Bias-Dependent Electron Velocity Extracted From AlGaN/GaN HFETs and Its Impact on gm and fT

Bias-dependent electron velocity (
${v}_{\text {e}}$
) was extracted from AlGaN/GaN HFETs with different gate lengths (
${L}_{\text {g}}$
) by S-parameter measurement. A peak
${v}_{\text {e}}$
was observed at a channel electron density (
${n}_{\text {s}}{)}$
of
$3.5\times 10^{{12}}$
cm−2 in a
$0.25~\mu \text{m}~{L}_{\text {g}}$
HFET. This
${v}_{\text {e}}$
behavior can explain the extrinsic transconductance (
${g}_{\text {m}}{)}$
and current gain cutoff frequency (
${f}_{\text {T}}{)}$
in different
${L}_{\text {g}}$
AlGaN/GaN HFETs with different
${V}_{\text {gs}}$
based on time delay methods and drift-diffusion solver coupled to Monte Carlo (MC) simulation. As
${L}_{\text {g}}$
is scaled down from
$0.5~\mu \text{m}$
to
$0.25~\mu \text{m}$
, the device exhibits largely increased
${g}_{\text {m}}$
and
${f}_{\text {T}}$
. The better
${g}_{\text {m}}$
is related to the increased
${v}_{\text {e}}$
by the enhanced electric field (
${E}{)}$
and the increased positive
$\triangle {v}_{\text {e}}/\triangle {V}_{\text {gs}}$
by the enhanced polarization Coulomb field (PCF) scattering in the short-
${L}_{\text {g}}$
device. The better
${f}_{\text {T}}$
is related to the increased
${v}_{\text {e}}$
,
${g}_{\text {m,}}$
and decreased gate-source capacitance (
${C}_{\text {gs}}{)}$
. These results highlight
$\triangle {v}_{\text {e}}/\triangle {V}_{\text {gs}}$
as a vital parameter to effectively modulate the device performances of GaN HFETs.