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GaN field-effect transistors with impressive power switching characteristics have been demonstrated. Preventing their widespread field deployment are reliability and instability concerns. Some emanate from the use of a dielectric in the gate stack. Under typical operation, the gate dielectric comes periodically under intense electric field. This causes trapping and detrapping of electrons and introduces transient shifts in the threshold voltage, a phenomenon known as Bias-Temperature Instability (BTI). A high electric field also results in the formation of defects inside the dielectric. Over time, the defects accumulate and eventually result in the abrupt creation of a conducting path that shorts the dielectric and renders the device inoperable. This process, known as Time-Dependent Dielectric Breakdown (TDDB), often imposes a maximum lifetime for the FET technology. This article presents a methodology for the study of BTI and TDDB in insulated-gate GaN FETs. Our findings paint a picture of BTI and TDDB that in many respects is similar to that of Si transistors but with some unique characteristics. Understanding the physics and developing appropriate lifetime models is essential to enabling the deployment of this important new power electronics technology.

I. INTRODUCTION

In the last few years, GaN has emerged as a semiconductor with extraordinary capabilities in photonics and electronics. GaN packs a unique set of physical attributes: a wide and large direct band gap, high breakdown field, excellent thermal conductivity, and a relatively high electron mobility.1 GaN belongs to a family of ternaries, including InGaN and AlGaN, which span a wide range of bandgaps and make possible GaN-based light-emitting diodes (LED) with colors from red to ultra-violet.2 GaN LEDs today represent a multibillion dollar market. Blue GaN lasers are also commercial for high-density recording. Heterostructures based on GaN and AlGaN also lead to the formation of a two-dimensional electron gas (2DEG) on the GaN side of the interface even in the absence of doping.3 This is due to the large spontaneous polarization of AlGaN as well as stress-induced polarization in AlGaN that arises from its lattice mismatch with GaN. The lack of doping confers electrons with very high mobility, about 2000 cm2/V.s at room temperature. The excellent quality of this heterostructure system has been Contributing Editor: Don W. Shaw a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2017.363

demonstrated in the observation of the quantum-Hall effect and the fractional quantum-Hall effect.4 AlGaN/GaN High Electron Mobility Transistors (HEMT) were first demonstrated in 1993.5 The combination of a high breakdown field in GaN with the high mobility and good electron confinement of the 2DEG at the AlGaN/GaN interface brings about a unique suitability of GaN HEMTS for power amplifier applications in the radio frequency (RF), microwave, and millimeter-wave regimes.6 Electrical power ma

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