For high-power rF applications, aluminum-gallium- nitride (AlGaN)/GaN high-electron mobility transistors (HEMTs) are predominantly fabricated on silicon or silicon-carbide substrates. The choice of substrate represents an array of essential material properties. Those properties, in turn, affect device performance and reliability. In application note AN-011, titled "Substrates for GaN RF Devices," Nitronex Corp. explains why it has adopted 100- mm silicon as its substrate of choice.
Although GaN HEMTs have been demonstrated on silicon, silicon carbide, sapphire, and native GaN substrates, the company recommends silicon and silicon carbide for RF devices. Silicon substrates are economical, scalable, mature, high in quality, plentiful, and consistent. GaN is not a viable alternative because of the material and economical challenges faced by the GaN-substrate research and development community as well as the immaturity of bulk GaN crystal growth. For its part, sapphire has poor thermal conductivity compared to silicon and silicon carbide.
The firm details how the growth of high-quality GaN on silicon can be achieved by addressing both lattice misfit and thermal-expansion-coefficient mismatch. The capabilities of Nitronex's GaN HEMT epi-wafer technology stem from the development of transition layers, which accommodate the stresses that originated from the differences in properties of silicon substrates and GaN-based materials. Such transition layers result in smooth epitaxial films that are free from micropipes and other substrate imperfections, which reduce performance, reliability, and yield.
Nitronex Corp., 2305 Presidential Dr., Durham, NC 27703; (919) 807-9100, FAX: (919) 807-9200, Internet: www.nitronex.com.