Origin of the Increasing Access Resistance in AlGaN/GaN HEMTs

We have studied the origin of the increasing source access resistance in GaN HEMTs through a combination of theoretical modeling, simulations and experimental approaches. The reduction of the differential mobility as the electric field in the source region increases to 30 KV/cm in open channel conditions has been identified as the main cause of the increasing access resistance. Although excellent qualitative agreement between our modeling/simulations and the experimental results has been found, the quantitative results are very sensitive to the actual transport model in nitrides. From our results, the electron transport in the 10-30 kV/cm range is better described by a Trofimenkoff model than by the different Monte Carlo simulations reported in the literature, which indicates than current Monte Carlo techniques are overestimating the mobility at moderate electric fields (10-30 KV/cm). The design of new transistor structures with more constant access resistances is critical to improve the frequency performance and linearity of these devices and several new structures will be proposed at the meeting.