Semiconductor Electronics Notes

Introduction

Semiconductor electronics is a pivotal topic in NEET Physics, encompassing the principles and applications of semiconductors in electronic devices. Understanding this subject is crucial for aspiring medical students as it forms the basis for various diagnostic and therapeutic instruments. This study note will break down the complex ideas into digestible sections, ensuring a comprehensive understanding of semiconductor electronics.

1. Basics of Semiconductors

1.1 What is a Semiconductor?

A semiconductor is a material with electrical conductivity intermediate between a conductor and an insulator. The conductivity of semiconductors can be controlled by doping and external conditions such as temperature and light.

1.2 Types of Semiconductors

• Intrinsic Semiconductors: Pure semiconductors without any significant impurities. Example: Silicon (Si) and Germanium (Ge).
• Extrinsic Semiconductors: Semiconductors doped with impurities to alter their electrical properties. They are further classified into:
  • n-type Semiconductors: Doped with donor atoms (e.g., Phosphorus in Silicon), which provide extra electrons.
  • p-type Semiconductors: Doped with acceptor atoms (e.g., Boron in Silicon), which create holes by accepting electrons.

2. Energy Bands in Solids

2.1 Energy Band Theory

• Valence Band: The energy band containing valence electrons.
• Conduction Band: The energy band where electrons can move freely, contributing to conduction.
• Band Gap: The energy difference between the valence band and the conduction band.

For semiconductors, the band gap is small enough to allow thermal excitation of electrons from the valence band to the conduction band.

2.2 Conductivity in Semiconductors

The conductivity ($\sigma$) of a semiconductor can be expressed as: $$ \sigma = ne\mu_e + pe\mu_h $$ where:

• $n$ = number of free electrons
• $p$ = number of holes
• $e$ = charge of an electron
• $\mu_e$ = electron mobility
• $\mu_h$ = hole mobility

3. p-n Junction

3.1 Formation of p-n Junction

When a p-type semiconductor is joined with an n-type semiconductor, a p-n junction is formed. At the junction, electrons from the n-region diffuse into the p-region and recombine with holes, creating a depletion region.

3.2 Depletion Region

The depletion region is devoid of free charge carriers and acts as an insulator. It has an electric field due to the ionized donor and acceptor atoms.

3.3 Biasing of p-n Junction

• Forward Bias: The p-side is connected to the positive terminal, and the n-side to the negative terminal of a battery. This reduces the width of the depletion region and allows current to flow.
• Reverse Bias: The p-side is connected to the negative terminal, and the n-side to the positive terminal. This increases the width of the depletion region and prevents current flow.

4. Semiconductor Devices

4.1 Diodes

A diode is a semiconductor device that allows current to flow in one direction only. It has two terminals: anode and cathode.

• Forward Bias: Diode conducts.
• Reverse Bias: Diode does not conduct.

4.2 Transistors

Transistors are three-terminal devices used for amplification and switching. They come in two types:

• Bipolar Junction Transistor (BJT): Consists of emitter, base, and collector.
  • NPN Transistor: Current flows from collector to emitter when a small current is applied to the base.
  • PNP Transistor: Current flows from emitter to collector when a small current is applied to the base.
• Field Effect Transistor (FET): Consists of source, gate, and drain.
  • Junction FET (JFET) and Metal-Oxide-Semiconductor FET (MOSFET) are common types.

5. Applications of Semiconductor Devices

5.1 Rectifiers

Diodes are used in rectifiers to convert AC to DC. Types include:

• Half-Wave Rectifier: Uses a single diode.
• Full-Wave Rectifier: Uses multiple diodes to utilize both halves of the AC cycle.

5.2 Amplifiers

Transistors are used in amplifiers to increase the amplitude of signals. Common configurations include:

• Common Emitter: Provides high gain.
• Common Base: Provides low input impedance.
• Common Collector: Provides high input impedance.

5.3 Switching

Transistors are used as switches in digital circuits. They can turn on or off a circuit by applying a small control voltage.

Summary

Semiconductor electronics is a vast field with numerous applications in modern technology. Understanding the fundamental concepts such as types of semiconductors, p-n junctions, and semiconductor devices is essential for mastering this topic. This study note provides a structured approach to learning these concepts, ensuring a solid foundation for NEET Physics.