Field-effects in polycrystalline-silicon films

Abstract The effect of a normal electric field applied near the surface of a chemically deposited, polycrystalline-silicon film has been studied. Since such polycrystalline films generally contain large numbers of deep donor and acceptor defect levels within the forbidden gap, an analysis of the effect of these deep defect levels has been performed. This analysis shows that deep donors increase the magnitude of the threshold voltage of a p -channel MOS transistor constructed with the conducting channel in the polycrystalline-silicon film, while deep acceptors have the opposite effect. The effects partially cancel when both types of defects are present. Experimental investigation of threshold voltages of p channel MOS transistors constructed in doped films indicates that the magnitude of the threshold voltage is increased by defect levels for both n - and p -type films. The effect is generally greates for films with minimum dopant concentration, in agreement with the expected behavior. Field-effect mobilities of approximately 30 cm 2 /V-sec are observed for most films. Films deposited at various temperatures over the range 650–1100°C with no intentional doping show that the magnitude of the threshold voltage generally decreases with increasing deposition temperature.