The effects of hydrogen plasma passivation on n-channel polycrystalline silicon thin film transistors fabricated with a maximum temperature of 600°C are investigated. It is found that charge, sharing, avalanche multiplication, grain size, and gate edge in-situ passivation or even partial hydrogenation can all contribute to the small geometry effect. At low drain bias, the channel charges supported by the source and drain fringing fields cause threshold shifts for short channel devices; at high drain bias avalanche multiplication is the dominant effect. The charge sharing effect may be related to the unpassivated trap-states in the channel since thresholds of fully hydrogenated devices became independent of channel length at low drain bias. The threshold shifts of narrow width devices were also diminished with increasing hydrogenation time. An anomalous turnaround effect of the field effect mobility in saturation regime for the short channel devices is observed near saturated hydrogenation. This effect is probably due to the passivation of back-channel defects and the reduction in impact ionization rate by hydrogenation. Saturated hydrogenation not only improves device performance but also reduces small geometry effects. Avalanche multiplication remains the dominant mechanism for short channel effect in the saturated hydrogenated devices.