14. SEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES AND SIMPLE CIRCUITS

Devices in which a controlled flow of electrons can be obtained are the basic building blocks of all the electronic circuits. Before the discovery of transistor in 1948, such devices were mostly vacuum tubes (also called valves) like the vacuum diode which has two electrodes, viz., anode (often called plate) and cathode; triode which has three electrodes – cathode, plate and grid; tetrode and pentode (respectively with 4 and 5 electrodes). In a vacuum tube, the electrons are supplied by a heated cathode and the controlled flow of these electrons in vacuum is obtained by varying the voltage between its different electrodes. Vacuum is required in the inter-electrode space; otherwise the moving electrons may lose their energy on collision with the air molecules in their path.

This introduction discusses the evolution of electronic devices, highlighting the transition from vacuum tubes to semiconductor devices. Vacuum tubes, which were predominant before the transistor, operate on the principle of electron flow in a vacuum created between electrodes. The anode and cathode are essential components of vacuum tubes, where the former receives the electrons emitted by the heated cathode. The need for a vacuum is crucial to prevent electron energy loss due to air resistance.

Think of a vacuum tube as a train moving through a tunnel. If the tunnel (vacuum) is free of obstacles (air molecules), the train (electrons) can move swiftly without losing speed. If there were obstacles in the tunnel, the train would slow down and could even stop, similar to how electrons lose energy in air.

These vacuum tube devices are bulky, consume high power, operate generally at high voltages (~100 V) and have limited life and low reliability. The seed of the development of modern solid-state semiconductor electronics goes back to 1930’s when it was realised that some solid-state semiconductors and their junctions offer the possibility of controlling the number and the direction of flow of charge carriers through them.

This chunk outlines the disadvantages of vacuum tubes, such as their size, power consumption, voltage requirements, and reliability issues. The introduction of solid-state semiconductors began in the 1930s when scientists recognized that these materials could effectively manage electron flow, paving the way for the transistor's invention. Unlike vacuum tubes, which are cumbersome and inefficient, semiconductors can control electron flow more effectively and efficiently.

Consider vacuum tubes like old, heavy light bulbs that use a lot of electricity and have a limited lifespan. In contrast, semiconductors are like modern LED bulbs that are small, energy-efficient, and last much longer. This innovation transformed how we use electronic devices.

In the following sections, we will introduce the basic concepts of semiconductor physics and discuss some semiconductor devices like junction diodes (a 2-electrode device) and bipolar junction transistors (a 3-electrode device). A few circuits illustrating their applications will also be described.

This section sets the stage for delving deeper into semiconductor physics, focusing on key devices like junction diodes and bipolar junction transistors, each serving different functions in electronic circuits. The discussion will encompass the fundamental principles behind these semiconductors, how they are constructed, and their real-world applications in electronic devices.

Imagine semiconductors as the traffic controllers of an electronic circuit. Just as traffic lights allow cars to flow smoothly through intersections, junction diodes and transistors regulate the flow of electric current, ensuring everything runs efficiently in devices like radios and computers.

On the basis of conductivity, solids are broadly classified as: (i) Metals: They possess very low resistivity (or high conductivity). r ~ 10–2 – 10–8 Ω m; s ~ 102 – 108 S m–1; (ii) Semiconductors: They have resistivity or conductivity intermediate to metals and insulators. r ~ 10–5 – 106 Ω m; s ~ 105 – 10–6 S m–1; (iii) Insulators: They have high resistivity (or low conductivity). r ~ 1011 – 1019 Ω m; s ~ 10–11 – 10–19 S m–1.

In this chunk, materials are categorized based on their conductivity levels. Metals are highly conductive, allowing electric current to pass through easily. Semiconductors lie in between, having moderate conductivity that can be altered through doping, while insulators resist electric current due to their high resistivity. Understanding these classifications helps delineate how different materials can be utilized in electronic devices.

Think about a road system. Metals are like major highways that allow traffic (electricity) to flow freely. Insulators are like dead-end streets where no traffic can pass. Semiconductors are like side streets that can be opened or closed depending on traffic lights; they can direct traffic flow as needed.

Our interest in this chapter is in the study of semiconductors which could be: (i) Elemental semiconductors: Si and Ge; (ii) Compound semiconductors: Examples are: Inorganic: CdS, GaAs, CdSe, InP, etc.; Organic: anthracene, doped phthalocyanines, etc.

Here, the text discusses two primary types of semiconductors: elemental and compound. Elemental semiconductors, such as silicon (Si) and germanium (Ge), are composed of a single type of element. In contrast, compound semiconductors, like gallium arsenide (GaAs), comprise two or more elements. Each type has unique properties and applications that leverage their conductive abilities.

Consider elemental semiconductors like the basic ingredients in a recipe, such as flour (silicon) and eggs (germanium). Compound semiconductors are like a more complex dish, including both flour and eggs along with ingredients like sugar and milk (gallium and arsenic) that create something entirely new and useful.

Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level. These different energy levels with continuous energy variation form what are called energy bands. The energy band which includes the energy levels of the valence electrons is called the valence band. The energy band above the valence band is called the conduction band.

This chunk explains the concept of energy bands within semiconductors. As atoms come together, the energy levels of their electrons are affected by the presence of neighboring atoms, forming bands of energies. The valence band encompasses the energy levels of electrons involved in bonding, while the conduction band consists of energy levels where electrons can move freely and contribute to electrical conductivity.

Imagine a concert where the attendees (electrons) are seated in different sections (energy levels). The first section (valence band) is reserved for bond-holding attendees, while the section above (conduction band) allows fans who want to dance around freely. In a semiconductor, some attendees can get enough energy to move up to the conduction band and start dancing (conducting electricity).

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