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

Shubnikov-de Haas (SdH) oscillation and Hall measurement results were compared with HEMT de 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(2)/V/s and a sheet concentration of 9 x 10(12) cm(-2) at 300 K (7900 cm(2)/V/s and 8 x 10(12) cm(-2) at 80 K), but showed a high threshold voltage and high de 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(SdH) = 7.8 x 10(12) cm(-2) and a high quantum scattering lifetime of tau(q) = 1.5 x 10(-13) s at 4.2 K compared to n(SdH) = 8.24 x 10(12) cm(-2) and tau(q) = 1.72 x 10(-13) s for the thick AlGaN cap layer cm 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.