As the power consumption of data center applications has grown, power devices have found their way into products across different market segments such as information technology, electric motor drives, grid infrastructure, automotive, and aerospace. This resurgence of wide-bandgap devices is not only driving growth by enabling high volume manufacturing and reduction in cost, but also innovation in material and packaging technologies lead to improvement in reliability and novel device types.
The two main technologies responsible for the growing list of applications for power devices are SiC (silicon carbide) and GaN (gallium nitride) due to their unique material properties. SiC is often used in applications requiring 10kV or more of power, possessing lower switching losses and lower production costs. However, SiC is less reliable than GaN, which can function at higher temperatures. GaN is typically deployed in high-speed applications as a result of its ability to maintain high switching frequencies.
These two technologies have found their space in the applications as shown in figure 1.
As the implementation of power devices steadily increases, so does that of wide-bandgap semiconductors. Wide-bandgap devices offer significant advantages over their silicon counterparts.
Large bandgap and the critical electric field enable high voltages to be blocked with thin layers, resulting in lower resistance and associated conduction losses. Thin layers not only provide low on-resistance, but allow for smaller form factors and reduced capacitance, leading to higher frequency operation. Large bandgap also produces low intrinsic carrier concentration, resulting in low leakage and high temperature operation. Large thermal conductivity is a key benefit for high power operation with limited cooling requirements.
Reliability of Wide-bandgap Devices
Wide-bandgap devices can offer increased flexibility in applications, but they are susceptible to failure mechanisms such as gate-oxide breakdown, SiC breakdown, edge-termination breakdown, threshold voltage drift over time, or reduction in current flow due to increase in resistance. There are various accelerated life tests that can identify vulnerable parts of a wide-bandgap device; however, they are not always accurate in capturing intrinsic wearout or lifecycle failures.
Another important reliability concern is wide band gap devices’ susceptibility to terrestrial neutrons. When a wide band gap device is impacted by neutron particles, it can produce an ionization path resulting in large current transients causing gate burn out and thus, a gate-oxide failure.
There are some reliability issues with respect to packaging. Silicon packaging technology is not enough for wide band gap devices. In order to minimize thermal resistance and thermo-mechanical stresses generated due to high power dissipation, double-sided cooling is often required. Wire bond techniques typically used for low voltage logic devices are to be replaced with planar interconnects using topside copper tabs soldered to die. Encapsulation material ought to have required dielectric strength to meet the high operating voltage and temperature of the die. Finally, the die attachment and interface material must have a high melting point with good thermal conductivity.
Standard qualification does not suggest the part can meet a desired lifetime nor that it has lower defectivity. Demanding an often-different mission profile than what has been tested could produce field acceleration and failure modes that have not been screened by standard tests. The reliability challenges must be addressed with the right software analysis tools and techniques prior to product or system design to prevent expensive field failures.
When it comes to packaging for wide band gap devices, there are reliability challenges to consider as silicon packaging is insufficient. If designers wish to limit thermo-mechanical stresses as a result of high power dissipation, they may need to implement doubled-sided cooling. Wire bonding strategies used in low voltage devices should be substituted with planar interconnects featuring copper tabs soldered to die. With the higher operating voltages found in wide band gap devices, encapsulation material needs to meet a requisite strength to handle the voltage. Addressing these reliability concerns requires accurate software analysis tools and reliability techniques prior to system design or construction of prototype to prevent field failures. Other methods such as standards and qualifications do not address the complex and unique applications of these devices, rather they establish baseline, universal qualifications.
Challenges and Future Opportunities
As the needs develop for industries such as autonomous vehicles, aerospace, and data centers, wide-bandgap device capabilities must increase to demonstrate a high range of operation for higher quality products in which reliability is crucial. New technologies drive growth and opportunities will present themselves for advances in manufacturing and device design to enable higher volume with better quality and reduced cost. Unfortunately for many designers, they need a solution for the present as they strive to create robust and safe products. To that extent, accurate simulation and modeling is an impactful tool. With simulation and modeling, reliability engineers can manipulate inputs to recreate the field lifecycle for products utilizing wide band gap devices, offering a glimpse into how they perform in previously unexplored environments. As a result, designers can have higher confidence in their product and reduced risk.
To learn more about more Physics-of-Failure and Reliability in Power Semiconductors, register for a webinar on Nov. 19th, 2019.
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