Simulation Investigation of Laterally Downscaled N-Polar GaN HEMTs

Abstract

The N-polar GaN high-electron mobility transistors (HEMTs) have demonstrated a powerful performance as the Ga-polar GaN HEMTs. This investigation aims to show the direct current performance and cutoff frequency ( ${f}_{\text {T}}$ ) of the planar N-polar GaN HEMTs with gate lengths downscaling from $4~\mu \text{m}$ to 50 nm by 2-D device simulation. The impacts of traps and gate dielectrics and the roles of the field-dependent mobility and the source and drain series resistances are investigated. For our central-gated device with a 10-nm top GaN channel layer, a 30-nm Al0.3Ga0.7N barrier layer, and a 3-nm Al2O3 gate dielectric, the gate length ( ${L}_{\text {G}}$ ) as the transition point from the long-channel behavior to the short-channel one is found to be 200 nm. For ${L}_{\text {G}} nm, a linear ${f}_{\text {T}}^{-{1}}$ versus ${L}_{\text {G}}$ relation shows up, and notable short-channel effects appear, i.e., the negative shift of the threshold voltage, the increase of the drain-induced barrier lowering, and the almost ${L}_{\text {G}}$ -independent constant maximum transconductance in the saturation region. The degradation of the ${f}_{\text {T}}\times {L}_{\text {G}}$ product with the decrease in the aspect ratio between ${L}_{\text {G}}$ and the equivalent gate-channel distance ( ${t}_{t}$ ) is shown to be quite small. The ${L}_{\text {G}}/{t}_{t}$ ratio for 15% degradation of the ${f}_{\text {T}}\times {L}_{\text {G}}$ product from its upper limit ( $19.23~\text {GHz}\cdot \mu \text {m}$ ) is 5.5. This small degradation of the frequency characteristics in short-channel devices is attributed to the quite small fringing capacitance of the gate.