The degradation of hydrogenated amorphous silicon under an applied field is studied in an amorphous silicon thin‐film transistor. A possible mechanism for metastable defect creation due to trapping of electrons at weak bonds together with a bond‐switching event is investigated. The energy for the bond‐switching process is assumed to be supplied thermally. The rate equation is set up and it is shown that this new model for defect creation is capable of describing the experimentally observed slow field‐effect current transients at various temperatures.