物聯網時代來臨，在先進物聯網技術下，數據之儲存、傳輸、感測都需要超低功耗電晶體(ultra-low power transistor)作為基礎，傳統之金屬氧化物半導體場效電晶體(MOSFET)因受限於波茲曼限制(Boltzmann limit)，於室溫下之次臨界擺幅(subthreshold swing)永遠大於或等於60 mV/decade，為了實現超低功耗電晶體之目標，在超低操作電壓下將汲極電流提升並降低次臨界擺幅且低於60 mV/decade將是主要目標。

近年陸續研究出包括鰭式電晶體(Fin Field Effect Transistor, FinFET)、穿隧式電晶體(Tunnel Field Effect Transistor, TFET)、負電容場效電晶體(Negative Capacitance Field Effect Transistor, NCFET)等新世代超低功耗電晶體，負電容場效電晶體是唯一兼具同時提升開狀態汲極電流(I¬on)且降低次臨界擺幅潛力之超低功耗電晶體，也是本實驗之研究對象。

本次實驗將以負電容場效電晶體為基礎，將MIM架構之鐵電材料(HZO MIM)與金屬氧化物半導體場效電晶體做連接，並將傳統之閘極耦合式負電容場效電晶體(gate coupling NCFET)與本次實驗所研發之新式汲極/源極耦合式負電容場效電晶體(source/drain coupling NCFET)進行比較。實驗結果發現，傳統架構負電容場效電晶體之有效電荷遷移率( eff)受負電容效應影響而急劇降低，由HZO電偶極擾動產生之遠端散射(remote scattering)將在通道引發嚴重之聲子散射現象(phonon scattering event)，進而導致有效電荷遷移率降低，最終影響開狀態汲極電流的表現，而遠端散射現象可藉由將閘極氧化層電容提升，以極大之極化(polarization)能力將HZO之電偶極對齊，以降低遠端散射現象，進而使有效電荷遷移率之下降適度減緩，但此法牽涉到MIM鐵電材料與MOSFET閘極氧化層電容(Cox)之匹配問題，為求直接且全面的改善，本次實驗提出新式之並聯架構負電容場效電晶體。

以降低遠端散射現象並提升開狀態汲極電流(I¬on)為目標，本次實驗研發汲極/源極耦合式負電容場效電晶體(source / drain coupling NCFET)，經實驗證明，將MIM架構之HZO鐵電材料與傳統之金屬氧化物半導體場效電晶體進行並聯將有效降低由HZO電偶極所引發之遠端散射現象，通道載子將不再受聲子散射現象影響，明顯提升有效電荷遷移率，並實現同時放大開狀態汲極電流與降低次臨界擺幅(< 60 mV/decade)之目標。

除了上述由汲極/源極耦合式負電容場效電晶體所帶來之效益，本實驗亦針對短通道效應(short channel effect)進行探討，隨著汲極電壓增加，汲極引發之能障降低(DIBL)將更為嚴重，使閘極對於通道之控制能力降低，使得次臨界擺幅上升，而汲極/源極耦合式負電容場效電晶體已證明具有減緩汲極引發之能障降低之潛力，並能在施加適當之汲極電壓時達成零遲滯(non-hysteresis)之負電容場效電晶體。總之，本新穎結構的負電容場效電晶體，不只可以大幅提升Ion電流，亦有不錯的次臨界擺幅，適合於低電壓及低功耗用途。

In the internet of thing (IoT) era, more and more edge devices need to be accessed or transmit data to each other. The IoT is expected to be a smart and highly efficient platform in every industry. To realize IoT, ultra low power transistors are a prerequisite. In this point, negative capacitance field effect transistors (NCFETs) have come into the place because of their capability for achieving steeper subthreshold swing (< 60mV/decade) and larger on-current (Ion) simultaneously.

To realize a high-performance NCFET, not only the capacitance matching between ferroelectric HZO MIM and MOSFET but also how effective mobility is affected by HZO dipoles. Though the overall gate capacitance is greatly enhanced by NC effect, it does not guarantee the enhancement of drain current (Ion) because of the serious mobility degradation. In this work, we observed that NCFET with conventional architecture (gate coupling) has fair effective mobility ( eff), which eventually influenced the Ion¬ performance.

In order to rule out the issue of mobility degradation, we suggest a new architecture of source/drain coupling NCFET. With this approach, the channel carriers will not be affected by NC effect and subsequently enhanced the effective mobility. Furthermore, we have developed the experimental method to extract NC values, so not only the improvement of effective mobility, but also we have gained another benefit from the NC effect.

In the experiment of short channel effect, since the dipole built-in field of HZO enhances the gate controllability of MOSFET, drain coupling and source coupling show lower Vth. Moreover, they cause a decrease of DIBL with increasing Vds. Therefore, by integrating these desirable characteristics in NCFET, a high performance NCFET with Ion improvement and lower subthreshold swing can be achieved, which provides a potential candidate for low voltage and low power applications in the future.

英文文獻

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