On the correlation between intermodulation distortion and RF transconductance for microwave GaN HEMT
Résumé
This paper investigates the correlation between intermodulation distortion and RF transconductance in AlGaN/GaN high electron mobility transistor (HEMT) grown on SiC Substrate. Two-tone intermodulation distortion measurements were carried out across a comprehensive biasing range changing the frequency and ambient temperature and input power to point out this correlation. The main contribution is that, the correlation between intermodulation distortion and the behavior of transconductance and its derivatives (Gm, Gm2, and Gm3) is investigated and summarized. It is demonstrated that the nonlinearity is directly correlated to the RF transconductance behavior where the peak values of the fundamental IMD arise at the peak of Gm and the null points in IMD2 and IMD3 arises when Gm2 and Gm3 alters from positive to negative magnitudes. The changes in frequency, input power and temperature affect the threshold voltage VT which in turn impacts on that phenomenon. As the nulls/notches are crucial indicator for identifying minimum distortion level, an empirical model for null's current is develop in correlation with transconductance. Finally, the measured and simulated IMDs as well as the null's current shows very good agreement. In addition, the small-signal measurement and as well as the de-embedding of the device has been done before the IMD study. The power gain and its derivatives also compared to the transconductance. This analysis is important to know how the intermodulation distortion products behave and to identify a distortion free device at the particular operation point of interest.