DYNAMIC CHANGES IN CHARACTERISTICS OF A-SI TRANSISTORS DURING FAST PULSED OPERATION

The switching time of a-Si TFT's was measured. During pulsed operation there are dynamic changes of threshold voltage V(t) and gate capacitance C(g) which occur mostly on a time scale ranging from microseconds to milliseconds. These can be qualitatively explained in terms of the fraction of the induced channel charge which is trapped in deep states in the semiconductor, and its spatial distribution. The value of V(t) can decrease by a much as 3 V during a pulse and also depends on the duty cycle. In pulsed operation, V(t) is always less than the static value; hence the current output i(sd) will be higher than calculated from the static characteristics and will depend on the duty cycle. The effective mobility remains nearly constant with changes of operating cycle, showing that the thermal equilibrium between the free channel charge and the shallow traps does not change. The change in source-gate capacitance C(gs) confirms the inwards diffusion of the trapped charge which had been predicted by modeling studies of other investigators.