Bias-Dependent Electron Velocity Extracted From AlGaN/GaN HFETs and Its Impact on g_m and f_T

Bias-dependent electron velocity ( v(e) ) was extracted from AlGaN/GaN HFETs with different gate lengths ( L-g ) by S-parameter measurement. A peak v(e) was observed at a channel electron density ( ns) of 3.5x10(12) cm(-2) in a 0.25 mu m L-g HFET. This v(e) behavior can explain the extrinsic transconductance ( g(m)) and current gain cutoff frequency ( f(T)) in different Lg AlGaN/GaN HFETs with different V-gs based on time delay methods and drift-diffusion solver coupled to Monte Carlo (MC) simulation. As L-g is scaled down from 0.5 mu m to 0.25 mu m , the device exhibits largely increased gm and f(T) . The better gm is related to the increased v(e) by the enhanced electric field ( E) and the increased positive triangle v(e)/triangle V-gs by the enhanced polarization Coulomb field (PCF) scattering in the short- L-g device. The better f(T) is related to the increased v(e) , g(m), and decreased gate-source capacitance ( C-gs) . These results highlight triangle v(e)/triangle V(gs )as a vital parameter to effectively modulate the device performances of GaN HFETs.