講演情報
[10p-N301-6]Transport limitation in N-polar AlN-based thin GaN channel HEMTs
〇Markus Pristovsek1, Yoann Robin1, Xu Yang1, Itsuki Furuhashi1, Chengzhi Zhang2, Matthew D. Smith2, Martin Kuball2, Sheng Zhang3, Xinhua Wang3 (1.Nagoya Univ., 2.Bristol Univ., 3.Inst. Microelectr. CAS)
キーワード：
HEMT、N-polar、AlN
N-polar AlN-based high electron mobility transistors (HEMTs) have intrinsic advantages over GaN-based HEMTs like the highly confined Ga-channel with direct contacts, no leaking through a back-barrier and a simple buffer structure with low thermal resistance. Because the growth of smooth N-polar AlN has been only recently mastered, first depletion-mode HEMTs have been reported by Cornell and Yamaguchi Univ. in 2022/2023. The devices from Cornell on bulk AlN substrates show low sheet resistance and thus high saturation currents, high transconductance and good RF performance. The devices from Yamaguchi on sapphire had higher sheet resistance and thus lower saturation currents. All devices showed considerable source-drain leakage in the off state and consequently low on/off ratios below 100.
We have made enhancement-mode HEMTs on sapphire, processed at Bristol Univ. using a p-NiO Schottky gate. These devices show a very high on/off ratio of 107 and a threshold voltage of ~1 V with a saturation current of 90 mA/mm. Depletion-mode HEMT processed at CAS in Beijing showed higher saturation currents up to 250 mA/mm and on/off ratios of 1000-10,000. We attribute our high on/off ratio to a direct AlN/GaN interface, avoiding the AlN/AlGaN interface in the transition layer used by the other groups which could act as a buried gate or even as a second conducting channel.
The saturation currents of our HEMTs are limited by the sheet resistance Rs~2000 Ohms. Therefore, we have studied the impact of various parameters on Rs. The largest reduction was by the channel thickness. To understand this, we have performed variable temperature Hall-measurements together with band structure calculations using k*p theory with a Poisson solver. Our results indicate that the wave-function probability near the surface is much higher for excited states for thinner structures compared to thicker layers, also saturating at 10 nm.
We have made enhancement-mode HEMTs on sapphire, processed at Bristol Univ. using a p-NiO Schottky gate. These devices show a very high on/off ratio of 107 and a threshold voltage of ~1 V with a saturation current of 90 mA/mm. Depletion-mode HEMT processed at CAS in Beijing showed higher saturation currents up to 250 mA/mm and on/off ratios of 1000-10,000. We attribute our high on/off ratio to a direct AlN/GaN interface, avoiding the AlN/AlGaN interface in the transition layer used by the other groups which could act as a buried gate or even as a second conducting channel.
The saturation currents of our HEMTs are limited by the sheet resistance Rs~2000 Ohms. Therefore, we have studied the impact of various parameters on Rs. The largest reduction was by the channel thickness. To understand this, we have performed variable temperature Hall-measurements together with band structure calculations using k*p theory with a Poisson solver. Our results indicate that the wave-function probability near the surface is much higher for excited states for thinner structures compared to thicker layers, also saturating at 10 nm.