Study of direct tunneling through ultrathin gate oxide of field effect transistors using Monte Carlo simulation

Direct tunneling gate currents of ultrathin gate oxide thickness metal oxid e semiconductor field effect transistors (MOSFETs) are modeled in a two-ste p calculation procedure based on the treatment of physical microscopic data acquired during Monte Carlo device simulation. Gate currents are obtained by weighting the carrier perpendicular energy distribution at the Si/SiO2 a nd N+-poly-Si/SiO2 interfaces by the electron transmission probability, whi ch is calculated by the one-dimensional Schrodinger equation resolution wit h the transfer-matrix method. The procedure is applied to a 0.07 mu m gate length and 1.5 nm gate oxide thickness transistor, for which the gate and d rain voltage influences on gate currents are studied by assuming at first a uniform gate oxide layer. It is shown that the maximum gate current is obt ained for one of the two static points of complementary metal oxide semicon ductor inverters: V-GS=V-DD and V-DS=0, which raises a severe problem of st andby power consumption. The contribution of hot carriers to the tunnel cur rent is evaluated and is found to be small in case of such ultrathin oxide n-MOSFETs: contrary to thick (> 5 nm) gate oxide transistors, the maximum g ate current is not linked to the carrier energy peak in the channel but is located near the source well where the electron concentration is the larges t. Oxide thickness fluctuations are then considered by meshing the oxide su rface area and assuming a Gaussian law for the local oxide thickness deviat ion to the mean value. It is shown that a correct agreement is achieved wit h experimental published data when the oxide film nonuniformity is included in the calculation. Gate currents mapping for different bias conditions ar e given and analyzed, which show that very high current densities run throu gh the oxide layer in the vicinity of weak points. An estimate of the surfa ce through which flows the major part of the current is made, and a link be tween the highly nonuniform current leakage and the soft-breakdown mechanis m of the oxide layer is proposed. (C) 1999 American Institute of Physics. [ S0021-8979(99)01519-4].