Realisation of very high transconductance GaAs MESFETs

By using a model which considers velocity overshoot, it is shown that the performance of GaAs MESFETs in enhancement mode depends strongly on the geometrical and electrical characteristics of the access region between source and gate. The sheet resistance of the unrecessed epilayer, and the distance between the source-end of the recessed region and the gate, have to be as small as possible. 300 nm gate length MESFETs with very low values for these parameters were realised with an n-GaAs active layer (6×10¹⁷ cm^-3). These devices exhibit very high microwave transconductances (800 mS/mm) with good cutoff frequencies (up to 55 GHz). This result suggests that very high transconductance MESFETs can be fabricated from not-too-heavily doped active layers provided that the characteristics of the source-gate access region is properly optimised