtake sales of traditional incandescent bulbs by year 2018. By 2020, IHS/IMS Research estimates that LEDs will be found in four out of five sockets in developed regions, with incandescent bulbs covering just 2 percent, and CFLs making up the difference.
What is the driving force behind such widespread adoption despite a higher initial cost? The answer lies in the high efficiency, durability, and environmental friendliness of LED lighting. Significant energy savings and longer lifetimes make LED lighting attractive as an alternative to traditional, filament-based incandescent lighting. LED lighting also provides a mercury-free alternative to CFL bulbs. Incandescent lamps convert 90% of their energy to heat and 10% to light. LEDs convert 90% of their energy to light and 10% to heat.
LEDs also have applications beyond lighting. For example, the high switching speed of LEDs enable fast turn-on/off to produce desirable effects in television and other display applications. Backlighting for mobile phones, automobile lighting, aviation lighting, advertising displays, traffic signals, and even flashlights are other popular uses for LEDs.
GaN is also used to make blue, violet, and ultra-violet (UV) laser diodes. Blu-ray players, projection systems, laser printing, and medical imaging are all technologies using blue or violet lasers. Watermark inspection for anti-counterfeiting, medical instrument disinfection and sterilization, and water/air purification are applications for UV lasers.
Cree, Avago Technologies, Panasonic, LED ENGIN, OSRAM Opto Semiconductor, and Philips LumiLEDs are some of the major suppliers of WBG-based LEDs with products available through Mouser.
Conclusion
The worldwide semiconductor market is forecasted to hit $328 billion USD in 2015. A lot is at stake given the limitation of its current foundation, silicon. WBG semiconductors are here to start the migration to better materials, enabling forward movement at the rapid technological pace that we have come to expect. Initial devices are being proven in the areas of power, RF, illumination, and optoelectronics, laying the groundwork for opportunities in other areas, such as microcontrollers. The Panasonic 600V GaN suite of microcontrollers, for example, is optimized for GaN transistors to provide the industry’s smallest and highest efficiency power control solution. SiC power devices are expected to make the biggest impact in renewable energy applications such as solar and wind power generation systems and grid storage. Both SiC and GaN power devices are anticipated to be well adopted in automotive and transportation systems due to high heat tolerance, size and weight reduction, and efficiency gain. High performance-to-price-ratio GaN-based power and RF devices should see implementations in IT, communications, industrial, and consumer electronics, as well as make inroads into more general applications. GaN dominates the progression of technology in LED lighting and in blue, violet, and ultraviolet laser technology.
Keeping in mind that it has taken silicon nearly 60 years to get to where it is today though, it’s reasonable to envision that it will take some time for WBG and other emerging technologies to displace silicon. Amid the optimism surrounding WBG semiconductors are still some major challenges, not the least of which is cost reduction and the need to optimize packaging to allow the full realization of the potential of WBG materials. In addition, in the area of illumination, the long life of LEDs also presents a challenge, as the market saturates. Without continued innovation (such as development of white LEDs without phosphor) to drive turn over to new products, large, expensive LED fabs will have wasted capacity. What we can expect nevertheless, given the tremendous potential, is to see more of WBG semiconductors in our future.
Mouser is committed to supporting designers using WBG semiconductors in the areas of evaluation, design, and development. The latest products available in SiC and GaN can be found in Mouser’s broad WBG semiconductors portfolio.
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