Stability of nanocrystalline silicon bottom-gate thin film transistors with silicon nitride gate dielectric

We report on the stability of nanocrystalline silicon (nc-Si) bottom-gate (BG) thin film transistors (TFTs) with various compositions of hydrogenated amorphous silicon nitride (a-SiNx:H) gate dielectric. TFTs with nitrogen-rich nitride exhibit higher output transconductance, threshold voltage stability, and effective field effect mobility (mu(FE)) than the devices with silicon-rich gate dielectric. For example, mu(FE) drops from 0.75 to 0.2 cm(2)/V s when the gate dielectric composition [N]/[Si] changes from 1.3 to 1. The corresponding threshold voltages (V-T) are 4 and -2 V. Following 5 h electrical stress tests, the shift in threshold voltage (Delta V-T) is larger for dielectrics with lower [N]/[Si] content, regardless of the operating regime. Indeed, Delta V-T in the saturation regime is considerably less and correlates with the charge concentration in the channel, i.e., Delta V-T in saturation is about 2/3 of that in the linear regime. Relaxation tests on the stressed TFTs show that the charge trapping is the instability mechanism in nc-Si BG TFTs.