New hot-carrier degradation mode in PMOSFETs with W gate electrodes

Hot-carrier degradation of W gate PMOSFETs, which are surface-channel devices because of the work function of W, has been investigated in comparison with polycide (WSi_x/n⁺ poly-Si) ones. In W gate PMOSFETs, transconductance g_m and threshold voltage V_th decrease on the drain avalanche hot-carrier (DAHC) stress, and Δg_m /g_m0 and ΔV_th become minimum at V_G?V_D/2. By using the charge-pumping technique, it is found that, after stressing at the same stress condition, the interface state density of W gate devices is about 10 times larger than that of polycide ones but the densities of trapped electrons are almost equal. These results indicate that the difference of hot-carrier degradation between W and polycide gate devices is mainly caused by the difference of the interface state density