隨著製程的演進，超低電壓的應用與需求越來越為廣泛，諸如可攜式電子產品、醫療／生醫電子裝置、無線感測元件等，這些產品的產生主要為了因應越來越嚴苛的能源要求。在這類電路中，有些電路的操作電壓低於電晶體的門檻電壓，這類的電路我們稱為低門檻電壓電路，其最大的特點為透過降低電壓使得功率的消耗能夠有平方倍的下降，但所需克服的難題除了本身電晶體特性的改變外，還有電路操作過程中會產生的漏電流，如何有效率的使用能源，是該類電路的挑戰。數位電路在一個系統中扮演著很重要的角色，本篇主要是探討如何透過模擬與分析，改良數位電路標準元件使其在超低門檻電壓的應用更有效率。

Emerging ultra Low-Power applications such as portable devices, medical instruments, and wireless sensor networks demand extremely low supply voltages in order to meet stringent power budgets. Sub-Threshold circuits, which operate at supply voltage lower than the threshold voltage (Vth), are considered to be promising candidates for ultra Low-Power systems where operating speed is not the primary design concern. Digital circuit is an important part in SOC system, so digital circuits operating in the sub-threshold region provide the minimum energy solution for applications which strict energy constrains.