Silicon has been the basis of semiconductor technology for nearly 60 years. In over half a century, however, engineers and manufacturers have made vast strides in silicon manufacturing, integrated circuit design, and semiconductor applications. Moore's Law shows that researchers are close to reaching silicon-based semiconductors' theoretical limit.
Although silicon semiconductor substrate is perfect for some electronics applications, research scientists and semiconductor manufacturers have long searched for more robust alternatives to silicon for specific applications. These scientists have experienced varying degrees of success over the last few decades, but one strong contending alternative to silicon has emerged: gallium nitride (GaN).
Gallium nitride crystal structure
Gallium nitride is a wurtzite crystal structured semiconductor that is manufactured using metal-organic chemical vapor deposition (MOCVD). In this process, gallium and nitrogen combine to form the crystal. Various mixtures for this synthesis exist, but one example of GaN synthesis employs the use of ammonia (NH3) as the nitrogen source and a gallium source such as trimethylgallium.
GaN's crystalline structure has some uniformity issues, sometimes reaching millions of defects per centimeter range. However, the most modern MOCVD techniques have been able to reduce the number of defects per centimeter to anywhere between 100 and 1000, allowing them to grow and utilize larger GaN crystals as wafers. When scientists are able to synthesize GaN to a low degree of error, the compound has several distinct crystalline properties that give it desirable traits in semiconductor applications.
Bandgap of semiconductors: Advantage GaN
One of the most significant advantages of gallium nitride over silicon is its bandgap, which gives it various electrical properties that equip it for higher power applications. Gallium nitride has a bandgap of 3.2 electronvolts (eV), while silicon's bandgap is only 1.1 eV. Since GaN has a bandgap that's nearly triple silicon's, it uses significantly more energy to excite a valence electron into the conducting band of the semiconductor. This feature limits GaN's usage in very low voltage applications, but it allows GaN larger breakdown voltages and more thermal stability at higher temperatures.
GaN breakdown field
GaN's breakdown field is 3.3 MV/cm, while silicon has a breakdown field of 0.3 MV/cm. That makes gallium nitride ten times more capable of supporting high voltage designs before failing. A higher breakdown field means that gallium nitride is superior over silicon in high voltage circuits such as high-power products. Manufacturers and engineers can also use GaN in similar voltage applications while maintaining a significantly smaller footprint. Silicon, by comparison, has a drastically higher power density.
Silicon vs. GaN electron mobility
Silicon has an electron mobility of 1500 cm2/Vs, while gallium nitride has an electron mobility of 2000 cm2/Vs. Therefore, the electrons in gallium nitride crystals can move over 30% faster than silicon's electrons. This electron mobility gives gallium nitride a distinct advantage for use in RF components, as it can handle higher switching frequencies than silicon.
Thermal conductivity comparison
One disadvantage of gallium nitride over silicon is its lower thermal conductivity. Gallium nitride has a thermal conductivity of 1.3 W/cmK, while silicon has a thermal conductivity of just 1.5 W/cmK. While gallium nitride may not be as well equipped to handle high thermal loads, GaN's efficiency at comparable voltages actually reduces the thermal loads created by the circuit, meaning it will run cooler than silicon.
Check out one example of this thermal conductivity difference, where EPC Corporation shows a demonstration 40% power loss reduction when comparing a120V to 12V 12A GaN FET versus MOSFET. In turn, the GaN FET runs nearly ten degrees cooler and saves energy while running.
Manufacturability of silicon and GaN
Gallium nitride's technological bane is its manufacturing process, especially when compared to silicon's widely adopted, commoditized manufacturing process. Gallium nitride, for example, contains a massive number of crystal defects over a small area. By comparison, Silicon can contain as low as 100 defects per square centimeter. Before this century, engineers had never been able to manufacture GaN substrates with fewer than one billion defects/cm.
Obviously, this large quantity of defects/area is incredibly ineffective given most semiconductor manufacturing design requirements. The defects also limited GaN semiconductor substrates by their physical size alone. While new manufacturing techniques have lowered the number of defects to more efficient numbers, the cost to produce the same amount of GaN wafers still can't compare to silicon.
Summary
GaN has distinct advantages over silicon when being used for semiconductor applications. There are two main hurdles for gallium nitride:
- Defect control in manufacturing
- Maintaining cost-effectiveness
Gallium nitride is more efficient, more thermally stable, and certainly more capable for use in power devices that demand more load or higher frequencies at higher temperatures. Gallium nitride is a future-proofed material for the semiconductor world and will lead to more readily available small, high-frequency products. Explore a variety of power products, including GaN MOSFETs.