Microwave Power Performance of Buffer-Free AlGaN/GaN MISHEMT With MOCVD Grown Ex Situ SiN

Abstract

This work investigates the effect of ex situ MOCVD-grown SiNx gate dielectric and PECVD SiNx passivation layer on the microwave power performance of buffer-free AlGaN/GaN MISHEMT. We fabricated devices on a series of four samples diced from a 4 inch epi: first two samples had no gate dielectric while the latter two had upto 3 nm of ex situ SiNx as the gate dielectric. In each of the categories, one sample had baseline 100 nm SiNx passivation deposited under high frequency plasma conditions, while the other sample had 100 nm bilayer SiNx passivation. Devices had gate length of $0.22~\mu $ m and width of $2\times 50~\mu $ m. Samples with no gate dielectric exhibited low 2DEG density $\sim 7\times 10^{{12}}$ cm ${}^{-{2}}$ possibly due to significant charge depletion by the gate Fermi pinning owing to a thin barrier (10 nm, Al0.3Ga0.7N). Moreover, these HEMTs exhibited high current collapse (CC) $\ge 38$ % irrespective of the passivation scheme when subjected to $1~\mu $ s pulse. The MISHEMTs on the other hand, exhibited high 2DEG density $\sim 10^{{13}}$ cm ${}^{-{2}}$ which also manifested as increased threshold voltage. Gate lag reduced to a mere 4%–6% in MISHEMTs, while under gate-drain double pulsing, the collapse was found to be 27% and 11% for different passivation schemes. Drain current transient (DCT) studies revealed suppression of surface states upon ex situ SiN deposition, and hinted at localized defects. Finally, under 28 V Class B operation at 6 and 10 GHz, output power of 8.1 W/mm power added efficiency (PAE 68%) and 5.2 W/mm (PAE 47%) respectively were measured under pulsed loadpull conditions.