In NMOS transistors, the threshold voltage can be increased by using transistors with a higher Vt value. This can be accomplished by adjusting the doping profile or the channel length of the transistor. Higher Vt transistors have a higher threshold voltage, which means that they require a larger input voltage to turn on. This helps in reducing leakage current and improving noise margins.
Similarly, in PMOS transistors, the threshold voltage can be increased by using transistors with a lower Vt value. Lower Vt transistors have a higher threshold voltage in the case of PMOS. By using PMOS transistors with a lower Vt, you can achieve a higher threshold voltage, which again reduces leakage current and improves noise margins.
The use of different Vt values for NMOS and PMOS transistors allows designers to optimize the performance of the circuit by carefully selecting the appropriate transistors for each part of the design. This technique helps in achieving a balance between power consumption and speed, as well as improving the overall reliability of the circuit.