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Physics of electrical degradation in GaN high electron mobility transistors

The deployment of GaN high electron mobility
transistors (HEMT) in RF power applications is currently
bottlenecked by their limited reliability. Obtaining the required
reliability is a difficult issue due to the high voltage of
operation. In order to improve reliability, it is essential to
develop detailed physical understanding of the fundamental
degradation mechanisms. In this thesis, we investigate the physical
mechanisms behind the electrical degradation of GaN HEMTs by
performing systematic stress experiments on devices provided by our
industrial collaborators. These devices are electrically stressed
under various bias conditions while regularly characterized by a
benign characterization suite. We observe that electrical stress
beyond a critical voltage results in an increase in drain
resistance, a decrease in maximum drain current, and a sharp
increase in reverse gate current. We show that this mode of
degradation is driven by electric field and that current is less
relevant. Behind this degradation is trap formation that occurs at
the critical voltage. To understand this, we have developed a new
trap-analysis methodology. It is found that under stress, the
density of traps increases in the AlGaN barrier layer in the
proximity to the gate edge on the drain side of the device. We show
that this degradation is enhanced under mechanical uniaxial tensile
strain that is externally applied to the device. From our
experiments, we propose a degradation mechanism of defect formation
through the inverse piezoelectric. In this mechanism, high vertical
electric field at the gate edge under high voltage increases
tensile stress in the AlGaN layer due to piezoelectricity of the
material.