Understanding the impact of bulk traps on GaN HEMT DC and RF characteristics

Abstract

The demand for high power high frequency semiconductor devices has led to the development of microwave power devices using GaN and SiC. AlGaN/GaN HEMTs have shown power densities of 9.8 W/mm at 8 GHz. Although these results are very encouraging, significant work needs to be done to improve performance. It is generally recognized that trapping effects limit the performance of these devices. In this dissertation we study the impact of bulk traps on three distinct characteristics of these devices. These 3 mechanisms are: degradation in the IV characteristics, self-heating and gate-lag. Displacement-damage induced degradation in AlGaN/AlN/GaN HEMTs with polarization charge induced 2DEGs is examined using simulations and experiments. Carrier removal in the unintentionally doped AlGaN layer changes the space charge in the structure and this changes the band bending. The band bending decreases the 2DEG density, which in turn reduces the drain current in the device. The effect of the defect energy levels on the 2DEG density is also studied. The interplay between band bending, mobility degradation, and the charged defects is analyzed and quantified.

Experiments and TCAD simulations are used to study the relationship between bulk traps, self-heating and mobility degradation in AlGaN/AlN/GaN HEMTs. Bulk traps in the GaN channel region and other regions of the device degrade the 2DEG density and the mobility in the device. This in turn degrades the performance of the device. Mobility degradation is closely coupled with the self-heating in the device. The interplay between bulk traps, mobility degradation and self-heating is analyzed and quantified. Experiments and simulations showing the impact of proton irradiation induced bulk traps on gate lag in AlGaN/AlN/GaN HEMTs are analyzed. Pre-existing donor-like surface traps in the gate-drain and source-gate access regions cause the majority of the gate-lag in the device. The simulations indicate that these traps at the AlGaN/Nitride surface are very close to the valence band. Gate lag increases with increased bulk traps. This is due to the reduction in the 2DEG density as a result of band bending and mobility degradation. The experiments and simulations did not indicate any substantial hot electron induced current collapse due to bulk traps.