Semiconductor Electronics: Materials, Devices And Simple Circuits 14.1 Introduction

14.1 INTRODUCTION

Electronic devices that allow controlled flow of electrons form the foundation of all electronic circuits. Before the invention of the transistor in 1948, vacuum tubes (also called valves) were primarily used for this purpose. Vacuum tubes include devices such as vacuum diodes (with two electrodes: anode and cathode), triodes (three electrodes: cathode, plate, grid), tetrodes, and pentodes (with four and five electrodes respectively). In these devices, electrons are emitted from a heated cathode and their flow in the vacuum between electrodes is controlled by varying voltages applied to different electrodes. The vacuum is necessary to prevent electrons from losing energy due to collisions with air molecules. These devices allow electron flow only from cathode to anode, hence the term 'valve'. However, vacuum tubes are bulky, consume high power, operate at high voltages (~100 V), have limited life, and low reliability. The development of modern solid-state semiconductor electronics began in the 1930s when it was realized that some solid-state semiconductors and their junctions could control the number and direction of charge carriers flowing through them. Unlike vacuum tubes, semiconductor devices operate within the solid material itself without the need for external heating or vacuum. They are compact, consume low power, operate at low voltages, and have long life and high reliability. For example, cathode ray tubes (CRTs) used in televisions and monitors, which operate on vacuum tube principles, are being replaced by liquid crystal display (LCD) monitors supported by solid-state electronics. Historically, a naturally occurring crystal of galena (lead sulphide, PbS) with a metal point contact was used as a detector of radio waves, illustrating early semiconductor device concepts. This chapter introduces the basic concepts of semiconductor physics and discusses semiconductor devices such as junction diodes (two-electrode devices) and bipolar junction transistors (three-electrode devices). It also describes simple circuits illustrating their applications.

• Vacuum tubes were the primary devices before transistors for controlling electron flow.
• Vacuum tubes require heated cathodes and vacuum to operate, allowing electron flow in one direction.
• Vacuum tubes are bulky, consume high power, operate at high voltages, and have limited life.
• Semiconductor devices control charge carriers within the solid without vacuum or heating.
• Semiconductor devices are small, low power, low voltage, reliable, and long-lasting.
• Early semiconductor detectors used natural crystals like galena with metal contacts.
• 📌 Vacuum tube (Valve): An electronic device controlling electron flow in vacuum between electrodes.
• 📌 Semiconductor: A solid material whose electrical conductivity can be controlled by impurities and external stimuli.
• 📌 Transistor: A semiconductor device with three electrodes used to amplify or switch electronic signals.

14.2 CLASSIFICATION OF METALS, CONDUCTORS AND SEMICONDUCTORS

Solids can be broadly classified based on their electrical conductivity (σ) or resistivity (ρ = 1/σ) into metals, semiconductors, and insulators. (i) Metals: Metals have very low resistivity and correspondingly high conductivity. Their resistivity ranges approximately from 10⁻⁸ to 10⁻² Ω·m, and conductivity ranges from 10² to 10⁸ S·m⁻¹. Metals have partially filled conduction bands or overlapping conduction and valence bands, allowing free movement of electrons. (ii) Semiconductors: Semiconductors have resistivity and conductivity values intermediate between metals and insulators. Their resistivity ranges roughly from 10⁻⁵ to 10⁶ Ω·m, and conductivity from 10⁻⁶ to 10⁵ S·m⁻¹. Semiconductors can be elemental (like Silicon (Si) and Germanium (Ge)) or compound semiconductors (inorganic such as CdS, GaAs, CdSe, InP; organic such as anthracene, doped pthalocyanines; and organic polymers like polypyrrole, polyaniline, polythiophene). (iii) Insulators: Insulators have very high resistivity and very low conductivity. Their resistivity ranges from 10¹¹ to 10¹⁹ Ω·m, and conductivity from 10⁻¹¹ to 10⁻¹⁹ S·m⁻¹. They have a large energy gap between valence and conduction bands, preventing free electron movement. The classification based on energy bands explains these differences. In isolated atoms, electrons occupy discrete energy levels. When atoms form solids, their outer electron orbits overlap, forming continuous energy bands. The valence band contains valence electrons, and the conduction band lies above it. Metals have overlapping or partially filled bands allowing free electron movement. Insulators have a large band gap (>3 eV) preventing electron excitation to the conduction band. Semiconductors have a smaller band gap (<3 eV), allowing some electrons to thermally excite to the conduction band at room temperature, enabling conduction. Silicon and Germanium have diamond-like crystal structures where each atom shares four valence electrons with neighbors forming covalent bonds. Their energy bands split into valence and conduction bands separated by an energy gap Eg. The valence band is fully occupied at absolute zero, and the conduction band is empty. The band gap Eg for Si is about 1.1 eV and for Ge about 0.7 eV. This section sets the foundation for understanding conduction in semiconductors by classifying materials and explaining their band structures.

• Metals have low resistivity (~10⁻⁸ to 10⁻² Ω·m) and high conductivity (10² to 10⁸ S·m⁻¹).
• Semiconductors have intermediate resistivity (~10⁻⁵ to 10⁶ Ω·m) and conductivity (10⁻⁶ to 10⁵ S·m⁻¹).
• Insulators have very high resistivity (~10¹¹ to 10¹⁹ Ω·m) and very low conductivity (10⁻¹¹ to 10⁻¹⁹ S·m⁻¹).
• Energy bands form in solids due to overlapping atomic orbitals; valence and conduction bands are key.
• Metals have overlapping or partially filled bands; insulators have large band gaps; semiconductors have small band gaps.
• Silicon and Germanium have diamond-like structures with an energy gap between valence and conduction bands.
• 📌 Valence band: Energy band containing valence electrons, usually fully occupied at 0 K.
• 📌 Conduction band: Energy band above valence band where electrons are free to move and conduct electricity.
• 📌 Energy band gap (Eg): Energy difference between conduction band minimum and valence band maximum.