Jawaharlal Nehru Technological University Hyderabad B. Tech III VLSI DESIGN Previous Question Paper (February - 2023 - R18)

The threshold voltage (V_th) is the minimum gate-to-source voltage (V_GS) required to create a conducting inversion layer (channel) between the source and drain terminals of a MOSFET. When V_GS is less than V_th, the device is in the cut-off region; when V_GS exceeds V_th, the transistor enters the strong inversion region, allowing significant current to flow.

A pass transistor is a MOSFET used as a switch to pass or block logic signals between nodes in a digital circuit. It operates by connecting the drain to the input signal and the source to the output node, with the gate controlled by a clock or logic signal. When the gate voltage is high (for an NMOS), the transistor conducts, allowing the input signal to pass to the output; when low, it acts as an open circuit.

The advantages of using pass transistors include:

• Reduced Transistor Count: Logic functions such as multiplexers and XOR gates can be implemented with fewer transistors compared to standard CMOS logic, leading to smaller silicon area.
• Lower Power Consumption: Due to the reduced number of transistors and lower parasitic capacitance, the overall power dissipation of the circuit is significantly decreased.
• Higher Speed: The reduced input capacitance and simplified circuit topology allow for faster switching transitions in specific logic applications.