Wide-bandgap GaN-based high electron mobility transistors (HEMTs) have been widely explored as promising candidates for high-frequency power amplifiers and high-voltage power switching applications owing to their superior material and device properties, including high breakdown electric field, high saturation velocity, and low on-resistance. Conventional GaN-based HEMTs are intrinsically depletion-mode because of spontaneous and piezoelectric polarization-induced two-dimensional electron-gas (2DEG) at the heterostructure interface. However, in power switching applications, normally-off GaN transistors are highly preferred for the inherent fail-safe operation and simpler gate drive circuit configurations. One of the most adopted approaches to realizing normally-off operation is to fabricate MOS-channel-HEMT (MOSC-HEMT) devices that feature heterostructure access regions and a gate region where the III-nitride barrier layer is fully recessed, usually through dry etching and replaced by an insulating gate dielectric. The poor controllability of the recess depth and dry-etching-induced damage may lead to poor uniformity in device performance and degraded electron mobility. Besides, the newly created gate-dielectric/GaN interface may present new challenges in suppressing/reducing the detrimental Ga-O bond-induced interface traps and the obtained stable device operation. In the first part of this thesis, the focus is on the comprehensive investigation of a Al
2O
3/AlN/GaN MOS structure that features an AlN interfacial layer grown in situ with Al
2O
3 in a PEALD system. An in situ remote plasma pretreatment (RPP) was first applied to remove the surface native oxide and then a monolayer-AlN was deposited prior to Al
2O
3 deposition in a plasma enhanced atomic layer deposition (PEALD) system. The structural and chemical composition characterizations of the gate-dielectric/GaN interface show a sharp and monocrystal-like interface with effective suppression of GaN surface oxidation. Frequency-dependent measurements of capacitance and conductance exhibit that the AlN interfacial layer not only results in lower interface trap density, but also enables lower border traps in proximity to the gate-dielectric/GaN interface. In the second part of this body of work, by adopting the high-quality Al
2O
3/AlN/GaN interface structure, a novel normally-off Al
2O
3/AlN/GaN MOS-Channel-HEMT was developed. The normally-off operation is realized by a digital etching process to precisely control recess depth and minimize etching-induced damages. Compared to a device without the AlN interfacial layer, the device with the AlN interfacial layer exhibits reduced threshold voltage (V
th) hysteresis because of the low-trap-density gate-dielectric/GaN interface. The device also demonstrates improved maximum drain current, enhanced maximum field-effect mobility, suppressed dynamic on-resistance degradation, and superior V
th thermal stability characteristics. In addition, the impact of the V
th shift on R
on was also established by a fast dynamic gate stress test. The third section of this thesis focuses on the optimization schemes. Using a combination of dry etching and digital etching techniques, the AlGaN barrier in the gate region was recessed into 2 nm to fabricate a normally-off recessed-thin-barrier GaN MIS-HEMT. The dry etching process was able to enhance etching efficiency, whereas digital etching was capable of mitigating the dry-etching-induced damages and precisely control the recess depth. The thin AlGaN barrier layer not only permitted a normally-off channel, but also simultaneously reserved the original hetero-interface, bringing in high channel electron mobility and reduced on-resistance.
| Date of Award | 2016 |
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| Original language | English |
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| Awarding Institution | - The Hong Kong University of Science and Technology
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