There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

In this paper, we present a comprehensive reviewand discussion of the state-of-the-art device technology and application development of GaN-on-Si power electronics. Several device technologies for realizing normally off operation that is highly desirable for power switching applications are presented. In addition, the examples of circuit applications that can greatly benefit from the superior performance of GaN power devices are demonstrated. Comparisonwith other competingpower device technology, such as Si superjunction-MOSFET and SiC MOSFET, is also presented and analyzed. Critical issues for commercialization of GaN-on-Si power devices are discussed with regard to cost, reliability, and ease of use.

GaN-on-Si Power Technology: Devices and Applications

Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

/pdf/the-2018-gan-power-electronics-roadmap-2iunv619fh.pdf

The 2018 GaN power electronics roadmap

In this letter, 600-V normally-OFF SiNx/AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT) is reported. Normally-OFF operation and low OFF-state gate leakage are obtained by using fluorine plasma ion implantation in conjunction with the adoption of a 17-nm SiNx thin film grown by plasma-enhanced chemical vapor deposition as the gate insulator. The normally-OFF MIS-HEMT exhibits a threshold voltage of +3.6 V, a drive current of 430 mA/mm at a gate bias of 14 V, a specific ON-resistance of 2.1 mΩ·cm2 and an OFF-state breakdown voltage of 604 V at a drain leakage current of 1 μA/mm with VGS=0 V, and the substrate grounded. Effective current collapse suppression is obtained by AlN/SiNx passivation as proved by high-speed pulsed I-V and low-speed high-voltage switching measurement results.

600-V Normally Off ${\rm SiN}_{x}$ /AlGaN/GaN MIS-HEMT With Large Gate Swing and Low Current Collapse

Ohmic contacts to Gallium Nitride materials

In this letter, enhancement-mode (E-mode) AlGaN/ GaN high electron mobility transistors (HEMTs) were demon- strated based on lateral scaling of the 2-D electron gas channel using nanochannel array (NCA) structure. The NCA structure consists of multiple parallel channels with nanoscale width defined by electron-beam lithography and dry etching. Because of the improved gate control from the channel sidewalls and partially relaxed piezoelectric polarization, the fabricated 2 μm-gate-length NCA-HEMT with a nanochannel width of 64 nm showed a thresh- old voltage of +0.6 V and a higher extrinsic transconductance of 123 mS/mm, compared to -1.6 V and 106 mS/mm for the conventional HEMT with μm-scale channel width. The scaling of threshold voltages, peak transconductance, and gate leakage as a function of the nanochannel width were investigated. Small-signal RF performance of NCA-HEMTs were characterized for the first time and compared with those of conventional HEMTs.

Enhancement-Mode Operation of Nanochannel Array (NCA) AlGaN/GaN HEMTs

We report a high-performance normally-off Al
 2
O
 3
/AlN/GaN MOS-channel-high electron mobility transistor (MOSC-HEMT) featuring a monocrystalline AlN interfacial layer inserted between the amorphous Al
 2
O
 3
 gate dielectric and the GaN channel. The AlN interfacial layer effectively blocks oxygen from the GaN surface and prevents the formation of detrimental Ga-O bonds. Frequency-dispersion in C-V characteristics and threshold voltage hysteresis are effectively suppressed, owing to improved interface quality. The new MOSC-HEMTs exhibit a maximum drain current of 660 mA/mm, a field-effect mobility of 165 cm
 2
/V·s, a high on/off drain current ratio of ~10
 10
, and low dynamic on-resistance degradation.

Al 2 O 3 /AlN/GaN MOS-Channel-HEMTs With an AlN Interfacial Layer

An effective passivation technique that yields low off-state leakage and low current collapse simultaneously in high-voltage (600-V) AlGaN/GaN high-electron-mobility transistors (HEMTs) is reported in this letter. The passivation structure consists of an AlN/SiNx stack with 4-nm AlN deposited by plasma-enhanced atomic layer deposition and 50-nm SiNx deposited by PECVD. The AlN/ SiNx-passivated HEMTs with a gate-drain distance of 15 μm exhibit a high maximum drain current of 900 mA/mm, a low off-state current of 0.7 μA/mm at VDS = 600 V, and a steep subthreshold slope of 63 mV/dec. Compared with the static on-resistance of 1.3 mΩ·cm2, the dynamic on-resistance after high off-state drain bias stress at 650 V only increases to 2.1 mΩ·cm2. A high breakdown voltage of 632 V is achieved at a drain leakage current of 1 μA/mm .

High-Voltage (600-V) Low-Leakage Low-Current-Collapse AlGaN/GaN HEMTs with AlN/SiN x Passivation

In this paper, we systematically investigated the leakage and breakdown mechanisms of the low-pressure chemical vapor deposition (LPCVD) silicon nitride thin film deposited on AlGaN/GaN heterostructures. The LPCVD-SiN x gate dielectric exhibits low leakage and high breakdown electric field. The dominant mechanism of the leakage current through LPCVD-SiN x gate dielectric is identified to be Poole–Frenkel emission at low electric field and Fowler–Nordheim tunneling at high electric field. Both electric-field-accelerated and temperature-accelerated time-dependent dielectric breakdown of the LPCVD-SiN x gate dielectric were also investigated.

Characterization of Leakage and Reliability of SiN x Gate Dielectric by Low-Pressure Chemical Vapor Deposition for GaN-based MIS-HEMTs

A mechanism of the formation of the bulges on the surface of Ti/Al/Ni/Au Ohmic contact in AlGaN/GaN high electron mobility transistors is proposed. According to the analysis of TEM images and corresponding electron dispersive x-ray spectra, the bulges were found to consist of Ni–Al alloy in the body and Au–Al alloy surrounding. We deduce that the bulges were formed due to Ni–Al alloy aggregation in some local areas during the rapid thermal annealing process, which accounts for the rough surface morphology.

Shenghou Liu

Papers

Enhancement-Mode Operation of Nanochannel Array (NCA) AlGaN/GaN HEMTs

Al 2 O 3 /AlN/GaN MOS-Channel-HEMTs With an AlN Interfacial Layer

High-Voltage (600-V) Low-Leakage Low-Current-Collapse AlGaN/GaN HEMTs with AlN/SiN x Passivation

Characterization of Leakage and Reliability of SiN x Gate Dielectric by Low-Pressure Chemical Vapor Deposition for GaN-based MIS-HEMTs

Analysis of surface roughness in Ti/Al/Ni/Au Ohmic contact to AlGaN/GaN high electron mobility transistors