3D GaN nanoarchitecture for field-effect transistors

The three-dimensionality of 3D GaN field-effect transistors (FETs) provides them with unique advantages compared to their planar counterparts, introducing a promising path towards future FETs beyond Moore's law. Similar to today's Si processor technology, 3D GaN FETs offer multi-gate structures that provide excellent electrostatic control over the channel and enable very low subthreshold swing values close to the theoretical limit. Various concepts have been demonstrated, including both lateral and vertical devices with GaN nanowire (NW) or nanofin (NF) geometries. Outstanding transport properties were achieved with laterally contacted NWs that were grown in a bottom-up approach and transferred onto an insulating substrate. For higher power application, vertical FETs based on regular arrays of GaN nanostructures are particularly promising due to their parallel integration capability and large sidewall surfaces, which can be utilized as channel area. In this paper, we review the current status of 3D GaN FETs and discuss their concepts, fabrication techniques, and performances. In addition to the potential benefits, reliability issues and difficulties that may arise in complex 3D processing are discussed, which need to be tackled to pave the way for future switching applications

GaN-based power devices: Physics, reliability, and perspectives

Over the last decade, gallium nitride (GaN) has emerged as an excellent material for the fabrication of power devices. Among the semicon- ductors for which power devices are already available in the market, GaN has the widest energy gap, the largest critical field, and the highest saturation velocity, thus representing an excellent material for the fabrication of high-speed/high-voltage components. The presence of spon- taneous and piezoelectric polarization allows us to create a two-dimensional electron gas, with high mobility and large channel density, in the absence of any doping, thanks to the use of AlGaN/GaN heterostructures. This contributes to minimize resistive losses; at the same time, for GaN transistors, switching losses are very low, thanks to the small parasitic capacitances and switching charges. Device scaling and monolithic integration enable a high-frequency operation, with consequent advantages in terms of miniaturization. For high power/high- voltage operation, vertical device architectures are being proposed and investigated, and three-dimensional structures—fin-shaped, trench- structured, nanowire-based—are demonstrating great potential. Contrary to Si, GaN is a relatively young material: trapping and degradation processes must be understood and described in detail, with the aim of optimizing device stability and reliability. This Tutorial describes the physics, technology, and reliability of GaN-based power devices: in the first part of the article, starting from a discussion of the main proper- ties of the material, the characteristics of lateral and vertical GaN transistors are discussed in detail to provide guidance in this complex and interesting field. The second part of the paper focuses on trapping and reliability aspects: the physical origin of traps in GaN and the main degradation mechanisms are discussed in detail. The wide set of referenced papers and the insight into the most relevant aspects gives the reader a comprehensive overview on the present and next-generation GaN electronics

Modelling the cryogenic properties of germanium for emerging liquid hydrogen power applications

Ph. D. ThesisIn recent years, there has been an increase in research focused towards the reduction and/or elimination of greenhouse emissions from applications used in everyday life. In addressing this, liquid hydrogen has been highlighted as an attractive alternative fuel source for commercial vehicles due to it’s lower weight, higher power density and zero greenhouse emissions in comparison to petrol and diesel fuels. Incorporating such a fuel source however introduces a cryogenic environment of 20 K affecting the power electronics used to deliver the power from source to load. Herein, the physical properties of semiconductors influencing the overall efficiency of devices within an H-bridge circuit are considered. From this, germanium is hypothesised to be the most suitable semiconductor for power devices at or near temperatures of 20 K. Closed-loop models are developed for the carrier concentration, carrier mobility, carrier velocity, for both electrons and holes as a function of doping concentration and temperature with critical analysis of the range of suitability for each. Multiple models are also developed for both carrier concentration and carrier mobility which offer a trade off depending on whether one requires accuracy or simplicity in calculation. A significant influence on the device characteristics of MOSFETs is that of the oxide/semiconductor interface. For the first time, ZrO2 is fabricated directly on germanium substrates through the thermal oxidation of zirconium on germanium. The interface state density of these capacitors are comparable to literature values offering a much cheaper and simpler fabrication method for high-κ dielectric formation on germanium substrates. The leakage current density of the ZrO2 MOS capacitors are low in comparison to reported values and are shown to decrease with decreasing temperature. With the physical models of both bulk and interfacial germanium, multiple PiN germanium diodes are simulated using technology computer aided design (TCAD) that show the potential for germanium power devices with breakdown voltages in excess of 800 V at room temperature and 400 V at 20 K. Simulations of vertical power MOSFETs incorporating a ZrO2 interlayer show great promise for low temperature power electronics at or near 20 K where other commercial devices experience significant resistive losses. With the work conducted here, vertical power MOSFETs fabricated using germanium and ZrO2 open the gateway for low voltage applications incorporating liquid hydrogen fuel cells.Engineering and Physical Sciences Research Counci