AlGaN/GaN HEMTs have played an important role in microwave and millimeter-wave power application due to the excellent characteristics, such as a wide bandgap, a high breakdown field, a high saturation velocity and a high thermal conductivity.
In this work, fabrication processes of AlGaN/GaN HEMT are optimized. Mesa isolation is achieved by $Ar (6 sccm)/Cl_2 (4 sccm)$ mixed gases based remote ion beam etching (RIBE) with beam voltage of 1000 V and an etching rate of 400 A/min for GaN material. Ohmic contact for source and drain are formed using the Ti/Al/Ti/Au (150/1000/450/550 A), which are evaporated, and annealed at 850$\circ$C for 30 sec in $N_2$ ambient. Contact resistance of 0.34 $\Ohm$ㆍmm is obtained from a standard TLM measurement. Finally, Schottky contact for gate is formed by evaporating Ni/Au (200/3000 A), which has an ideality factor of 1.4. Using the optimized fabrication processes, the AlGaN/GaN HEMTs are fabricated and good characteristics, such as maximum drain current of 683 mA/mm, transconductance of 170 mS/mm, current-gain cutoff frequency of 10.6GHz, and maximum oscillation frequency of 32.3 Ghz, are measured.
In addition, to reduce the dispersion problem, a photosensitive Benzocyclobutene (BCB) material, which have advantage of much simpler fabrication processes, a lower dielectric constant, and a lower surface damage, is proposed and measured by means of DC, RF, pulse, noise and large-signal microwave power characteristics to investigate the effects of passivation. Also, the SiN passivated AlGaN/GaN HEMTs as conventional method are fabricated and compared with the proposed method. Comparing of measurement results, the photosensitive BCB passivated AlGaN/GaN HEMTs demonstrated similar DC, pulse and microwave power characteristics and better RF and noise characteristics.
To improve breakdown characteristic, field plate (FP) structure is fabricated and investigated. AlGaN/GaN HEMTs with field plate are shown better breakdown characteristic, but, worse RF characteristics than AlGaN/GaN HEMTs without field plate. Finally, double field plate (DFP) structure is proposed. Comparing to the field plate structure, the double filed plate structure exhibits better breakdown characteristic and similar RF characteristics.
