Current collapse and kink effect in GaN RF HEMTs: the key role of the epitaxial buffer

Abstract

GaN RF HEMTs all require a semi-insulating GaN layer to confine the electrons in the channel, and this is frequently achieved by incorporating iron as a deep acceptor. However, smaller densities of unintentionally incorporated carbon can actually have a larger effect than the iron on the dynamic behavior. We report simulations which include not only conventional deep level statistics but also hopping conduction along threading dislocations allowing a convincing representation of the floating p-type region which results from carbon co-doping. A wide range of experimental observations relating to important parameters and effects can be explained quantitatively. These include dynamic RON, dispersion in pulse I-V, kink effect, electric field crowding at the drain terminal, and complex drain current transients. It is strongly recommended that the incorporated carbon is taken more seriously and should be measured and reported much more widely than is currently the case.