The dominant theme in advancing semiconductor technology has always been: “Smaller, faster, cheaper.” Power electronics is no exception, except that one might add “more robust, more efficient—and with a wider bandgap.”
Bandgap—the energy needed to transport an electron from the valence band of a semiconductor material to the material’s conduction band (thereby making it mobile)—is a key determinant of the semiconductor’s characteristics.
Compared to silicon, wide-bandgap (WBG) materials can deliver impressive improvements in conductivity and switching speeds. They can also operate at higher frequencies and withstand higher voltages and temperatures than silicon. Faster switching speeds also give them a competitive advantage in RF applications in terms of bandwidth.
Twilight of the MOS generation
The ability of metal-on-silicon power transistors (MOSFETs) to deliver improved performance at constantly declining cost is close to hitting a wall. MOSFETs will continue to have many viable markets, of course, but system design engineers are increasingly looking for power transistors that are, in effect, ideal: ones with infinitely fast switching speed, no electrical resistance and lower cost.
In the past several years, WBG materials such as gallium nitride (GaN) and silicon carbide (SiC) have emerged as viable alternatives to silicon and GaAs as well. The figure below shows the life cycles of various technologies used in power semiconductors over the past half-century.
Figure: Life cycles of power device technologies. Courtesy: Yole Développement
Before discussing SiC and GaN, however, it should be noted that in RF applications, GaAs-based components have supplanted silicon in power circuits for decades. Commonly known as monolithic microwave ICs (MMICs), these GaAs-based devices typically incorporate amplifiers and switches. The transistors themselves are usually fabricated as metal-semiconductor field-effect transistor (MESFET) or heterojunction bipolar transistor (HBT) structures. Today, GaAs is also losing ground to WBG competitors in specific applications.
The theoretical high-frequency cutoff for GaAs transistors is about 150 GHz. But the MMIC amplifiers that integrate them tend to top out at about 100 GHz. GaAs power amplifiers have output power ratings of about 5 W, which is roughly their upper limit. But by using multiple devices in parallel or combining amplifier outputs in transformers or networks, power levels up to about 20 W to 40 W are possible.
GaAs is well-suited to high-frequency, small-signal semiconductor devices, especially where low-noise performance is essential—such as in receiver front ends. Their “natural-fit” applications are those with the low power, voltage and current requirements that can use a battery as a power source—and this is one of the reasons why they are at risk from WBG competitors.
To put the bandgap differences into perspective, Silicon’s bandgap is 1.1 electronvolts (eV), compared to 1.4 eV for GaAs, 3.3 eV for SiC and 3.4 eV for GaN. So while GaAs delivers a 50-percent improvement over silicon, SiC and GaN are more on the order of 3X.
SiC leads the way
Low-volume sales of SiC JFETS began in 2007, which makes it is the most mature of WBG technologies. When 6-inch SiC wafers went into production in 2013, an important cost-of-production hurdle was cleared and avalanche of several types of SiC devices entered mass production.
SiC-based devices offer ultra-low on-resistance and lower output capacitance, both of which help minimize power dissipation.
Key benefits of high-voltage SiC MOSFETs are:
- Lower losses, which enables higher system efficiency. According to market research firm Yole Développement, SiC (and GaN) can increase DC-to-DC conversion efficiency from 85 percent to 95 percent and AC-to-DC conversion efficiency from 85 percent to 90 percent.
- Higher switching frequencies reduce passive components and enable more compact designs.
- Breakdown voltages in the tens of kilovolts.
Because SiC boasts three-times-greater thermal conductivity than silicon, temperature does not impact switching performance or on-resistance. The result: SiC devices operate efficiently at temperatures above 150°C and system cooling requirements are lower, which eliminates fans and heat sinks.
High thermal conductivity also allows for the fabrication of vertical power devices that distribute heat effectively across the die. These devices can withstand high current surges and high transient voltages, which makes them very well-suited for high-power (>1,200 V, >100 kW), high-temperature (200°C to 400°C) applications such as solar inverters, hybrid and electric vehicles, many military systems and industrial motor drives.
GaN: high performance at reduced cost
SiC and GaN devices are still more expensive than silicon and, as a result, are used in specialized applications. But there is some reason to believe that GaN can become a cost-effective alternative to GaAs as it goes into higher-volume wafer production. High voltage operation, high switching frequency and many of the other advantages of SiC are already making GaN-based power devices attractive for some applications.
It’s usually the superior performance that can give GaN an edge in a design—and sometimes its performance edge translates into fewer components in the system, which reduces system bill of materials cost.
It has at least a 5:1 power density advantage over GaAs and its thermal conductivity is more than three times that of GaAs. This translates into the lower temperature rise at conduction—which, in turn, enables GaN devices to handle higher power levels than GaAs.
GaN-based high-electron-mobility transistor (HEMT) devices conduct electrons more than 1,000 times more efficiently than silicon. This higher electron mobility means that it can amplify signals well into the upper-gigahertz ranges. Other benefits are similar to SiC: lower on- resistance; lower conductance losses; faster devices with lower switching losses; less power needed to drive the circuit; and smaller devices that take up less space on the printed circuit board.
Amplifiers that integrate GaN devices easily operate at +48 V DC and higher. These high-power/high-voltage capabilities make GaN suitable for applications such as power amplifiers in wireless base stations, radar, satellite, communications and electronic warfare.
To gauge the performance advantages of GaN, consider Analog Devices’ HMC8205BF10 power amplifier. This standard product operates from a 50-V power supply and provides 35 W of RF power at 35-percent nominal efficiency with ~20 dB of power gain covering over a decade of bandwidth. It provides roughly 10 times more power compared to similar approaches in GaAs. In years past, this would have required a complicated combining scheme of GaAs die that would not have been able to reach the same efficiency.
But raw power is not always the be-all and end-all. GaAs amplifiers tend to be more linear and have less distortion than GaN amplifiers.
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Conclusion
Silicon and GaAs still have a lot to offer system designers in terms of cost, design familiarity and reliability. Their continued popularity is built on years of use in the field. In many applications, there is no pressing need to move to components based on more recent materials technology developments. But as applications with very special needs grow—such as the high voltages used in electric vehicles or the extremely efficient devices used for energy harvesting—there will increasingly be a place for SiC and GaN.
