645 V quasi-vertical GaN power transistors on silicon substrates

In this paper, we present GaN-on-Si vertical transistors consisting of a 6.7 pm thick n-p-n heterostructure grown on 6-inch silicon substrates by MOCVD. The fabricated vertical trench gate MOSFETs exhibited E-mode operation with a threshold voltage of 3.3 V and an on/off ratio of over 108. A specific on-resistance of 6.8 mSL"cm2 and a high off-state breakdown voltage of 645 V were achieved. These results show the great potential of the GaN-on-Si platform for the next generation of cost-effective power electronics

GaN-on-Si Quasi-Vertical Power MOSFETs

We demonstrate the first GaN vertical transistor on silicon, based on a 6.7-mu m-thick n-p-n heterostructure grown on 6-inch silicon substrate by metal organic chemical-vapor deposition. The devices consist of trench-gate quasi-vertical metal-oxide-semiconductor field-effect transistors with a 4-mu m-thick drift layer, exhibiting enhancement-mode operation with a threshold voltage of 6.3V and an ON/OFF ratio of over 10(8). A high OFF-state breakdown voltage of 645 V along with a specific ON-resistance of 6.8 m Omega.cm(2) were achieved thanks to the high-quality 4-mu m-thick GaN drift layer, presenting a relatively low defect density and very high electron mobility (720 cm(2)/V.s). This excellent performance represents a major step toward the realization of high-performance GaN vertical power transistors on low-cost silicon substrates

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

GaN vertical power devices on silicon substrates

Gallium Nitride (GaN) is a wonder material which has widely transformed the world by enabling energy-efficient white light-emitting diodes. Over the past decade, GaN has also emerged as one of the most promising materials for developing power devices which can operate at significantly higher power densities, higher temperatures, and higher frequencies, thanks to inherently superior material properties like higher bandgap, 10x higher critical electric field, and 3x higher electron saturation velocity, compared to silicon. Lateral GaN high electron mobility transistors (HEMTs) based on the AlGaN/GaN heterostructures capable of switching at high frequencies over 10 MHz have been already commercialized and are the device of choice for implementing modern-day adapters and for wireless charging solutions. However, for high-voltage and high-current applications, it is envisaged that vertical GaN power devices will play a crucial role given that these devices dont scale in size for increasing the BV unlike HEMTs, and are not affected by surface trap related reliability issues. The main bottleneck towards the commercialization of vertical GaN devices on bulk GaN substrates is the high cost and small size availability of these substrates. Similar to lateral GaN HEMTs, GaN epitaxial layers grown on silicon substrate could also become a game-changer for vertical GaN power devices considering that silicon substrates are significantly cheaper and are available in large sizes up to 12-inch diameters which can greatly accelerate viable commercialization. However, there are several roadblocks arising from the growth as well as fabrication perspective that has limited the demonstration of high-performance power devices on GaN-on-Si. In this thesis, we discuss the key hindrances and our solutions for improving the feasibility of GaN-on-Si vertical power devices. All the necessary fabrication steps were first optimized from scratch to develop state-of-the-art power devices. As a first demonstration, we could develop a GaN p-i-n diode with an ultra-low Ron,sp of 0.33 mohmcm2 and record BV of 820 V with a voltage blocking GaN layer of just 4 um. A quasi-vertical MOSFET was then demonstrated for the first time on GaN-on-Si platform with excellent ON- and OFF-state performances. We then probed the limits of current crowding, a main deterrent to the current up-scaling of quasi-vertical devices by exploring large area quasi-vertical MOSFETs. A novel and robust method for achieving a fully-vertical design for GaN-on-Si devices was developed which led to an exemplary improvement in the ON-state performance of quasi-vertical MOSFETs. Device integration has been identified by many leading power semiconductor companies as the way forward due to significant advantages to be had, as a result of lower parasitics and simplified packaging. Taking a cue from these developments, we demonstrated vertical GaN power MOSFETs with integrated freewheeling diodes and reverse blocking capability as described in Chapter 4. In the last chapter, we introduce p-type NiO as a possible substitute for p-GaN for realizing high-performance p-i-n diodes and as junction termination extensions (JTEs) for Schottky barrier diodes. Our initial results point to a strong future for p-NiO to be used for realizing a myriad of reliable GaN power devices

High performance Fully-vertical GaN-on-Si power MOSFETs

We report the first demonstration of fully-vertical power MOSFETs on 6.6-μm-thick GaN grown on a 6-inch Si substrate by metal-organic chemical vapor deposition (MOCVD). A robust fabrication method was developed based on a selective and local removal of the Si substrate as well as the resistive GaN buffer layers, followed by a conformal deposition of a 35-μm-thick copper layer on the backside by electroplating, which provides excellent mechanical stability and electrical contact to the drain terminal. The fabrication process of the gate trench was optimized, improving considerably the effective mobility at the p-GaN channel and the output current of the devices. High performance fully-vertical GaN-on-Si MOSFETs are presented, with low specific on-resistance (R on,sp ) of 5 mΩcm 2 and high off-state breakdown voltage (BV) of 520 V. Our results reveal a major step towards the realization of high performance GaN vertical power devices on cost-effective Si substrates

Quasi-vertical GaN-on-Si reverse blocking power MOSFETs

We demonstrate quasi-vertical reverse blocking (RB) MOSFETs on 6.7 mu m thick GaN grown on a 6 inch Si substrate by metalorganic chemical vapor deposition. The RB capability was achieved by replacing the ohmic drain with a quasi-vertical Schottky drain, resulting in a RB voltage of similar to 300 V while preserving the ON-resistance (R-on,R-sp). Schottky contacts on etched i-GaN surface were realized through an optimized fabrication process based on tetramethylammonium hydroxide treatments. The fabricated RB-MOSFET had a low R-on,R-sp of 4.75 m omega cm(2), current density of similar to 0.9 kA cm(-2) and a forward blocking voltage of 570 V