非晶矽薄膜電晶體在閘級交流電壓下之不穩定性探討

Abstract

在本篇論文中,我們探討了氫化非晶矽薄膜電晶體之元件特性有關的各項 因素,比較實驗結果可以發現改變閘極介電層及氫化非晶矽層的組成時,確 實會影響其元件特性,而其中的不穩定機制為電荷攫取及能態生成;電荷攫 取主要發生於閘極介電層及閘極介電層/氫化非晶矽層之介面,它會對電晶 體的轉換特性造成平移的效果,而能態生成則跟矽懸浮鍵與氫群集有較密 切的關係,它會對次臨界區的特性造成妨害.當正或負的交流偏壓施加於閘 極時,兩者得到的特性會有很大的差異,我們推測那是因為在N型薄膜電晶 體中的電洞是少數載子,所以當所施加的負偏壓頻率太高時,並沒有足夠的 電洞能跟隨脈波的起伏而有所反應,因此造成在交流偏壓下所得兩者結果 之不同,這是在直流偏壓下觀察不到的現象.另一方面,改變施加交流偏壓 的工作周期也會影響到量測的結果,我們可以用等效的總脈波時間來解釋 此一現象,如果總和的時間愈長,則所得到的臨界電壓偏移量便愈明顯.在 本篇論文中,我們利用自己建立的一套系統來進行非晶矽薄膜電晶體的可 靠度分析實驗,希望從中得到改善電晶體特性的方法,並且進而應用於生產 線上的產品,將來此套測試方法亦可應用於其它元件的量測,以期能達到改 善其特性之目的. The characteristics of the a-Si:H TFTs can be affected by the composition of the gate dielectric and a-Si:H layers. By comparing the different samples of a-Si:H TFTs, it is confirmed that the instability mechanisms are charge trapping and state creation. Charge trapping occurs in the gate dielectric and gate dielectric/a-Si:H film interface while the carriers are moving through the active region, and it can rigidly shift the transfer characteristics of the a-Si:h TFTs. On the other hand, state creation can degrade the subthreshold behaviors of the TFTs. It is related the the silicon dangling bonds and hydrogen clusters in the a-Si:H film which act as a defect pool. Quite different unstable characteristics were found when positive and negative gate bias stresses were applied, possibly it is related to the nature of the n-type a-Si:H TFTs. Holes are much fewer than electrons and there are not sufficient holes to follow the negative gate bias pulese when high frequency is chosen. An interesting phenomenon was also investigated. The absolute value of the threshold voltage shift increases when the total width or the duty cycle of the applied pulse width becomes larger. So it can be said that the instability characteristics depend on not only the period but also the duty cycle of the pulses. A testing system was bulit to make the reliability experiments and the factors which can affect the characteristics of a-Si:H TFTs have been widely studied. Finally, from the discussions the thesis, the electrical properties of a-Si:H TFTs are hoped to have improvements and can be applied on the real production lines. In addition, the procedures of testiong may be conducted on other transistors, such as poly-Si TFTs or single-crystalline Si transistors to know the reliability issues of them more.