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 oxide semiconductor field effect transistors (MOSFETs) are modeled in a two-step 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 and N⁺-poly–Si/SiO₂ interfaces by the electron transmission probability, which is calculated by the one-dimensional Schrödinger equation resolution with the transfer-matrix method. The procedure is applied to a 0.07 μm gate length and 1.5 nm gate oxide thickness transistor, for which the gate and drain 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 obtained for one of the two static points of complementary metal oxide semiconductor inverters: V_GS=V_DD and V_DS=0, which raises a severe problem of standby power consumption. The contribution of hot carriers to the tunnel current 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 gate 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 largest. Oxide thickness fluctuations are then considered by meshing the oxide surface area and assuming a Gaussian law for the local oxide thickness deviation to the mean value. It is shown that a correct agreement is achieved with experimental published data when the oxide film nonuniformity is included in the calculation. Gate currents mapping for different bias conditions are giv- en and analyzed, which show that very high current densities run through the oxide layer in the vicinity of weak points. An estimate of the surface through which flows the major part of the current is made, and a link between the highly nonuniform current leakage and the soft-breakdown mechanism of the oxide layer is proposed. © 1999 American Institute of Physics.