Design of ultra-scaled-channel N-polar GaN HEMTs with high charge density: A systematic study of hole traps and their impact on charge density in the channel

Abstract

It has been previously shown that hole/donor traps at the (Al, Ga) N/GaN interface cause DC-RF dispersion and output conductance in N-polar GaN high electron mobility transistors (HEMTs). In this work, we systematically studied the impact of hole trap energy and density on two-dimensional electron gas (2DEG) density using Silvaco Atlas. Simulation results revealed that the exclusion of the hole traps in the model results in an underestimation of 2DEG density compared to experimentally obtained 2DEG density. By comparing simulations with the experimental results, a hole trap with a density of 3 × 1013 cm−2 at 280 meV above the valence band at the AlN/GaN negative polarization interface was estimated. Three different silicon doping schemes were then examined to suppress the effect of traps. Delta doping (15 nm) at the GaN buffer and barrier interface (doping Scheme-A) is effective in compensating traps present at that interface but insufficient to compensate traps near the GaN channel. Similarly, doping the back-barrier (doping scheme-B) is sufficient to neutralize traps in the middle of the back-barrier and close to the channel but inadequate to neutralize traps at the buffer–barrier interface. Series-C doping employs a combination of doping schemes A and B that effectively neutralizes traps present at all interfaces while simultaneously modulating the 2DEG charge density. An ultra-scaled 5-nm-thick GaN HEMT epitaxial structure was also designed by band engineering that can maintain high 2DEG density in the channel (2 × 013 cm−2) with less than 5% parasitic charge and trap ionization over a wide range of doping from 6 × 1018 cm−3 to 1 × 1019 cm−3.