Explain the construction of n channel and p channel FET.

1. n-channel Field Effect Transistor (FET) Construction:

An n-channel FET consists of a semiconductor substrate (typically silicon) with three terminals: the drain, source, and gate. The substrate is lightly doped with acceptor impurities (p-type), creating a region with holes as majority carriers.

a) Substrate Preparation:

• The process begins with a p-type silicon wafer.
• The wafer is cleaned and polished to remove impurities and defects.
• A thin layer of silicon dioxide (SiO2) is grown on the surface by thermal oxidation. This serves as the gate insulator.

b) Source and Drain Formation:

• Two heavily doped (n-type) regions, called the source and drain, are created on the wafer surface.
• This is usually achieved by ion implantation or epitaxial growth techniques.
• The n-type regions are formed on either side of the gate insulator.

c) Gate Formation:

• A gate material, typically polysilicon (polycrystalline silicon), is deposited on the gate insulator.
• The gate material is patterned and etched to define the gate region.
• The gate is connected to the external circuit through metal contacts.

d) Contact Formation:

• Ohmic contacts are formed between the source, drain, and gate terminals and the semiconductor.
• Metal layers (such as aluminum or gold) are deposited and patterned to make these contacts.

2. p-channel FET Construction:

The construction of a p-channel FET is similar to that of an n-channel FET, with the following key differences:

a) Substrate Preparation:

• Instead of a p-type substrate, an n-type silicon wafer is used.
• This creates a substrate with electrons as majority carriers.

b) Source and Drain Formation:

• In a p-channel FET, the source and drain regions are heavily doped with acceptor impurities (p-type).
• This results in p-type regions on either side of the gate insulator.

Overall, the basic structure and fabrication steps of an n-channel FET and a p-channel FET are very similar, with the main difference being the doping types of the substrate, source, and drain regions.