Carrier transport related analysis of high-power AlGaN/GaN HEMT structures

Shubnikov-de Haas (SdH) oscillation and Hall measurement results were compared with HEMT DC and RF characteristics for two different MOCVD grown AlGaN-GaN HEMT structures on semiinsulating 4H-SiC substrates. A HEMT with a 40-nm, highly doped AlGaN cap layer exhibited an electron mobility of 1500 cm/sup 2//V/s and a sheet concentration of 9/spl times/10/sup 12/ cm at 300 K (7900 cm/sup 2//V/s and 8/spl times/10/sup 12/ cm/sup -2/ at 80 K), but showed a high threshold voltage and high DC output conductance. A 27-nm AlGaN cap with a thinner, lightly doped donor layer yielded similar Hall values, but lower threshold voltage and output conductance and demonstrated a high CW power density of 6.9 W/mm at 10 GHz. The 2DEG of this improved structure had a sheet concentration of n/sub SdH/=7.8/spl times/10/sup 12/ cm/sup -2/ and a high quantum scattering lifetime of /spl tau//sub q/=1.5/spl times/10/sup -13/ s at 4.2 K compared to n/sub SdH/=8.24/spl times/10/sup 12/ cm/sup -2/ and /spl tau//sub q/=1.72/spl times/10/sup -13/ s for the thick AlGaN cap layer structure, Despite the excellent characteristics of the films, the SdH oscillations still indicate a slight parallel conduction and a weak localization of electrons. These results indicate that good channel quality and high sheet carrier density are not the only HEMT attributes required for good transistor performance.