JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III VLSI DESIGN Previous Question Paper (August - 2022 - R18)

The n-well CMOS process is a standard fabrication technique where n-type wells are created in a p-type substrate to house PMOS transistors, while NMOS transistors are formed directly in the p-substrate. The process steps are as follows:

1. Substrate Preparation: A p-type silicon wafer is used as the starting material.
2. n-well Formation: An oxide layer is grown, patterned, and an n-type dopant (typically phosphorus) is implanted and driven in to form the n-well.
3. Active Area Definition: Field oxide (FOX) is grown using the LOCOS process to isolate transistor regions.
4. Gate Oxide and Polysilicon: A thin gate oxide is grown, followed by the deposition and patterning of polysilicon to form the transistor gates.
5. Source/Drain Implantation: Self-aligned ion implantation is performed. Boron is used for p+ source/drain regions in the n-well, and arsenic or phosphorus is used for n+ source/drain regions in the p-substrate.
6. Metallization: An insulating layer (PSG) is deposited, contact holes are etched, and aluminum or copper is deposited to create interconnects.

The structure consists of the p-substrate, the n-well, field oxide regions, gate electrodes, and source/drain diffusion regions, with metal layers providing the final connections.

When an external bias voltage (V_G) is applied to the gate of a MOS system, it alters the energy band structure and charge distribution at the silicon surface. The three primary operating regimes are:

1. Accumulation (V_G < V_FB): For a p-type substrate, a negative gate voltage attracts majority carriers (holes) to the oxide-semiconductor interface, increasing the hole concentration above the equilibrium level.
2. Depletion (V_FB < V_G < V_T): A positive gate voltage pushes holes away from the interface, creating a region depleted of mobile carriers, leaving behind negatively charged fixed acceptor ions.
3. Inversion (V_G > V_T): As the gate voltage increases further, the surface potential becomes high enough to attract minority carriers (electrons) to the interface. When the electron concentration at the surface exceeds the hole concentration in the bulk, the surface is considered inverted.

The MOS system acts as a capacitor where the gate oxide serves as the dielectric, and the gate electrode and the silicon substrate act as the two plates. The total capacitance is a function of the gate voltage and the resulting charge distribution.