JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV LOW POWER VLSI DESIGN Previous Question Paper (July/August - 2022 - R18)

The demand for low power design in VLSI circuits is primarily driven by the need for increased integration density, portability, and reliability. As the number of transistors on a chip grows, power consumption increases, leading to thermal management challenges and the requirement for expensive cooling systems. For mobile devices, power consumption directly dictates battery life, making efficiency critical. Furthermore, high power dissipation leads to increased junction temperatures, which accelerates degradation mechanisms like electromigration and hot-carrier injection, thereby reducing the operational lifespan and reliability of the device.

The threshold voltage (Vth) of a MOSFET is the gate-source voltage required to create a conductive channel. It is defined by the following expression:

The factors influencing Vth include:

• Flat-band voltage (Vfb): Determined by the work function difference between the gate and the substrate and the presence of oxide charges.
• Surface potential (2φf): Dependent on the doping concentration (NA) of the substrate.
• Oxide capacitance (Cox): Inversely proportional to the gate oxide thickness (tox); thinner oxides reduce Vth.
• Body effect: If the source-bulk voltage is non-zero, the depletion region charge increases, raising the threshold voltage.