few materials that have both a wide and direct bandgap (almost twice that of SiC and GaN) and large thermal conductivity. This is due to its small atomic mass, strong interatomic bonds, and simple crystal structure. This property makes AlN attractive for application in high speed and high power communication networks. Many devices handle and manipulate large amounts of energy in small volumes and at high speeds, so due to the electrically insulating nature and high thermal conductivity of AlN, it becomes a potential material for high-power power electronics. Among group III-nitride materials, AlN has higher thermal conductivity compared to gallium nitride (GaN). Therefore, AlN is more advantageous than GaN in terms of heat dissipation in many power and radio frequency electronic devices. Thermal expansivity is another critical property for high temperature applications. The calculated thermal expansion coefficients of AlN at 300 K are 4.2×10-6 K-1along a-axis and 5.3×10-6 K-1 along c-axis. Stability and chemical properties Aluminium nitride is stable at high temperatures in inert atmospheres and melts at about 2200 °C. In a vacuum, AlN decomposes at ~1800 °C. In the air, surface oxidation occurs above 700 °C, and even at room temperature, surface oxide layers of 5–10 nm thickness have been detected. This oxide layer protects the material up to 1370 °C. Above this temperature bulk oxidation occurs. Aluminium nitride is stable in hydrogen and carbon-dioxide atmospheres up to 980 °C. The material dissolves slowly in mineral acids through grain-boundary attack and in strong alkalies through attack on the aluminium-nitride grains. The material hydrolyzes slowly in water. Aluminium nitride is resistant to attack from most molten salts, including chlorides and cryolite. Aluminium nitride can be patterned with a Cl2-based reactive ion etch. Manufacture AlN is synthesized by the carbothermal reduction of aluminium oxide in the presence of gaseous nitrogen or ammonia or by direct nitridation of aluminium. The use of sintering aids, such as Y2O3 or CaO, and hot pressing is required to produce a dense technical-grade material. Applications Epitaxially grown thin film crystalline aluminium nitride is used for surface acoustic wave sensors (SAWs) deposited on silicon wafers because of AlN's piezoelectric properties. One application is an RF filter, widely used in mobile phones, which is called a thin-film bulk acoustic resonator (FBAR). This is a MEMS device that uses aluminium nitride sandwiched between two metal layers. AlN is also used to build piezoelectric micromachined ultrasound transducers, which