Characterization and modeling of large signal dynamics of GaN HEMTs
Résumé
RF GaN power technologies are emerging for future equipment in the defence, space and telecommunications domains. Indeed the unique properties of GaN Field Effect Transistors in terms of breakdown voltage associated to high cut-off frequencies make possible a breakthrough for the development of new Solid State Power Amplifiers (SSPA) for Radars and Telecommunications applications. In both domains those SSPA are fed by modulated signals such as versatile pulses in the case of Radars or highly modulated signals in the case of telecommunications applications. Therefore it is essential to assess the dynamical behavior of those RF devices fed by large modulated signals. Contrary to CW conditions, the dynamics of parasitic created by trapping and thermal effects can no longer be ignored. In order to get insights into the impact of those effects on large signal characteristics, several characterization techniques are required. Classical DC/pulsed IV characteristics, S-parameters or CW load pull measurement don't provide enough information to characterize the large signal dynamics of the devices. Therefore one has to rely on Low Frequency characterization as well as on Load-Pull measurement with modulated signals. In this presentation several techniques will be reviewed which allow to extract and verify the non linear models of GaN HEMTs in real working conditions. Several characteristics must be obtained concerning the thermal behavior or the trapping behavior of the device. Those characteristics are the inputs of a nonlinear model which is able to reproduce the dynamics of the device. Different experimental test benches will be described: ??? Ultra wide band (10Hz-40GHz) S-parameters measurement set-up: this set-up allows to obtain the S-parameters of the transistor not only in the high frequency range to extract the RF equivalent circuit, but also in the very low frequency range to characterize the dispersion phenomenon due to the traps. [1,2] ??? A dedicated test benc- to directly measure the thermal impedance of the device. This bench relies on the 3 ?? method which takes advantage of the generation of harmonics due to the self heating of the device to obtain the thermal impedance. [3] ??? A time domain load pull system which allows to perform load pull measurement with pulsed signals and multitone telecommunications signals. [4-8]. This test set-up allows to assess large signal dynamics of GaN HEMTs as well as to identify traps characteristics [9]. All these measurement systems, associated to classical ones (Pulsed IV, S-parameters, CW load pull) allow a complete characterization of the small signal and large signal characteristics. From those characteristics, the extraction of an electrothermal nonlinear model will be described. This model takes into account the nonlinear dynamics of the traps and it will be shown that it is able to reproduce measured characteristics both in the CW mode and in the pulsed mode [10-12]. Some conclusions will be drawn to address the challenge of characterization and modeling of GaN HEMTs for the design of RF power amplifiers.