Equivalent Circuit Description of Threshold Voltage Shift in a-Si:H TFTs From a Probabilistic Analysis of Carrier Population Dynamics

Abstract

Amorphous hydrogenated silicon thin-film transistors (TFTs) are critical components in large area display and sensor systems, and the need for TFT circuits has been increasing. However, the intrinsic metastability associated with the TFT leads to a threshold voltage shift (VT shift) with time, under prolonged gate bias. For design of reliable TFT circuits, it is imperative to accurately predict this instability for time-varying analog gate bias. In this paper, the author model the threshold voltage variation using a probabilistic analysis of the electron population dynamics as prescribed by the defect pool and charge trapping mechanisms. The model is then extended for prediction of the effect of variable gate bias, and in particular the device history, on the VT shift. Based on this model, a passive equivalent circuit is synthesized to accurately predict the VT shift in TFTs for applications in active matrix organic light emitting diode displays and sensors