Hot carrier effects in devices and circuits formed from poly-Si

Abstract
Hot carrier induced degradation effects have been characterised in both discrete poly-Si TFTs and poly-Si circuits. DLTS measurements on individual TFTs have been used to confirm an increase in fast trapping state density which was consistent with the DC current/voltage characteristics of the TFTs. The increase in fast trapping state density occurred in the drain half of the device and was continuous in energy across the band gap between at least 0.1 and 0.5 eV below the conduction band. The effects observed were qualitatively similar in both as-deposited columnar poly-Si TFTs and poly-Si TFTs fabricated from furnace crystallised films deposited in an initially amorphous state. Digital NMOS poly-Si circuits running under accelerated drive conditions undergo degradation caused by hot carrier effects in the TFTs which support the high drain biases. However, a stable dynamic shift register suitable for LC-TV row drive applications has been demonstrated. This was achieved by using the fact that devices with a longer total channel length are more stable with respect to hot carrier induced degradation than short channel length devices.

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