What makes SiC and GaN suitable for high-power designs
Article By : Bonnie Baker
There are several characteristics when analyzing silicon, silicon carbide (SiC), and gallium nitride (GaN) capabilities for a power system.
There are several characteristics when analyzing silicon (Si), silicon carbide (SiC), and gallium nitride (GaN) capabilities for a power system.
Silicon process is most familiar to designers as it has been the process of choice for as long as I can remember. How far back does the semiconductor silicon go? In 1824, Jöns Jacob Berzelius, a Swedish chemist, applied heat to chips of potassium in a silica container and then carefully washed away the residual by-products.
The first commercial SiC semiconductor material was patented by Henry Harrison Chase Dunwoody in 1906; it was used in a crystal radio “carborundum” detector diode—a synthetic silicon carbide.
Next, at University of California, Santa Barbra (UCSB) College of Engineering, professors Steven DenBaars, James Speck and Umesh Mishra unveiled their work on GaN as a semiconductor medium in 1993.
Figure 1 These are the three basic semiconductor materials currently serving the high-voltage applications. Source: Bonnie Baker
These three device materials generate a broad spectrum of characteristics. It’s best to reveal the differences through the electrical specifications, physical sizes, and cost. These differences will help to understand how to apply these semiconductor materials effectively.
Electrical differences among Si, SiC and GaN
The electrical specifications for wide bandgap (WBG) semiconductors include frequency range, breakdown field strength, high electron mobility transistors (HEMT), and thermal conductivity (Figure 2).
Figure 2 Power versus frequency plot reveals the fundamental characteristics of silicon, SiC and GaN semiconductors. Source: Texas Instruments
Semiconductors with wide bandgap
Gallium nitride and silicon carbide have similar bandgap and breakdown fields. The bandgap for GaN is 3.2 eV, while SiC has a bandgap of 3.4 eV. While these values appear identical, maxim SiC breakdown voltages at approximately 1,700 V are considerably higher than GaN’s 650 V.
With silicon at 1.1 eV bandgap, there is nearly a three times difference compared to GaN and SiC. Therefore, while the higher GaN and SiC bandgap comfortably supports higher voltage circuits, these semiconductors cannot support the lower voltage circuitry like silicon.
Next, breakdown voltage, BVDSS, comes into play when a significant current flows between the source and drain by the avalanche multiplication process. The reverse-biased body-drift diode breaks down, and in contrast, the gate and source are shorted together.
Breakdown fields in SiC and GaN are close, with SiC boasting a breakdown field of 3.5 MV/cm and GaN having a breakdown field of 3.3 MV/cm. In contrast, the breakdown field in silicon devices is 0.3 MV/cm, meaning GaN and SiC are over 10 times more capable of maintaining higher voltages. So, compared to silicon, these breakdown fields make GaN and SiC significantly better equipped to handle higher voltages.
Commercial SiC transistors—JFET, MOSFET—can block voltage up to 1,700 V and GaN transistors (HEMT) can withstand a maximal voltage of 650 V, while each can conduct current from a few amperes to a few tens of amperes. The silicon breakdown voltage is from 40 V to 400 V.
High electron mobility transistor (HEMT)
Gallium nitride HEMTs offer their best performance in the 600 V to 1,200 V voltage class. Vertical device structures become preferable as the breakdown voltage increases, and here, 3.3 kV to 10 kV GaN MOSFETs are the best performers.
A significant difference between SiC and GaN is in their electron mobility. The electron mobility specifies the electron speed through the semiconductor material. The electron mobility speed of silicon is 1,500 cm2/Vs. The GaN electron mobility is 2,000 cm2/Vs. This level of mobility means electrons can move over 30% faster than silicon.
On the other hand, the electron mobility of SiC is 650 cm2/Vs. The SiC electrons are slower moving than both GaN and silicon. That makes GaN three times more suitable for high-frequency applications because of its elevated electron mobility.
SiC and GaN: Other performance parameters
Thermal conductivity:
Thermal conductivity of a material is its ability to transfer heat through itself. Given the circumstances of use, thermal conductivity directly influences the material’s temperature. Inefficiencies in materials will create heat in high-power applications. So, high power circuits increase the temperature of the material to change the electrical characteristics.
Thermal conductivity of GaN is 1.3 W/cmK (watts per centimeter-kelvin). This level of conductivity is worse than silicon’s 1.5 W/cmK conductivity. However, SiC’s thermal conductivity of 5 W/cmK is three times better at transferring thermal loads. This thermal conductivity feature makes SiC highly advantageous in high-power, high-temperature applications.
Manufacturing process, size and cost:
The other performance parameters to consider when using these devices are manufacturing process, size, and cost. The manufacturing process impacts reliability and ultimate yields. As a result, the manufacturing process affects the final price of the devices.
Wafer manufacturing process of semiconductors:
The limiting factor in the current manufacturing processes is GaN and SiC materials. Silicon manufacturing processes are either less expensive and more accurate or they are more energy intensive. For example, GaN contains a massive number of crystal defects over a small area. In contrast, silicon can have as few as 100 defects per square centimeter.
On the other hand, before the year 2000, manufacturers could not create GaN substrates with fewer than one billion defects/cm. The source of defects made GaN semiconductors incredibly ineffective. There is still a rubble as GaN semiconductors struggle to meet the stringent defect criteria.
Where SiC and GaN have potential
Compared to silicon, current manufacturing techniques of GaN and SiC limit the cost-effectiveness, making both high-power materials more expensive. However, both materials have impressive semiconductor application advantages.
Silicon carbide may be a more effective product and less expensive in the short term. It’s easier to manufacture more extensive, more uniform wafers of SiC than GaN. Over time, given its higher electron mobility, GaN will find its place in small, high-frequency products. SiC will be preferable in more oversized power products given its power capabilities and higher thermal conductivity than GaN.
Silicon is not meeting the demands for higher voltage circuitry. At the same time, SiC and GaN are moving ahead in manufacturing defect density reduction as a genuine semiconductor material replacement. Both these semiconductors withstand higher voltages, higher frequencies, and more complex electronics. These factors point strongly to the adoption of SiC and GaN devices across the high-voltage electronics market.
This article was originally published on Planet Analog.
Bonnie Baker is a seasoned analog, mixed-signal, and signal chain professional and electronics engineer. She has published and authored hundreds of technical articles and blogs in industry publications. Baker is also the author of the book “A Baker’s Dozen: Real Analog Solutions for Digital Designers” as well as coauthor of several other books.
