Investigation of traps in AlGaN/GaN high electron mobility transistors by sub-bandgap optical pumping

Abstract

Sub-bandgap optical pumping with wavelengths of 671 and 532 nm was employed to study traps in AlGaN/GaN High Electron Mobility Transistors (HEMTs). The trap activation energies were extracted from the Arrhenius plots with transient drain current upon sub-band optical pumping with different laser fluxes. A trap state at 0.73 eV below the conduction band was realized for the 671 nm laser pumping. For photon flux larger than 5.8 × 1016 mm-1sec-1, two states at 0.73 and 1.91 eV below the conduction band were identified for the use of 532 nm laser. However, only the trap state corresponding to 1.91 eV were observed, when the laser fluxes smaller than 5.8 × 1016 mm-1sec-1. This indicates that the trap with 1.91 eV is likely to exist at the surface of the HEMTs, while the trap with Ea =0.73 eV is not. Gate-lag measurements with and without laser illumination were also conducted, and the result confirmed with the optical pumping test. This approach to selectively excite the trap states with different fluxes of sub-band lasers provides a non-destructive method of probing the trap levels and spatial location of traps of AlGaN/GaN HEMTs.