Physical Mechanisms Affecting the Reliability of GaN-based High Electron Mobility Transistors
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Physical Mechanisms Affecting the Reliability of GaN-based High Electron Mobility Transistors R. D. Schrimpf1, D. M. Fleetwood1, S. T. Pantelides1, Y.S. Puzyrev1, S. Mukherjee1, R. A. Reed1, J. S. Speck2, and U. K. Mishra2 1 2
Vanderbilt University, Nashville, TN, 37212, United States.
University of California Santa Barbara, Santa Barbara, CA, 93106, United States.
ABSTRACT The physical mechanisms responsible for electrically-induced parametric degradation in GaN-based high electron mobility transistors are examined using a combination of experiments, device simulation, and first-principles defect analysis. A relatively simple formulation is developed under the assumption that the hot-electron scattering cross-section is independent of the electron energy. In this case, one can relate the change in defect concentration to the operational characteristics of a device, such as the spatial and energy distribution of electrons (electron temperature), electric field distribution, and electron energy loss to the lattice. INTRODUCTION GaN-based high electron mobility transistors (HEMTs) offer excellent high-power and high-frequency performance, allowing them to amplify high power signals at microwave frequencies very efficiently [1, 2]. The current in GaN HEMTs flows in a two-dimensional electron gas at the interface between GaN and AlGaN layers, which offers excellent electrical properties (high carrier density and mobility). Understanding the physics of failure in these devices remains an important issue, with both abrupt failures and gradual parametric degradation having been observed [3-6]. In particular, understanding the relationship between defect formation and device degradation is challenging [7]. These issues also have been addressed for defects formed by neutron irradiation [8] and proton irradiation [9-13]. GaN/AlGaN HEMTs may in some cases show sudden and permanent damage when subjected to very high reverse bias-stress. While this failure mechanism has been addressed by improvements in processing technology, the parametric degradation that occurs in the “semi-ON” state at moderate drain biases remains an issue [14]. In this condition, the device is biased close to pinch-off, but the relatively small numbers of electrons that are flowing acquire high kinetic energies in the relatively high electric field that exists near the drain. The energetic carriers can activate, by dehydrogenation, or reconfigure defects near the interface [15]. The defect generation rate is usually greatest at the end of the gate on the gate-drain side, where the lateral electric field is at its maximum. The defects that form may lead to significant reductions in drain current and transconductance, as well as shifts in threshold voltage.
GaN/AlGaN HEMTs grown under various conditions (i.e., gallium-rich, nitrogen-rich, and ammonia-rich) have been analyzed and the defects responsible for degradation in each device type have been identified [15-17]. The atomic-scale nature of the traps that produce changes in threshold voltage, leakage
