STUDY OF LEAKAGE CURRENT IN N-CHANNEL AND P-CHANNEL POLYCRYSTALLINE SILICON THIN-FILM TRANSISTORS BY CONDUCTION AND LOW-FREQUENCY NOISE MEASUREMENTS

The off-state current in n- and p-channel polycrystalline silicon thin -film transistors (polysilicon TFTs) is investigated systematically by conduction measurements at various temperatures and low-frequency noi se measurements at room temperature. It is demonstrated that the leaka ge current is controlled by the reverse biased drain junction. The mai n conduction mechanisms are due to thermal generation at low electric: fields and Poole-Frenkel accompanied by thermionic filed emission at high electric fields. The leakage current is correlated with the traps present in the polysilicon bulk and at the gate oxide/polysilicon int erface which are estimated from the on-state current activation energy data. Analysis of the leakage current noise spectral density confirms that deep levels with uniform energy distribution in the silicon band gap are the main factors in determining the leakage current. The dens ity of deep levels determined from noise analysis is in agreement with the value obtained from conductance activation energy analysis. The s ubstantially lower leakage current observed in the n-channel polysilic on TFT is explained by the development. of positive fixed charges at t he interface near the drain junction which suppress the electric field . (C) 1997 American Institute of Physics.