Notes on Semiconductor Electronics: Materials, Devices and Simple Circuits

14.1 Introduction

The chapter discusses the evolution of electronic devices, moving from vacuum tubes to semiconductor devices, which have revolutionized electronic circuits. Key aspects include:

• Vacuum tubes like diodes and triodes were the predominant devices before transistors. They are bulky, inefficient, and unreliable.
• Creation of solid-state electronics began in the 1930s, realizing that certain materials could manipulate charge flow without the need for vacuum.
• Semiconductors are smaller, consume less power, are more reliable, and can operate at lower voltages. Devices like LCDs have started replacing CRT monitors based on the inefficient vacuum tube technology.
• The chapter aims to introduce basic semiconductor physics, key devices like junction diodes and bipolar junction transistors, and their applications.

14.2 Classification of Metals, Conductors, and Semiconductors

Conductivity Classification:

• Metals: High conductivity, low resistivity (10^-2 to 10^-8 Ωm).
• Semiconductors: Intermediate conductivity (10^-5 to 10^6 Ωm).
• Insulators: Very low conductivity and high resistivity (>10^11 Ωm).

Types of Semiconductors:

• Elemental: Silicon (Si), Germanium (Ge).
• Compound: Include both inorganic (e.g., GaAs, CdS) and organic (polymers like polyaniline).

Energy Bands:

• Energy bands arise when atoms form solids, with valence bands fully occupied and conduction bands potentially empty.
• Band Gap: The difference in energy between the valence band and the conduction band; affects conductivity:
  • Metals: Overlapping bands allow free flow of electrons.
  • Insulators: Large band gap (>3 eV) inhibits flow.
  • Semiconductors: Small band gap (0.2 - 3 eV) allows some conduction under certain conditions.

14.3 Intrinsic Semiconductors

• Intrinsic Semiconductors like Si and Ge form a diamond-like structure, sharing their four valence electrons with neighboring atoms.
• At absolute zero, there are no free charge carriers. As temperature rises, electrons gain enough energy to jump into the conduction band, creating holes (vacancies).
• The intrinsic carrier concentration of electrons equals that of holes: n = p = ni (intrinsic carrier concentration).

14.4 Extrinsic Semiconductors

• Doping is the introduction of impurities to increase conductivity.
• n-type semiconductors are formed by adding pentavalent impurities (e.g., P, As) which donate extra electrons, leading to electron majority and hole minority: n >> p.
• p-type semiconductors involve trivalent impurities (e.g., B, Al), creating holes as majority carriers: p >> n.
• The charge neutrality and relationship n*p = ni² hold true for both types.

14.5 p-n Junction

• A p-n junction forms when p-type and n-type semiconductors are in contact.
• Diffusion of charge carriers leads to the formation of a depletion region devoid of free carriers, creating an electric field across the junction.
• Equilibrium: At equilibrium, diffusion and drift currents balance, creating a built-in potential barrier.

14.6 Diodes and their Operation

• A semiconductor diode is a p-n junction that conducts current in one direction (forward bias).
• Forward Bias: Decreases the barrier potential allowing carrier injection and increased current.
• Reverse Bias: Increases the barrier potential, leading to a negligible current flow, only due to minority carriers.
• Diodes are crucial for rectification in circuits, converting AC to DC through devices like **half-wave and full-wave rectifiers

14.7 Applications of Diodes

Rectification:

• Half-wave rectification allows current to flow only during positive cycles, while full-wave rectification utilizes both cycles.
• Filters, like capacitors, smooth the pulsating rectified voltage to obtain a steady DC output.

Summary

1. Semiconductors form the basis of modern electronics, facilitating control over electricity.
2. Conductivity classifies materials into metals, semiconductors, and insulators based on resistivity.
3. Intrinsic and extrinsic types define semiconductor behavior with electrons and holes.
4. The p-n junction is critical in semiconductor devices, enabling diodes and transistors to function under changing conditions.
5. Diodes allow current flow in one direction, essential for converting AC to DC via rectifiers and filtering methods.

Key Points to Remember:

1. Semiconductors are critical components in modern electronics.
2. Intrinsically pure semiconductors have equal numbers of holes and electrons.
3. Doping introduces impurities to enhance conductivity, leading to n-type and p-type semiconductors.
4. A p-n junction is formed at the interface of n-type and p-type materials, crucial for diode operation.
5. In forward bias, the barrier potential decreases, increasing current flow; reverse bias increases it.
6. Diodes rectify current, allowing it to flow in one direction only, essential for power supply applications.

1. Semiconductors are foundational for modern electronics.
2. Intrinsic semiconductors have balanced electron-hole pairs.
3. Doping modifies conductivity, resulting in n-type and p-type materials.
4. A p-n junction is essential for diode functionality.
5. Current conduction varies with forward and reverse bias conditions.
6. Diodes serve as rectifiers, converting AC to DC.