Low frequency noise in hydrogenated amorphous silicon thin-film transistors

Abstract

Hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) are used as charge switches in flat-panel X-ray detectors. The inherent noise in the TFTs contributes to the overall noise figure of the detectors and degrades the image quality. Measurements of the noise provide an important parameter for modeling the performance of the detectors and are a sensitive diagnostic tool for device quality. Furthermore, understanding the origins of the noise could lead to change a method of a-Si:H deposition resulting in a reduction of the noise level. This thesis contains measurements of the low-frequency noise in a-Si:H TFTs with an inverted staggered structure. The noise power density spectrum fits well to a 1/ƒᵅ power law with α near one. The normalized noise power is inversely proportional to gate voltage and also inversely proportional to channel length in both the linear and saturation regions. The noise is nearly independent of the drain-source voltage and drain-source current. The noise is unaffected by degrading the amorphous silicon through gate-biasing stress. Hooge's parameter is in the range 1-2×10⁻³ or 2-4×10⁻⁴ depending on whether the parameter is calculated using the total number of charge carriers in the accumulation layer or just the number of free carriers. As an example, the signal to noise ratio is calculated for photodiode detector gated by a TFT using the results from the noise measurements.