Piezoelectric effect suppression in GaAs FETs by using Y-shaped gate structures

The overlayer stress dependence and the gate structure dependence of the threshold voltage shift (ΔV_T ) caused by the piezoelectric effect are investigated. The reduction of the overlayer stress is effective to reduce ΔV_T in long gate-length FETs, but in short gate-length FETs ΔV_T remains high. In FETs with a Y-shaped gate, ΔV_T is smaller than that of rectangular gate FETs. This is because the Y-shaped gate wings relax the stress under the gate electrode. Experiments and two-dimensional simulations show that ΔV_T decreases as the length (L_w ) between the gate edge adjoining the substrate and the Y-shaped gate wing edge increases. They also show that ΔV_T is reduced to almost 0 V when L_w is greater than 0.7 μm.