Large area GaN HEMT power devices for power electronic applications: switching and temperature characteristics

Large area AlGaN/GaN high electron mobility transistors (HEMT) for power electronic applications have been fabricated. These power devices offer lower on-resistance and higher switching speed than SiC devices due to higher electron mobility and high channel charge density achieved by a heterojunction. The GaN epi-layers were grown on semiinsulating 4H-SiC substrate by metal organic chemical vapor deposition (MOCVD) technique. The device structure was grown on SiC substrates due to its high thermal conductivity. The devices have been optimised with respect to electron mobility, sheet concentration, voltage breakdown, on-resistance and dispersion. Voltage breakdown of 1300 V was achieved on small devices while breakdown in the range 600-900 V was achieved on packaged devices depending on the number of devices that have been paralleled. The power device figure of merit V_BR²/R_on=9.94×10⁸ [V²·Ω^-1 cm^-2], where V_BR is the breakdown voltage and R_on is the on-resistance, is the highest among any reported switching devices. Switching losses of large area 600 V/2.5 A power devices were measured using resistive and inductive loading. Switching times of <30 ns were achieved with an on-resistance of 0.4 Ω (specific on-resistance=1.7 mΩ·cm²). The static and dynamic characteristics of GaN HEMT devices were also measured as a function of temperature up to 200°C. Finally, the temperature distributions in the active device area were measured using Raman spectroscopy (pyrospectroscopy). This technique can be used to measure temperatures with a spatial resolution of 1-2 μm. Device temperatures from both the active areas and SiC substrates have been measured.