Impact of Diamond Passivation on fT and fmax of mm-wave N-Polar GaN HEMTs

This article presents a modeling approach and its implementation to study the impact of the top-side diamond integration on the
${f}_{\text {T}}$
and
${f}_{\text {max}}$
performance of a millimeter-wave (mm-wave) N-polar gallium nitride (GaN) high-electron-mobility transistor (HEMT). This approach uses a co-simulation model formed by an equivalent small-signal circuit model of the device implemented in PathWave advanced design system (ADS) and full-wave simulations of 3-D modeling diamond passivation from Ansys high frequency simulation software (HFSS). Thin-film diamond as a passivation layer and a heat spreader on top of the device channel is explored as a function of the diamond’s dielectric constant and its thickness to understand how it affects the device’s
${f}_{\text {T}}$
and
${f}_{\text {max}}$
. The simulation results serve as a guide to the optimization of the radio frequency (RF) performance of mm-wave HEMT devices, aiding the device design of the diamond passivation. The designed methodology was applied to other passivation, such as benzocyclobutene (BCB) for benchmarking. This method allowed us to estimate the tradeoff in electrical performance for anticipated thermal benefits. A maximum reduction of 23.6% in
${f}_{\text {T}}$
and 21.8% in
${f}_{\text {max}}$
was obtained when the diamond passivation thickness is
$2.7 \mu \text{m}$
with a dielectric constant of 4.