GaN-based FETs for microwave power amplification

We review advances in GaN-based microwave power, field-effect-transistors ( FETs). Evolution in device technology included metal-semiconductor-field-ef fect-transistors (MESFETs), heterostructure-field-effect-transistors (HFETs ), modulation-doped-field-effect-transistors (MODFETs) or high-mobility-tra nsistors (HEMT), HEMTs with high Al contents, HEMTs with gate recess and Ga N-channel HEMTs grown on SiC substrates. The power density was first report ed as 1.1W/mm at 2 GHz using an AlGaN/GaN HEMT structure grown on sapphire substrate, and was subsequently improved to 1.5-1.7W/mm at 4-10 GHz by refi nement in device structure and processing techniques. This was advanced to 2.6-3.3 W/mm at 8-18 GHz by adopting: a high-Al-content AlGaN barrier layer . Success in gate recess helped to further increase the power density of th ese GaN HEMTs on sapphire substrates to 4.6 W/mm at 6 GHz. Substrate replac ement of sapphire by SiC, for excellent thermal dissipation, has boosted pe rformance to 6.9 W/mm at 10 GHz, which is higher than GaAs-based FETs by a factor of 6. Device periphery was scaled up to obtain high total output pow er. On one hand: GaN HEMTs on sapphire, using a flip-chip bonding technolog y for thermal management, have generated 7.6 W at 4 GHz. On another hand, G aN HEMTs on SiC; taking advantage of the high substrate thermal conductivit y, have achieved 9.1 W at 7.4 GHz. Two types of initial GaN-based power amp lifiers were also demonstrated using a flip-chip TC scheme. The transistors used were 0.7 to 0.8-mu m-long-gate GaN HEMTs. Bandwidths of 1-8 GHz and 3 -9 GHz were achieved with gains up to 11.5 dB. The output power levels rang ed from 3.2 to 4.6 W using devices with 2 and 3-mm gate peripheries, which were higher than that achievable with GaAs-bassed HEMTs of the same size by a factor of 2. Traps,in the device structure currently limit performance o f most GaN FETs. These traps cause dispersion in the T-V characteristics, w hich increases knee voltage and reduces channel current under RF gate drive . However, they are believed to be not inherent in the GaN semiconductor sy stem and can be minimized as the technology matures.