Wide Bandgap Semiconductors: Thermal Metrology Challenges

January 25, 2023by John Gaskins

“The quest for more-efficient electronics centers on power devices, and semiconductor materials are at the vanguard of the R&D activity. Silicon’s low cost and wide availability enabled it to overtake germanium years ago as the dominant power semiconductor material. Today, however, silicon is ceding its dominance in power devices to two higher-efficiency alternatives: silicon carbide (SiC) and gallium nitride (GaN).”

“The Potential of Wide-Bandgap Semiconductors,” Power Electronics News

Wide bandgap (WBG) semiconductors are capable of much greater power handling than traditional semiconductor designs while offering improved size, reliability, efficiency, and heat tolerance. These benefits are instrumental for common WBG applications such as LED lights, lasers, electric vehicles and radar systems, and wide bandgap semiconductors are increasingly emerging as critical components for green energy infrastructure. 

In this blog, we explore what makes wide bandgap semiconductors different, the advantages they can offer, and some unique thermal metrology challenges associated with these materials.

What are wide bandgap semiconductors?

In short, wide bandgap semiconductors are those which exhibit a bandgap in excess of 2 eV (electronvolts), substantially higher than that of traditional materials like silicon (1.1 eV), germanium (.7 eV), and gallium arsenide (1.4 eV).

All semiconductor materials have a bandgap (sometimes referred to as an “energy gap”), which refers to the minimum amount of energy needed for the electrons in a given material to enter an excited state which allows for conduction. Electrons exist in different bands of energy, and the bandgap refers specifically to the amount of energy an electron needs to jump from the valence band (EV, the lowest energy level band) to the conduction band (EC) in a material. 

This bandgap results in the core property which makes semiconductor materials so valuable: their ability to function as an insulator up to some point of excitation, and a conductor past it. Consequently, bandgap energy is one of the most important factors that determine how a given semiconductor material will perform:  the larger the bandgap, the greater a material’s power handling ability (current and voltage). As we explore below, a variety of WBG semiconductors offer bandgaps several times greater than that of traditional materials.

What materials are used to make wide bandgap semiconductors?

Compared to the most common materials for smaller bandgap semiconductors (silicon, gallium arsenide, and traditionally germanium), different materials are used in WBG designs, including:

  • Gallium nitride (GaN) and silicon carbide (SiC), the most commonly used materials for WBG semiconductors. Both exhibit bandgaps about 3x that of traditional silicon. GaN offers exceptional electron mobility which is a strong fit for high-frequency applications, while SiC’s high thermal conductivity is harnessed in higher power applications like electric vehicles and data centers.
  • Aluminum nitride (AlN), used in some electronic components designed for high-temperature operation.
  • Boron nitride (BN), with a number of emerging use cases such as ultra-violet range electronics. 
  • Gallium phosphide (GaP), used in the manufacture of red, orange, and green LEDs.
  • Diamond, another material with emerging applications as a silicon substitute in high-powered electronic devices. 

What are the key advantages of wide bandgap semiconductors?

In general, wide bandgap materials can be operated at much higher power levels than traditional, smaller bandgap designs, allowing for much greater tolerances for voltage (about 10x that of Si) and switching frequency (also about 10x), and ultimately greater efficiency and power density. Because bandgaps generally shrink when a device is operating at a high temperature, WBG designs can offer substantially improved reliability in high-temperature applications. Silicon carbide, for example, can perform reliably in temperatures ranging up to 400 °C. Improvements in overall efficiency also allow WBG components to be made smaller and lighter than their traditional silicon equivalent. 

WBG designs are found in a variety of components designed for high-power, high-temperature operation. For example, they are used to create short-wavelength LEDs, key to the white LEDs which have become commonplace lighting sources.  Without the onset of the GaN blue LED, we wouldn’t have white LED lights—or the violet lasers used in familiar devices such as Blu-Ray players. WBG semiconductors can also be found in ultra-high-frequency applications such as military radar. Because of their exceptional efficiency, wide bandgap materials are also expected to be a critical pillar for more efficient grid infrastructure and alternative energy solutions. 

According to the US Department of Energy, WBG-based electronic devices can offer a 90% reduction of power loss in AC-DC conversion compared to small bandgap materials. This advantage is critical to the efficient conversion of distributed power generation technologies like wind and solar. In electrical vehicle charging, WBG components can cut electricity losses by 66%. The Department of Energy also estimates that WBG designs have the potential to decrease transformer size by 10x or more and could be a key enabler for future high-efficiency DC transmission lines.

Important Thermal Metrology Challenges for Wide Bandgap Materials

While the benefits are numerous, wide bandgap semiconductors introduce some unique challenges for semiconductor manufacturers, including thermal metrology. Given the high frequency and high-power applications where WBG materials are so critical, the dissipation of the heat generated during operation is the critical component in improving device performance.  For example, to quote DARPA’s recent THREADS BAA¹:

“The operating output power densities achieved in today’s DoD RF transmitters are thermally limited to values substantially below theoretical electronic limits. Wide bandgap (WBG) transistors, such as gallium nitride (GaN), were developed specifically to improve output power density in power amplifiers. But while it is known that a further order-of-magnitude increase in output power is possible in GaN, this cannot be realized in sustained operation today due to excessive waste heat in the transistor channel layer.”

Thus, accurate measurements of the thermal resistances, thermal conductivities, and thermal boundary resistances of WBG film and interfaces are paramount to ensuring robust and optimal operation of WBG-based devices. However, these thermal measurements suffer from major roadblocks using traditional measurement techniques.  

In general, WBG materials are thermally conductive, and thus bulk metrologies fail to accurately measure their thermal resistances, especially in thin film form, as contact resistances will dominate.  Even in thin film form, the thermal measurement technique must be able to accurately separate thermal resistances of the WBG material from the interfaces to function as a robust thermal metrology for WBG materials in device geometries and length scales.  Additionally, the often anisotropic thermal conductivities of thin films, especially those demonstrated in WBG materials due to the location and density of defects that form during thin film growth, make it all the more critical for thermal metrologies to be able to properly assess both in-plane and cross-plane thermal conductivities to evaluate the efficacy of heat sinking and thermal spreading, both crucial processes in WBG-based devices. This, in tandem with the critical importance of thermal boundary resistances in dictating temperature rises and power dissipation abilities in WBG-based applications (e.g., RF and power devices and batteries) make optical-based pump-probe thermoreflectance techniques ideal as thermal metrologies to measure pertinent WBG-material based thermal resistances.  

_______________________________

¹DARPABroad Agency Announcement. “Technologies for Heat Removal in Electronics at the Device Scale (THREADS)”. Microsystems Technology Office HR001123S0013. November 18, 2022

Laser Thermal’s Steady-State Thermoreflectance in Fiber Optics (SSTR-F) is a novel thermal resistance testing technique that is well suited to the complexities of WBG metrology. You can learn more about this technique (and how it can be used to address other critical challenges in semiconductor testing) in our article here. 

John Gaskins

As Co-Founder and CEO of Laser Thermal, John has conducted research characterizing the mechanical, optical and thermal properties of materials for almost two decades. John received his Ph.D. in Mechanical and Aerospace Engineering from the University of Virginia in 2013.  His work developed methodologies for testing the size dependent properties in thin film structures.  After his Ph.D. John joined Patrick Hopkins’ group extending his Ph.D. work focused on mechanical property characterization to optical and thermal properties. John is leading Laser Thermal’s development of SSTR-F to provide accessible small-scale thermal conductivity measurements to industrial and academic partners.