The emergence of GaN-based devices promises a revolution in areas requiring high performance electronics, such as high speed earth and space-based communication systems, advanced radar, integrated sensors, high temperature electronics, and utility power switching. The properties of this system make it ideally suited for operation at elevated temperatures and at voltage and current levels well beyond that accessible by Si. Recent improvements in material quality and device performance are rapidly opening the door to commercialization, and III-N technologies are demonstrating exciting developments of late.
Though devices are entering commercialization, there is still some work to be studied. In particular, GaN high electron mobility transistors (HEMTs) show potential as gas sensors, which will be relevant to the emerging hydrogen fuel cell vehicle market. Devices were fabricated and show a very low detection limit of <10 ppm, and have been integrated in a wireless network that is currently undergoing field testing. Devices have also been fabricated on different substrates such as Si (low-cost), SiC (high performance), and SopSiC (novel material – potentially combines advantages of Si and SiC) to study the effects on device performance and their compatibility with processing for 3-D integration. Reliability testing is a major area of interest. A 32-channel stress test system has been designed and is currently being built.
A 3-D integration project is being undertaken to achieve 3-D bonding of integrated circuits and other components fabricated from dissimilar materials. The thermal design of a vertically integrated multi-chip-module (MCM) based on GaN High Electron Mobility Transistor (HEMT) power amplifiers (PA) on SiC substrates with a backside heat sink/antenna and Si modulator, bonded to a common ground plane using polydimethylsiloxane (PDMS), was studied. Heat transfer was estimated using finite element modeling for different PA power densities, HEMT gate finger pitch, layer thickness, the presence or absence of the thermally insulating layers, and the thickness of dielectric isolation interlayers.
Laser drilling is a promising method for through hole via formation in SiC, presenting several advantages over dry etching, the most important being considerably higher etch rates. Studies have been undertaken to minimize the surface contamination and semiconductor degradation due to this process and, ultimately, make this process competitive with conventional dry etch techniques. By using a UV excimer laser source (193 nm) for drilling, we can achieve considerably better smoothness inside the holes and minimize surface contamination, compared to the use of the more common Nd:YVO4 laser. The device characteristics of AlGaN/GaN HEMT layers grown on SiC substrates were similar after formation of vias by 193 nm laser drilling to those from an undrilled reference sample. By sharp contrast, 1064, 532, and 355 nm laser drilling produces significant redepostion of ablated material around the via and degrades the electrical properties of the HEMT layers.
Flip-chip bonding is critical for 3-D integration is it is the method that actually forms the bonds. Studies have been performed to look at various materials systems, particularly In, Au, and SU-8 polymer, as candidate materials for flip-chip bonding applications. Bonding protocols have been developed to optimize the mechanical strength of the bond for a MEMS application.