Low-frequency noise measurements are performed in two types of low temperature polysilicon thin film transistors (TFTs). For the first TFT process, the polysilicon two layer structure induces large values of the channel access resistances, whose contribution to noise is dominant for large gate bias. For the second TFT process, the polysilicon single layer structure induces small access resistances and the measured noise is mainly due to channel sources. For small voltages, the channel noise spectral density evolution with gate bias agrees with the mobility fluctuation model and is identical for both processes. For large voltages (>2 V), the channel noise spectral density evolution, observed only in the case of the single layer structure, seems to agree with the fluctuations of carrier density. However, this interpretation is discussed. The results of static characterization show that the quality of the channel active layer is quite different from the two layer structure to the single layer structure. in agreement with these observations, the observed evolution of the relative noise with increasing gate bias in TFTs can be interpreted from intergrain potential lowering.
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