  We report on the direct measurement of two-dimensional sheet charge density
dependence of electron transport in AlGaN/GaN high electron mobility
transistors. Pulsed IV measurements established increasing electron velocities
with decreasing sheet charge densities, resulting in saturation velocity of 1.9
x 10^7 cm/s at a low sheet charge density of 7.8 x 10^11 cm-2. A new optical
phonon emission-based electron velocity model for GaN is also presented. It
accommodates stimulated LO phonon emission which clamps the electron velocity
with strong electron-phonon interaction and long LO phonon lifetime in GaN. A
comparison with the measured density-dependent saturation velocity shows that
it captures the dependence rather well. Finally, the experimental result is
applied in TCAD-based device simulator to predict DC and small signal
characteristics of a reported GaN HEMT. Good agreement between the simulated
and reported experimental results validated the measurement presented in this
report and established accurate modeling of GaN HEMTs.
