HOT-CARRIER EFFECTS IN DEVICES AND CIRCUITS FORMED FROM POLY-SI

Hot carrier induced degradation effects have been characterised in bot h discrete poly-Si TFTs and poly-Si circuits. DLTS measurements on ind ividual TFTs have been used to confirm an increase in fast trapping st ate density which was consistent with the DC current/voltage character istics of the TFTs. The increase in fast trapping state density occurr ed 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 ban d. The effects observed were qualitatively similar in both as-deposite d columnar poly-Si TFTs and poly-Si TFTs fabricated from furnace cryst allised films deposited in an initially amorphous state. Digital NMOS poly-Si circuits running under accelerated drive conditions undergo de gradation caused by hot carrier effects in the TFTs which support the high drain biases. However, a stable dynamic shift register suitable f or LC-TV row drive applications has been demonstrated. This was achiev ed by using the fact that devices with a longer total channel length a re more stable with respect to hot carrier induced degradation than sh ort channel length devices.