A semiconductor device includes: a cell region including active regions where components of transistors are formed; the cell region are arranged to function as a D flip-flop that includes a primary latch (having a first sleepy inverter and a first non-sleepy (NS) inverter), a secondary latch (having a second sleepy inverter and a second NS inverter), and a clock buffer (having third and fourth NS inverters). The transistors are grouped: a first group has a standard threshold voltage (Vt_std); a second group has a low threshold voltage (Vt_low); and an optional third group has a high threshold voltage (Vt_high). The transistors which comprise the first or second NS inverter have Vt_low. Alternatively, the transistors of the cell region are further arranged to function as a scan-insertion type of D flip-flop (SDFQ) that further includes a multiplexer; and the transistors of the multiplexer have Vt_low.