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Metamorphic GaAs HEMTs의 전기적 특성 및 신뢰성 평가

Metamorphic GaAs HEMTs의 전기적 특성 및 신뢰성 평가
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In this study, the 35-nm T-gate In0.52Al0.48As/In0.53Ga0.47As metamorphic GaAs high electron mobility transistor (mHEMT) are fabricated and its DC, radio frequency (RF) and low-frequency characteristics are measured and analyzed. To reduce gate resistance and gate length at once, T-shaped gate structure is used. Two-step electron-beam (e-beam) lithography on the tri-layer e-beam resists is used to fabricate T-gate pattern. It is difficult to fabricate the T-gate under 50 nm because the short gate foot cannot support its big head. To fabricate mechanically stable T-gate without supporting layer, the zigzag-shape foot is introduced instead of a conventional straight foot.The maximum transconductance and the current gain cutoff frequency of the fabricated device are 1010 mS/mm and 311 GHz respectively. The hot electron effects on DC and low-frequency noise characteristics are also analyzed. Hot-electron stress causes the increase of drain current and transconductance. After the 1000 seconds of hot electron stress, the threshold voltage of the device is negatively shifted about 55 mV. After the relaxation, the 45 mV of threshold shift is not recovered. The impact ionized holes are trapped and accumulated at the layer above the channel. Before the hot-electron stress, the low-frequency noise spectral density uniformly decreases with however after the hot-electron stress, the slope of noise spectral density changes. The change of low-frequency noise characteristic shows the trap density of the barrier layer increased. The increase of trap density near gate is higher than near the channel.
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