Modeling 1/f Noise in a-Si:H Field-Effect Transistors

by Xu, Yang

Hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) are used as switching elements in large area active matrix liquid crystal displays and various image sensing devices for radiation detection. The noise inherent in the a-Si:H TFTs contributes to the overall noise figure of such devices and degrades the signal to noise ratio; therefore, the noise is an important factor in the design of the devices. The noise of the a-Si:H TFTs has been studied experimentally, but the origin of the noise is not understood.

This work calculates the noise of the a-Si:H TFTs based on a simulation of operation of the TFTs and the hypothesis that the device noise is due to the intrinsic noise of the a-Si:H material. An a-Si:H TFT with an inverted-staggered structure has been simulated by numerically solving the fundamental transport equations for various gate and drain-source voltages. The drain-source curves derived from the simulation agree qualitatively with the experimental results: both the linear and saturated regions are observed. The low frequency noise was calculated based on the charge density distribution in the channel obtained from the simulation and the known dependence of the noise in the a-Si:H on the charge density, Hooges relation. The calculated noise power increases with the drain-source voltage and is inversely proportional to the gate voltage or the effective channel length. The curves agree qualitatively with the experimental results. The calculated noise power agrees quantitatively with the experiments when the scaling parameter in Hooges relation, , is set to . This value agrees with the experimentally determined value for a-Si:H. The results are consistent with the hypothesis that the low frequency noise in the a-Si:H TFTs is due to the material itself.