Are SiC and GaN going to be replaced by ultra power semis?

With the recent announcement by Infineon that gallium nitride (GaN) use is reaching the “tipping point” for accelerated adoption, maybe it’s time to consider what’s next beyond wide bandgap (WBG) GaN and silicon carbide (SiC) devices. Next-generation ultra-wide bandgap (UWBG) power semiconductors are being developed that may replace current WBG options.

UWBG semiconductors have bandgap energies much greater than the 3.4 eV of GaN or 3.2 eV of SiC. Examples of UWBG semiconductors include diamond (bandgap 5.47 eV), gallium oxide (Ga2O3, bandgap 4.8 eV), cubic boron nitride (c-BN, bandgap 6.4 eV), and aluminum nitride (AlN, bandgap 6.2 eV).

In addition to larger bandgaps, UWBG materials have high breakdown strength and excellent thermal conductivity, making them candidates for high-power electronics and extreme-environment applications.

Baliga figure of merit

The Baliga figure of merit (BFOM) provides a baseline for comparing the performance of power semiconductor materials. It’s the ratio of the square of the breakdown voltage (VB) to the on-resistance (Ron): BFOM = VB² / Ron. The BFOM indicates how well a device can handle high power levels with minimal losses. A higher BFOM indicates higher-performance devices.

The BFOM relies on the fact that both VB and Ron are related to the doping concentration depletion region width, and both depend on the critical breakdown electric field, E_C. The value of E_C is dependent on the bandgap, hence the utility of the BFOM. BFOM curves for WBG and UWBG materials are shown in Figure 1.

Another performance perspective

Figure 1. BFOM values closer to the lower right of the chart indicate higher performance. (Image: MDPI materials)

The BFOM is an important metric, but other considerations exist when fabricating useful power semiconductor devices. The US Department of Defense (DoD) considers UWBGs for various applications, not just power conversion.

In addition to ultra-wide bandgaps, UWBG materials have high breakdown strength and thermal conductivity compared to traditional semiconductor and WBG materials (Figure 2). That can make them useful for deep ultraviolet (UV) LEDs and lasers for defense applications. Additional application possibilities include high-temperature electronics, sensors, and high-performance RF devices for radar and communication systems.

Figure 2. Performance comparison of WBG and UWBG material characteristics. The diameter of the circles is proportional to thermal conductivity. (Image: DARPA)

Barriers to realizing the promises

The Defense Advanced Research Projects Agency (DARPA) has identified a primary barrier and several secondary challenges to overcome before the promised performance of UWBGs can be reached and is sponsoring research to overcome those challenges.

The primary challenge is the availability of device-quality large-area (100 mm, 4-in diameter) UWBG substrates. Low defect density UWBG substrates are only available in smaller diameters that are not economically viable commercially. Current 100 mm UWBG substrates have high defect densities and are unsuitable for device fabrication.

Once the substrates are available, there are still challenges to device fabrication, including:

The efficient incorporation of dopants into UWBG materials is important because they allow precise control over the material’s electrical conductivity. Dopant incorporation efficiency for diamond is typically below 1%, and for AlN, it’s below 10% and is highly variable — poor dopant incorporation efficiency results in low material conductivity.
Creation of efficient device junctions. UWBG junctions formed in similar materials (homojunctions) exhibit low-defect densities but do not easily form abrupt junctions. While junctions in dissimilar materials (heterojunctions) can be formed with abrupt structures, they exhibit high junction defect densities. Lack of abruptness and/or high defect density of semiconductor junctions results in poor device performance, such as high junction capacitance, diode ideality factor, and leakage current.
Producing low-resistance electrical contacts. An intrinsically high barrier to current flow between UWBG materials and metal contacts results in high contact resistance. Current contact resistance for UWBGs can be 1000x higher than for mature semiconductor technologies. New technologies for contact fabrication will be required to make UWBG devices suitable for practical applications.

Summary

The adoption of SiC and GaN WBG devices is accelerating. What’s next? UWBG devices, including diamond, gallium oxide, cubic boron nitride, and aluminum nitride, are under development and promise even higher performance levels. But there’s still a lot of work to do before the promise of better devices for power, RF, and sensing applications can be transformed into reality.