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 (μFE) than the devices with silicon-rich gate dielectric. For example, μFE drops from 0.75to0.2cm2∕Vs when the gate dielectric composition [N]∕[Si] changes from 1.3 to 1. The corresponding threshold voltages (VT) are 4 and −2V. Following 5h electrical stress tests, the shift in threshold voltage (ΔVT) is larger for dielectrics with lower [N]∕[Si] content, regardless of the operating regime. Indeed, ΔVT in the saturation regime is considerably less and correlates with the charge concentration in the channel, i.e., ΔVT 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.