ELECTRICAL-PROPERTIES OF AMORPHOUS-SILICON TRANSISTORS AND MIS-DEVICES - COMPARATIVE-STUDY OF TOP NITRIDE AND BOTTOM NITRIDE CONFIGURATIONS

The electrical properties of silicon nitride/amorphous silicon structu res were investigated using thin film transistors (TFTs) and metal ins ulator semiconductor (MIS) devices employing either a top nitride (TN) or bottom nitride (BN) as gate insulator. The density of states (DOS) deduced from the subthreshold transfer characteristic of the TFTs is one to two orders of magnitude higher than that obtained from quasista tic C(V) measurements on the MIS structures. This difference is discus sed by considering the different thickness of the a-Si:H layers of the two devices and the role of a fixed charge at the rear interface. Bot h techniques indicate a DOS in BN devices which is only slightly lower than in TN devices, by less than a factor of two. The measured field effect mobility of BN TFTs is about 70% higher. The differences in the measured field effect mobility for TN and BN configuration are discus sed and ascribed to the source and drain parasitic resistances. The co nclusion is verified by the fabrication of a TN TFT with a pure phosph ine rear surface treatment, which exhibits performance comparable to B N TFTs.