JNTU B.Tech IV Year VLSI DESIGN Previous Question Paper (October/November - 2020 - R16)

The n-well fabrication process is a standard CMOS process used to create PMOS transistors within an n-type substrate region. The key steps are as follows:

1. Substrate Preparation: A lightly doped p-type silicon wafer is used as the starting substrate.
2. N-well Formation: A thick oxide layer is grown, followed by photolithography to define the n-well region. Phosphorus ions are implanted into the exposed silicon and driven in at high temperatures to form the n-well.
3. Active Area Definition: A thin layer of silicon dioxide (pad oxide) and silicon nitride (masking layer) is deposited. Photolithography and etching are used to define the active regions where transistors will be formed.
4. Field Oxide Growth: Local Oxidation of Silicon (LOCOS) is performed to grow thick field oxide in non-active areas for device isolation.
5. Gate Oxide and Polysilicon Deposition: The nitride mask is removed, and a high-quality thin gate oxide is grown. Polysilicon is then deposited and patterned to form the gate electrodes for the PMOS transistors.
6. Source/Drain Formation: A photoresist mask is applied to cover the n-well region except for the PMOS source and drain areas. Boron ions are implanted to create p+ source and drain regions within the n-well.
7. Metallization: An insulating layer (e.g., BPSG) is deposited, contact holes are etched, and metal (aluminum or copper) is deposited and patterned to provide electrical interconnections.

The primary difference between enhancement and depletion mode MOS devices lies in the state of the channel at zero gate-to-source voltage (Vgs=0).

In enhancement mode, the device is normally off and requires an applied gate voltage exceeding the threshold to induce a conductive channel. In depletion mode, the device is fabricated with an ion-implanted channel that allows current to flow even when Vgs is zero; a reverse bias voltage is required to deplete the carriers and shut off the current flow.