TiW/Si self-aligned gate for GaAs MESFETs

A major difficulty with fabricating GaAs digital logic circuits using enhancement-mode MESFETs has been the large gate-source and gate-drain parasitic resistances inherent in conventional designs. A self-aligned gate process is presented, which incorporates a mushroom gate structure for self-aligning both an n+ implant and the source/drain contacts to the gate, thus minimizing the parasitic resistances. The mushroom gate consists of a two-layer TiW/Si metallization in which the bottom TiW layer is undercut with a closely controllable chemical etch. The process is compatible with the high temperature anneal necessary to activate ion-implanted GaAs.