複晶矽薄膜電晶體短通道效應之研究

Abstract

在本論文中，我們想對複晶矽薄膜電晶體的短通道效應做些探討。我們將比較不同尺寸的元件，複晶矽薄膜晶粒不同大小尺寸，和製程上造成的不同晶粒邊界缺陷密度，對複晶矽薄膜電晶體所造成的特性差異。實驗發現，複晶矽薄膜電晶體隨著通道長度減少，有明顯導通電流上升和臨界電壓下降的趨勢。此外我們也觀察到載子撞擊游離造成嚴重的kink 效應。再者，對通道而言，浮動基極效應對絕緣層上的元件(SoI)可照成寄生雙極性介面電晶體（BJT）。應此我們也觀察到在不同通道寬度的短通道元件有拴鎖現像，這在複晶矽薄膜電晶體研究中，是相當罕見且有趣的。我們發現在不同的掃描方向觀察到磁滯效應，這個效應跟基極內電洞的復合有關。 為了瞭解載子撞擊游離對複晶矽薄膜電晶體的影響效應，可藉由Body contact 直接量測基極電流清楚得知。在高閘極電壓下可觀察到不尋常的基極電流。在反轉區跟基極的寄生穿隧效應可以解釋此現像。最後，根據物理觀念對所量測到的的基極電流建立一個模型。在導入包括最大電場效應載子撞擊游離模型的垂直電場散射效應後，可得到很好的一致性。

In this thesis, short channel effects in polycrystalline silicon thin-film transistors are investigated by the direct measurement through T-gate body contact . To study the short channel mechanisms more carefully, devices with different channel dimension, varying grain size, and different grain boundary trap density are fabricated and compared. It is found that with decreasing channel length, the devices exhibit improved normalized turn on current and decreased threshold voltage. However, severe kink effect which generated by the impact ionization also observed. Moreover, the floating body under the channel region serve as a parasitic BJT as in silicon-on-insulator devices. The related single transistor latch-up is observed and discussed for short-channel devices with various channel width. And a Hysteresis phenomenon is obvious. This is due to the recombination of the accumulation hole at the body. And it is found different behaviors with various channel width. The severe impact ionization effects in polycrystalline silicon thin-film transistors are investigated and characterized. By directly measuring the substrate current from conventional TFTs with body contact, the impact-ionization effects can be characterized and analyzed very clearly. An anomalous substrate current under high gate voltage is observed. The parasitic tunneling effect between inversion region and body region is proposed to explain this phenomenon. Finally, a physically-based model is established and compared with the measured substrate current. Good agreements are found when the vertical field scattering effect is included into the maximum electric field impact ionization model