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Soft compression and the origins of nonlinear behavior of GaN HEMTs

Pedro, J. C. ; Nunes, L. C. ; Cabral, P. M.

Soft compression and the origins of nonlinear behavior of GaN HEMTs , Proc European Microwave Conf. - EMC, Roma, Italy, Vol. 1, pp. 1297 - 1300, October, 2014.

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This work responds to the commonly formulated question in PA design of “Why do GaN HEMTs' nonlinear behavior seems so distinct from the one of their Si LDMOS, GaAs MESFETs or HEMTs counterparts?”. Starting from some recent results on the origin of AM/AM and AM/PM distortion in power amplifiers made for these two device types, we demonstrate that there is no fundamental reason why the devices should be different, except that the formers suffer from a noticeable low-frequency dispersion. Then, we dig into this particular aspect of GaN HEMT operation, to show that this charge trapping-related phenomenon is responsible for a severe self-biasing capable of inducing soft-compression of an otherwise almost flat AM/AM gain plot - when measured with static CW tests - which, in fact, corresponds to a severe gain expansion when the AM/AM is assessed with more realistic dynamic tests performed with real communication signals. And it is this class-C PA like AM/AM that is responsible for the recognized GaN HEMT nonlinearity.