Research On Electrical Properties And Stability Of Polysilicon Thin Film Transistors

As switching devices and peripheral circuit,Low-temperature polycrystalline silicon Thin Film Transistors?LTPS TFT?has been widely used in high-performance flat panel display,wearable devices,and integrated sensor,etc.However,there are always some instability problems such as the degradation of electrical properties in the practical application.The electrical properties and stability of LTPS-TFT have been studied in this paper.The main research contents and results of this paper are as follows:?1?The influence on channel length of the basic electrical performance?threshold voltage,field-effect mobility,and sub-threshold swing?and low-frequency noise characteristics of LTPS-TFT in P channel have been studied.The experimental results have shown that the threshold voltage of the devices is not affected by the channel length,and length and are maintained near-4V.The sub-threshold swing amplitude fluctuates slightly between 0.3 and0.35V/dec.However,the field-effect mobility of the device increases with the increase of channel length,which is due to the change of channel length,and the parasitic leakage/source?D/S?contact resistance forms a series with channel resistance,resulting in a great decrease in the field-effect mobility of the device in the shorter channel.The low-frequency noise?LFN?characteristics of the device have been measured and the noise parameters such as the spectral density of the flat band voltage noise and the Coulomb scattering coefficient have been extracted.The experimental results show that the dependence coefficient between the power spectral density of the leakage current and the effective gate voltage is between-1 and-2,and the Hooge factor remains unchanged in the device with a long channel device,indicating that the noise is mainly controlled by the mobility fluctuation mechanism at this time.However,in short channel devices,the Hooge factor decays with the increase of the effective gate voltage,indicating that the noise conforms to the carrier number fluctuation model.?2?The effects of temperature?10 K-300 K?on the basic electrical properties and noise characteristics of LTPS-TFT have been studied.The device transfer characteristics at different temperatures have been measured,and it is found that the threshold voltage shifted to the negative gate bias with the decrease of temperature,changing from-7.74 V at 300 K to-10.01V at 10 K.The mobility first increases and then decreases with the decrease of temperature,and it reaches the maximum value(?100 cm²V^-1s^-1)near 140 K.This is because the mobility change in the high-temperature region is dominated by lattice scattering,while that in the low-temperature region is dominated by the scattering of ionized impurities.The low-frequency noise characteristics of the device at different temperatures have been measured,and the noise parameters such as the flat-band voltage power spectral density and Hooge factor have been extracted.The experimental results show that the noise of the device is mainly caused by the capture/release of the carrier near the interface at a high temperature.At lower temperatures,the noise of the device is mainly caused by the fluctuation of mobility.?3?The influence on negative gate bias stress on the electrical properties of LTPS-TFT in P channel and its internal mechanism has been studied.The experimental results show that under the same gate bias stress,with the increase of stress time,the transfer characteristic curve shifts to the direction of negative gate pressure.For different gate bias stresses,the relationship between the threshold voltage change and the stress time follows a power exponential relationship,and the time constant increases with the increase of the bias stress,and its value is between 1/4 and 1/3.The swing amplitude of subthreshold value increases with the increase of stress time,and the mobility decreases with the increase of stress time.The negative charge generation model has been used to explain the bias stress effect,and the noise characteristics of the device at different temperatures have been tested and analyzed,which further proved the correctness of the model.