Stability issues in digital circuits in amorphous silicon technology

Hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs) are attractive for flat-panel liquid crystal displays (LCDs) and imaging sensor arrays in view of their large area capability. In large area imaging arrays or displays, the need for multiplexer circuits arises because of the large number of external gate and data line connections. This work presents a multiplexer (MUX) design using pass transistor logic (PTL) with a-Si:H TFTs used as switches. This research investigates inherent a-Si:H TFT switch metastability issues such as threshold voltage shifts due to prolonged TFT gate bias. In a typical MUX application, most of the TFTs are stressed under pulse bias instead of constant bias. Differences in stability behavior are observed under pulse and constant bias stress in a-Si:H TFTs. In this paper, we present constant and pulse bias stress measurement data performed on in-house fabricated a-Si:H TFTs.