emit and receive ultrasound and which can be used for in-air rangefinding over distances of up to a meter. Metallization methods are available to allow AlN to be used in electronics applications similar to those of alumina and beryllium oxide. AlN nanotubes as inorganic quasi-one-dimensional nanotubes, which are isoelectronic with carbon nanotubes, have been suggested as chemical sensors for toxic gases. Currently there is much research into developing light-emitting diodes to operate in the ultraviolet using gallium nitride based semiconductors and, using the alloy aluminium gallium nitride, wavelengths as short as 250 nm have been achieved. In 2006, an inefficient AlN LED emission at 210 nm was reported. AlN-based high electron mobility transistors (HEMTs) have attracted a high level of attention due to AlN’s superior properties, such as better thermal management, reduced buffer leakage, and excellent integration for all nitride electronics. AlN buffer layer is a critical building block for AlN-based HEMTs, and it has been grown by using MOCVD or MBE on different substrates. Building on top of AlN buffer, n-channel devices with 2D electron gas (2DEG) and p-channel devices with 2D hole gas (2DHG) have been demonstrated. The combination of high-density 2DEG and 2DHG on the same semiconductor platform makes it a potential candidate for CMOS devices. Among the applications of AlN are opto-electronics, dielectric layers in optical storage media, electronic substrates, chip carriers where high thermal conductivity is essential, military applications, as a crucible to grow crystals of gallium arsenide, steel and semiconductor manufacturing. See also Boron nitride Aluminium phosphide Indium nitride Aluminium oxynitride Titanium aluminium nitride, TiAlN or AlTiN References Cited sources Nitrides Aluminium compounds Piezoelectric materials III-V semiconductors Light-emitting diode materials III-V compounds Wurtzite structure type