Impact of Drain-Lag Induced Current Degradation for a Dynamically Operated GaN-HEMT Power Amplifier

In this paper, the impact of charge trapping effects in a GaN-HEMT power amplifier is analyzed and quantified, using a low complexity model for drain-lag effects. The peak drain-source voltage under modulated operation is used to define a dynamic shift of the threshold voltage, resulting in a variation of the bias point. The quantification is based on continuous wave (CW) measurements but is shown to allow accurate modeling of the drain current under modulated operation. Therefore, it allows to accurately predict the efficiency for wideband modulated operation. The theory extracted from the CW based model is verified by wideband modulated measurements using an eighttone signal with 28 MHz instantaneous modulation bandwidth. The peak-to-average power ratio of the signal is swept in a range from 2 to 9 dB to sweep the peak drain-source voltage at constant average output power. The findings bring further evidence that charging-times for some traps in GaN-HEMTs are in the picosecond range and thus fast enough so that, in the low GHz range, the RF swing can charge the traps.