| In recent years, the panel size of the flat panel display device is becominglarger and larger as the electronic technology develops quickly. At the same time,the color saturation and resolution of the display is constantly improving, and theresponse time of the device is reducing quickly. For all these reasons, we need highswitching and driving performance of the thin film transistor (TFT), which is thebasic component of the panel display device. Thus the perfect TFT should have idealparameters, such as the field effect mobility, working current, switch ratio and so on.However, the source-drain current offset happens very often in the TFT devices dueto the effects of the environment and preparation process. This source-drain currentoffset may lead to the electrical performance distort in the linear region, thereforecould affect the parameter extraction and abates the current drive capability of TFT.The oxide TFT, especial the ZnO TFT, has attracted many attentions for their virtuessuch as the high mobility, high transparency, and low process temperature.In thisthesis, Al-doped ZnO (ZAO) TFTs are fabricated using the pulse laser deposition(PLD) method. Then the effects of the annealing temperature, oxygen pressure, laserenergy, substrate temperature and the width to length ratio of the channel on the TFTelectrical properties are studied, with the expectation to find high performance ZAOTFT with no current offset. The main research contents and obtained results aresummarized as follows:1. ZAO films as well as the corresponding ZAO TFTs are prepared using thePLD method. The effects of the annealing temperature, oxygen pressure, laserenergy, substrate temperature and the width to length ratio of the channel on theTFT’s electrical properties are studied. The results show that ZAO TFTs annealed at200oC have serious Source-Drain current offset problem. And as the annealingtemperature increasing form200oC to500oC, the current offset extent is graduallydecreased. We also find that the Source-Drain current increase from17μA to51μAwhen the width to length ratio of the channel change from7.5to31.5.2. We prepared the ZAO films and the corresponding ZAO TFTs in the differentoxygen pressures, laser energies and substrate temperatures. And the effects of theseparameters on the micro-structure and current offset of the ZAO TFTs are studied.The experimental results show that the current offset extent decrease when the ZAOdeposition oxygen pressure changes from0mTorr to20mTorr. And the current offset extent increases when the laser energy rises from150mJ to450mJ. When thesubstrate temperature rises from50oC to300oC, the current offset extent decreases.The ZAO TFT device prepared at the substrate temperature of300oC nearly shows nocurrent offset. When the substrate temperature is at the range of300oC to500oC, theelectrical properties of the TFTs are improved with the temperature increasing, whilethe current offset extent increases with the temperature rising after annealing. TheZAO TFT prepared at the environment of a10mTorr oxygen pressure,250mJ laserenergy, and300oC substrate temperature exhibits very good electrical properties. Andthe ZAO TFT with a channel width to length ratio of7.5has a field effect mobility of3.43cm~2/V·s when the source-drain voltage is5V, and the work current with both thesource-drain voltage and gate voltage at20V is60μA.3. A source-drain current offset model based on the Petrosino model is developedto simulate effects of experimental parameters on the extent of the current offsets. Thesimulation results are in consistent with our experimental results which justifies thevalidity of our model. The simulation results show clearly that the current offsetextent increase with the raise of the gate leakage current and the source-drainelectrode potential difference. |
