We investigated the effect of hydrogen ion shower doping on polycrystalline silicon thin‐film transistors (p‐Si TFTs). Hydrogen atoms were introduced to the channel region ofp‐Si TFTs by PH3/H2ion shower doping of the source/drain contact. Hydrogen concentration in the channel region can be controlled by altering the gate metal thickness. Hydrogen atoms affect the TFT’s threshold voltage shifts until it becomes negative, inn‐type TFTs. The threshold voltage shift depends on the hydrogen content of the channel region inp‐Si TFTs. This is explained by the existence of Si−3trap states in the grain boundaries. ©1995 American Institute of Physics.