High speed FETs fabricated in GaAs/AlGaAs layered structures prepared by molecular beam epitaxy

The research on modulation doped AlGaAs/GaAs FETs (MODFETs) and on AlGaAs buffer GaAs MESFETs has continued with substantial progress. Current-voltage collapse observed in the drain characteristics of MODFETs in dark at 77K have for the first time been eliminated. The responsible mechanisms were found to relate to the defect concentration in AlGaAs and also to the gate recess width relative to the gate. Microwave S-parameter measurements were also made to deduce the device equivalent circuit parameters and compared to conventional GaAs MESFETs. The results are compared qualitatively to predictions. In GaAs FETs with high resistivity AlGaAs buffer layers it was found that no improvement (relative to GaAs buffer FETs) but degradation can result if the AlGaAs buffer GaAs active layer interface is not of high quality. The only way high quality can be assured has been found to be via the use of a thin superlattice at the heterointerface. Both optical and electrical properties of the GaAs layer grown on this superlattice are of sufficiently high quality to lead the expected improvements in the device performance. With the super lattice AlGaAs/GaAs MESFETs better RF performance has been obtained.