Non-uniform threshold voltage and non-saturating drain current in amorphous-Si TFT after saturation-mode bias temperature stress

We investigated the degradation effects on a-Si:H thin film transistors under a saturation-mode bias temperature stress. The simulated I-V characteristics using a non-uniform channel profile of V_t, with a maximum at source and minimum at drain, agreed very well with the measured data. The C-V characteristics were simulated using ATLAS based on this non-uniform V_t profile, which agreed well with data. The non-saturating reverse configuration I_d-V_ds characteristics was explained using the channel length modulation effect. Moreover, the device Vt extracted from linear I_d-V_gs characteristics can be modeled as a weighted average of the non-uniform V_t channel-profile. For short-channel devices, the experimental C-V data and ATLAS simulation show that the channel profiles of V_t, deep (NGA) and tail (NTA) state densities are affected significantly by the self heating effect during stress, where NGA (NTA) shows a flat maximum (minimum) level from source to mid channel.