Principal Investigator Jesus del Alamo
Energy-efficient electronics have been gaining attention as a solution to meet the growing demand for ener- gy and sustainability. GaN field-effect transistors (FET) show great promise as high-voltage power transistors due to their ability to withstand a large voltage and carry large current. However, at the present time, the GaN metal-insulator-semiconductor high-electron-mobility-transistor (MIS-HEMT), the device of choice for electric power management, is limited from commercialization due to many challenges, including gate dielectric reliability. Under continued gate bias, the dielectric ultimately experiences a catastrophic breakdown that renders the transistor useless, a phenomenon called time-dependent dielectric breakdown (TDDB).
One key issue is the impact of mechanical strain on TDDB. In particular, when studying OFF-state stress conditions where the drain-source bias is very positive and gate-source bias is negative, the presence of unknown traps at both the interfaces and the bulk of the heterolayers can detrimentally impact dielectric reliability. Mechanical strain introduced during fabrication steps may be causing further reliability problems by amplifying the presence of traps.
To understand the impact of mechanical strain on TDDB, we apply external strain by physically bending the devices. We compare the TDDB distributions which follow the Weibull statistical distribution at different external strain.
We show TDDB under ON-state stress conditions.Underthissituation,thegateisheldata positive bias while the drain and the source are grounded. Since the channel is not depleted, the electric field across the dielectric is distributed throughout the entire gate length and therefore traps make minimal impact on TDDB. Indeed, the breakdown statistics show that for two different mechanical strain, there is little change.
On the other hand, we can also show that TDDB under OFF-state stress condition changes with external strain. Under this stress condition, the majority of the electric field through the dielectric is focused at the gate/drain edge. As more of the electric field is focused in a small area, traps can play a significant role in TDDB.
Understanding the role of mechanical stress in amplifying trap effects will help the efforts to understand the physics behind TDDB.