AlGaN/GaN HEMTs are suitable for high power applications because
of their high breakdown voltages and high currents. However, the large
signal performance of these devices is not as good as one would expect
from the DC performance of the device. The large signal performance of
the device is improved by depositing a thin layer of SiN by PECVD. In
this work silicon nitride (SiN) was deposited in a variety of conditions
on AlGaN/GaN HEMT structures on sapphire substrates to study the impact
of the quality of SiN on the DC, breakdown, small signal and large
signal performance of the device. A maximum increase in the output power
from 1.3 W/mm to 2.3 W/mm at 4 GHz, 20 V bias was observed with a SiN
passivation film, grown in NH3 rich conditions and which gave
the least etch rate of 25 nm/min in buffered oxide etch (6:1). This
increase in power is mainly due to the increase in DC current. This is
clearly shown by plotting the RF dynamic loadline for an unpassivated
and passivated device
"The optimum thickness for passivating the GaN surface states is typically 100-150 nm which contributes to the overall gate capacitance . Ideally, one would want a low-κ, extremely thin dielectric which can suppress traps and decrease gate capacitance thus increasing small and large signal gain. "
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report on the dc/RF characteristics of AlInN/AlN/GaN HEMTs on SiC with novel 5-nm ultra-thin ALD Al 2 O 3 passivation for high frequency operation. The bar-rier layer contains a lower indium concentration (15%) which induces a lattice strain-induced piezoelectric field to increase the 2DEG. HEMTs were fabricated with 2x150-μm gate peri-phery and T-gate length L G ~ 90 nm. We also fabricated devices with 100-nm Si 3 N 4 passivation to compare small and large sig-nal gain as well as pulsed dc characteristics. Our results show a 25%+ decrease in parasitic capacitance and ~15% increase in small signal gain using ALD Al 2 O 3 . An intrinsic cutoff fre-quency up to ~130-GHz is reported which highlights the prom-ise of AlInN/GaN HEMTs with ultra-thin ALD Al 2 O 3 passiva-tion.
[Show abstract][Hide abstract] ABSTRACT: The dependence of current slump in AlGaN/GaN HEMTs on the thickness of the AlGaN barrier was observed. Power measurements on a 2/spl times/125/spl times/0.3 /spl mu/m AlGaN/GaN HEMT made on Silicon Carbide (SiC) substrates with an AlGaN thickness of 10 nm gave a saturated output power of 1.23 W/mm at 8 GHz whereas a device with the same dimensions fabricated on samples with an AlGaN barrier of 20 nm gave a saturated output power of 2.65 W/mm at the same frequency. RF load line measurements clearly show the reduction of RF full channel current as compared to dc full channel current and the increase in the RF knee voltage compared to the dc knee voltage, with the effect being more pronounced in thin barrier samples. Passivation improved the large signal performance of these devices. A 1/spl times/150/spl times/0.3 /spl mu/m transistor made on AlGaN(20 nm)/GaN structure gave a saturated output power of 10.7 W/mm (40% power added efficiency) at 10 GHz after passivation. This represents the state of the art microwave power density for AlGaN/GaN HEMTs. Heating of the transistors during high-power operation of these devices becomes the important factor in limiting their performance after passivation.
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